If you have been watching tweets from @DoubleXSci since early December, you’ll have noticed tweets about Notable Historical and Modern Women in Science. Nearly 100 women were presented over twitter. Those women will be presented in a series here on the blog with the original tweeted links and information as well as with some additional information not able to be presented in 140 characters. We hope you look up more on these women.
(1893-1975) Dr. Bilger received her PhD in chemistry from the University of Cinncinnati in 1916. She graduated and went straight into a position as head of the chemistry department at Sweet Briar College. A brief stint at the University of Cinncinnati gave her skills that she later used in her position as Chair of the Department of Chemistry at the University of Hawaii to design a new chemistry laboratory facility. Her post as University of Hawaii Department Head began in 1943 and lasted 11 years. Her research was on asymmetric nitrogen compounds, for which she won the Garvan Medal.
(1890-1972) Born in Bogota, Columbia of missionaries, she arrived in the U.S. to attend high school. Dr. Caldwell was supported by her family in her pursuit of education and science. Due to gender restrictions, Caldwell attended a women’s college and stayed on there for teaching initially. This gave her the start on what she is known for: being a role model and mentor for other women for six decades. She received her A.B in 1913 from Western College for Women, her master’s degree in 1919 from Columbia, and her PhD in 1921 from Columbia, where she stayed on to teach. She entered the relatively new at the time field of nutritional chemistry, laying the groundwork for those after her. While Caldwell was well-known for the quality of research and diligence in her work, she also maintained a work-life balance, as an avid hiker, doting aunt, and gardener.
(1880-1972) Emma Perry Carr first attended Mr. Holyoke College then transferred to and received her B.S. from the University of Chicago in 1905. After a short duration as an instructor at Mt. Holyoke, Dr. Carr returned to the University of Chicago to receive her PhD in 1910. She returned to Mt. Holyoke to become a full professor and head of the department by the age of 33, a post she held for 33 years. Dr. Carr was also a devoted aunt,a fashionable dresser, and a talented storyteller. She had a relationship with Mary Sherrill, another professor at Mt. Holyoke, whom she shared a residence with for 26 years. Emma Perry Carr was the first recipient of the Garvan Medal.
(1867-1934) Much has been written about Marie Curie. She is, perhaps, the first historical figure to come to mind when a person says “Notable Woman in Science.” She is the first person to have been a twice Nobel Laureate. Marya Sklodowska was born in Poland, and lived through the loss of her eldest sister and mother by age 11. After graduating first her in class from high school, she attended a secret university because Polish universities could not admit women. She wished to go to Paris to study, so she worked and saved her money to do so. She was the first women to receive her Licence es Sciences Physiques from the Sorbonne in 1893, graduating first in her class again. She received her Licence es Sciences Mathematiques in 1894 from the same institution. In 1903, she attained her PhD from the University of Parish, the same year she was awarded the Nobel Prize in Physics. Difficulties continued in her personal life, such as the death of her husband in 1906, her own ill health due to radiation poisoning, and her constant fight for her place in her work. She broke so many barriers, being the first woman in so many circumstances.
(1909-1997) Mary Feiser was encouraged by her parents to excel academically. She attended Bryn Mawr and received her B.S. in chemistry in 1930. She then attended Radcliffe college and worked on her master’s thesis in the lab of Louis F. Feiser at Harvard. She received her A.M. in 1931 and married in 1932. She opted to continue to work in her husband’s lab instead of pursue a PhD because of the funding and Harvard facilities. With her help, 15 papers and 17 books were published by Feiser. However, Harvard never granted her a salary nor official title for 29 years. Even at 85 years of age, Mary Feiser continued to write and publish organic chemistry books, which were well received.
(1876-1950) Dorothy Hahn received her B.A. in chemistry from Bryn Mawr and went to work at Mt. Holyoke College under the auspices of Emma Perry Carr. Together, the two women were a force producing many women chemists. While Dr. Carr ran the chemistry department, it is said Dr. Hahn ran the organic chemistry department. Dr. Hahn pursued and recieved her Ph.D. from Yale University in 1916 due to a fellowship from the AAUW (American Association of University Women). Hahn also preceeded well-known scientists Gilbert Lewis and Irving Langmuir on a theory of valence electrons. Professor Hahn was a huge influence on organic chemistry, teaching, and women in chemistry.
(1906-1995) Allene Rosaland Jeanes was born and raised in Texas. She received her A.B with highest honors from Baylor University in 1928. She graduated with her M.A. from the University of California – Berkeley in 1929. She taught for awhile in a few different colleges, then decided to return to graduate school. She attained her PhD from the University of Illinois in 1938. While she wanted to go into pharmaceutical research, opportunities were limited. She took a position at the National Institute of Health. Her research took her through several government positions and had applications in the food industry. She was honored with many awards, including the Garvan Medal and Federal Women’s Award from the U.S. Civil Service Commission.
(1879-1968) The story of Ellen Gleditsch is not well known in her native Norway nor abroad, and signifies how difficult it was for women to be recognized for their work. She received her degree in pharmacology in 1902. She worked with Marie Curie for 5 years, and received her Licencee es Sciences from the Sorbonne in 1912. She went to work at Yale University despite the animosity toward her from the men at the U.S. institutions of Yale and Harvard and received her D.Sc. form Smith College in 1914. In 1929, Oslo University became embroiled in controversy over the decision to advance Ellen Gleditsch to the position of professional chair, and it took a letter from Marie Curie to help quell the public outrage. During her time in Oslo, she also provided a home for scientists fleeing Nazi Germany. She continued to be an advocate and mentor for women in the sciences until her death at age 88.
(1912-1998) Born in Missouri, Anna Jane Harrison was raised on a farm and her childhood science education tended to be “go out and find caterpillars.” She learned about Caterpillar tractors from her father for that assignment. Her high school science teachers inspired her interest in science, so she went to the University of Missouri to earn a B.A. in chemistry in 1933, a B.S. in education in 1935, a M.A. in chemistry in 1937, and a Ph.D. in physical chemistry in 1940. She was the first woman to earn a PhD at the institution. After meeting Lucy Picket and Emma Carr at a meeting of the American Chemical Society (ACS), she went on to work at Mt. Holyoke College, carrying on the traditions established there by Emma Carr and Dorothy Hahn. She also has several more “firsts” including being the first woman to chair the Division of Chemical Education of the ACS and the first woman elected president of the ACS in the 102 year history of the organization up to then. She was honored with the honorary degree of D.Sc. from ten instutitions. She enjoyed traveling and once stated, “What I really like is to go places one isn’t supposed to go.”
The Garvan Medalis an award from the American Chemical Society to recognize distinguished service to chemistry by women chemists. Nobel Prize: From the site:
Every year since 1901 the Nobel Prize has been awarded for achievements in physics, chemistry, physiology or medicine, literature and for peace. The Nobel Prize is an international award administered by the Nobel Foundation in Stockholm, Sweden. In 1968, Sveriges Riksbank established The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel, founder of the Nobel Prize. Each prize consists of a medal, personal diploma, and a cash award.
The four basic categories of molecules for building life are carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates serve many purposes, from energy to structure to chemical communication, as monomers or polymers.
Lipids, which are hydrophobic, also have different purposes, including energy storage, structure, and signaling.
Proteins, made of amino acids in up to four structural levels, are involved in just about every process of life.
The nucleic acids DNA and RNA consist of four nucleotide building blocks, and each has different purposes.
The longer version
Life is so diverse and unwieldy, it may surprise you to learn that we can break it down into four basic categories of molecules. Possibly even more implausible is the fact that two of these categories of large molecules themselves break down into a surprisingly small number of building blocks. The proteins that make up all of the living things on this planet and ensure their appropriate structure and smooth function consist of only 20 different kinds of building blocks. Nucleic acids, specifically DNA, are even more basic: only four different kinds of molecules provide the materials to build the countless different genetic codes that translate into all the different walking, swimming, crawling, oozing, and/or photosynthesizing organisms that populate the third rock from the Sun.
Big Molecules with Small Building Blocks
The functional groups, assembled into building blocks on backbones of carbon atoms, can be bonded together to yield large molecules that we classify into four basic categories. These molecules, in many different permutations, are the basis for the diversity that we see among living things. They can consist of thousands of atoms, but only a handful of different kinds of atoms form them. It’s like building apartment buildings using a small selection of different materials: bricks, mortar, iron, glass, and wood. Arranged in different ways, these few materials can yield a huge variety of structures.
We encountered functional groups and the SPHONC in Chapter 3. These components form the four categories of molecules of life. These Big Four biological molecules are carbohydrates, lipids, proteins, and nucleic acids. They can have many roles, from giving an organism structure to being involved in one of the millions of processes of living. Let’s meet each category individually and discover the basic roles of each in the structure and function of life.
You have met carbohydrates before, whether you know it or not. We refer to them casually as “sugars,” molecules made of carbon, hydrogen, and oxygen. A sugar molecule has a carbon backbone, usually five or six carbons in the ones we’ll discuss here, but it can be as few as three. Sugar molecules can link together in pairs or in chains or branching “trees,” either for structure or energy storage.
When you look on a nutrition label, you’ll see reference to “sugars.” That term includes carbohydrates that provide energy, which we get from breaking the chemical bonds in a sugar called glucose. The “sugars” on a nutrition label also include those that give structure to a plant, which we call fiber. Both are important nutrients for people.
Sugars serve many purposes. They give crunch to the cell walls of a plant or the exoskeleton of a beetle and chemical energy to the marathon runner. When attached to other molecules, like proteins or fats, they aid in communication between cells. But before we get any further into their uses, let’s talk structure.
The sugars we encounter most in basic biology have their five or six carbons linked together in a ring. There’s no need to dive deep into organic chemistry, but there are a couple of essential things to know to interpret the standard representations of these molecules.
Check out the sugars depicted in the figure. The top-left molecule, glucose, has six carbons, which have been numbered. The sugar to its right is the same glucose, with all but one “C” removed. The other five carbons are still there but are inferred using the conventions of organic chemistry: Anywhere there is a corner, there’s a carbon unless otherwise indicated. It might be a good exercise for you to add in a “C” over each corner so that you gain a good understanding of this convention. You should end up adding in five carbon symbols; the sixth is already given because that is conventionally included when it occurs outside of the ring.
On the left is a glucose with all of its carbons indicated. They’re also numbered, which is important to understand now for information that comes later. On the right is the same molecule, glucose, without the carbons indicated (except for the sixth one). Wherever there is a corner, there is a carbon, unless otherwise indicated (as with the oxygen). On the bottom left is ribose, the sugar found in RNA. The sugar on the bottom right is deoxyribose. Note that at carbon 2 (*), the ribose and deoxyribose differ by a single oxygen.
The lower left sugar in the figure is a ribose. In this depiction, the carbons, except the one outside of the ring, have not been drawn in, and they are not numbered. This is the standard way sugars are presented in texts. Can you tell how many carbons there are in this sugar? Count the corners and don’t forget the one that’s already indicated!
If you said “five,” you are right. Ribose is a pentose (pent = five) and happens to be the sugar present in ribonucleic acid, or RNA. Think to yourself what the sugar might be in deoxyribonucleic acid, or DNA. If you thought, deoxyribose, you’d be right.
The fourth sugar given in the figure is a deoxyribose. In organic chemistry, it’s not enough to know that corners indicate carbons. Each carbon also has a specific number, which becomes important in discussions of nucleic acids. Luckily, we get to keep our carbon counting pretty simple in basic biology. To count carbons, you start with the carbon to the right of the non-carbon corner of the molecule. The deoxyribose or ribose always looks to me like a little cupcake with a cherry on top. The “cherry” is an oxygen. To the right of that oxygen, we start counting carbons, so that corner to the right of the “cherry” is the first carbon. Now, keep counting. Here’s a little test: What is hanging down from carbon 2 of the deoxyribose?
If you said a hydrogen (H), you are right! Now, compare the deoxyribose to the ribose. Do you see the difference in what hangs off of the carbon 2 of each sugar? You’ll see that the carbon 2 of ribose has an –OH, rather than an H. The reason the deoxyribose is called that is because the O on the second carbon of the ribose has been removed, leaving a “deoxyed” ribose. This tiny distinction between the sugars used in DNA and RNA is significant enough in biology that we use it to distinguish the two nucleic acids.
In fact, these subtle differences in sugars mean big differences for many biological molecules. Below, you’ll find a couple of ways that apparently small changes in a sugar molecule can mean big changes in what it does. These little changes make the difference between a delicious sugar cookie and the crunchy exoskeleton of a dung beetle.
Sugar and Fuel
A marathon runner keeps fuel on hand in the form of “carbs,” or sugars. These fuels provide the marathoner’s straining body with the energy it needs to keep the muscles pumping. When we take in sugar like this, it often comes in the form of glucose molecules attached together in a polymer called starch. We are especially equipped to start breaking off individual glucose molecules the minute we start chewing on a starch.
Double X Extra: A monomer is a building block (mono = one) and a polymer is a chain of monomers. With a few dozen monomers or building blocks, we get millions of different polymers. That may sound nutty until you think of the infinity of values that can be built using only the numbers 0 through 9 as building blocks or the intricate programming that is done using only a binary code of zeros and ones in different combinations.
Our bodies then can rapidly take the single molecules, or monomers, into cells and crack open the chemical bonds to transform the energy for use. The bonds of a sugar are packed with chemical energy that we capture to build a different kind of energy-containing molecule that our muscles access easily. Most species rely on this process of capturing energy from sugars and transforming it for specific purposes.
Polysaccharides: Fuel and Form
Plants use the Sun’s energy to make their own glucose, and starch is actually a plant’s way of storing up that sugar. Potatoes, for example, are quite good at packing away tons of glucose molecules and are known to dieticians as a “starchy” vegetable. The glucose molecules in starch are packed fairly closely together. A string of sugar molecules bonded together through dehydration synthesis, as they are in starch, is a polymer called a polysaccharide (poly = many; saccharide = sugar). When the monomers of the polysaccharide are released, as when our bodies break them up, the reaction that releases them is called hydrolysis.
Double X Extra: The specific reaction that hooks one monomer to another in a covalent bond is called dehydration synthesis because in making the bond–synthesizing the larger molecule–a molecule of water is removed (dehydration). The reverse is hydrolysis (hydro = water; lysis = breaking), which breaks the covalent bond by the addition of a molecule of water.
Although plants make their own glucose and animals acquire it by eating the plants, animals can also package away the glucose they eat for later use. Animals, including humans, store glucose in a polysaccharide called glycogen, which is more branched than starch. In us, we build this energy reserve primarily in the liver and access it when our glucose levels drop.
Whether starch or glycogen, the glucose molecules that are stored are bonded together so that all of the molecules are oriented the same way. If you view the sixth carbon of the glucose to be a “carbon flag,” you’ll see in the figure that all of the glucose molecules in starch are oriented with their carbon flags on the upper left.
The orientation of monomers of glucose in polysaccharides can make a big difference in the use of the polymer. The glucoses in the molecule on the top are all oriented “up” and form starch. The glucoses in the molecule on the bottom alternate orientation to form cellulose, which is quite different in its function from starch.
Storing up sugars for fuel and using them as fuel isn’t the end of the uses of sugar. In fact, sugars serve as structural molecules in a huge variety of organisms, including fungi, bacteria, plants, and insects.
The primary structural role of a sugar is as a component of the cell wall, giving the organism support against gravity. In plants, the familiar old glucose molecule serves as one building block of the plant cell wall, but with a catch: The molecules are oriented in an alternating up-down fashion. The resulting structural sugar is called cellulose.
That simple difference in orientation means the difference between a polysaccharide as fuel for us and a polysaccharide as structure. Insects take it step further with the polysaccharide that makes up their exoskeleton, or outer shell. Once again, the building block is glucose, arranged as it is in cellulose, in an alternating conformation. But in insects, each glucose has a little extra added on, a chemical group called an N-acetyl group. This addition of a single functional group alters the use of cellulose and turns it into a structural molecule that gives bugs that special crunchy sound when you accidentally…ahem…step on them.
These variations on the simple theme of a basic carbon-ring-as-building-block occur again and again in biological systems. In addition to serving roles in structure and as fuel, sugars also play a role in function. The attachment of subtly different sugar molecules to a protein or a lipid is one way cells communicate chemically with one another in refined, regulated interactions. It’s as though the cells talk with each other using a specialized, sugar-based vocabulary. Typically, cells display these sugary messages to the outside world, making them available to other cells that can recognize the molecular language.
Lipids: The Fatty Trifecta
Starch makes for good, accessible fuel, something that we immediately attack chemically and break up for quick energy. But fats are energy that we are supposed to bank away for a good long time and break out in times of deprivation. Like sugars, fats serve several purposes, including as a dense source of energy and as a universal structural component of cell membranes everywhere.
Fats: the Good, the Bad, the Neutral
Turn again to a nutrition label, and you’ll see a few references to fats, also known as lipids. (Fats are slightly less confusing that sugars in that they have only two names.) The label may break down fats into categories, including trans fats, saturated fats, unsaturated fats, and cholesterol. You may have learned that trans fats are “bad” and that there is good cholesterol and bad cholesterol, but what does it all mean?
Let’s start with what we mean when we say saturated fat. The question is, saturated with what? There is a specific kind of dietary fat call the triglyceride. As its name implies, it has a structural motif in which something is repeated three times. That something is a chain of carbons and hydrogens, hanging off in triplicate from a head made of glycerol, as the figure shows. Those three carbon-hydrogen chains, or fatty acids, are the “tri” in a triglyceride. Chains like this can be many carbons long.
Double X Extra: We call a fatty acid a fatty acid because it’s got a carboxylic acid attached to a fatty tail. A triglyceride consists of three of these fatty acids attached to a molecule called glycerol. Our dietary fat primarily consists of these triglycerides.
Triglycerides come in several forms. You may recall that carbon can form several different kinds of bonds, including single bonds, as with hydrogen, and double bonds, as with itself. A chain of carbon and hydrogens can have every single available carbon bond taken by a hydrogen in single covalent bond. This scenario of hydrogen saturation yields a saturated fat. The fat is saturated to its fullest with every covalent bond taken by hydrogens single bonded to the carbons.
Saturated fats have predictable characteristics. They lie flat easily and stick to each other, meaning that at room temperature, they form a dense solid. You will realize this if you find a little bit of fat on you to pinch. Does it feel pretty solid? That’s because animal fat is saturated fat. The fat on a steak is also solid at room temperature, and in fact, it takes a pretty high heat to loosen it up enough to become liquid. Animals are not the only organisms that produce saturated fat–avocados and coconuts also are known for their saturated fat content.
The top graphic above depicts a triglyceride with the glycerol, acid, and three hydrocarbon tails. The tails of this saturated fat, with every possible hydrogen space occupied, lie comparatively flat on one another, and this kind of fat is solid at room temperature. The fat on the bottom, however, is unsaturated, with bends or kinks wherever two carbons have double bonded, booting a couple of hydrogens and making this fat unsaturated, or lacking some hydrogens. Because of the space between the bumps, this fat is probably not solid at room temperature, but liquid.
You can probably now guess what an unsaturated fat is–one that has one or more hydrogens missing. Instead of single bonding with hydrogens at every available space, two or more carbons in an unsaturated fat chain will form a double bond with carbon, leaving no space for a hydrogen. Because some carbons in the chain share two pairs of electrons, they physically draw closer to one another than they do in a single bond. This tighter bonding result in a “kink” in the fatty acid chain.
In a fat with these kinks, the three fatty acids don’t lie as densely packed with each other as they do in a saturated fat. The kinks leave spaces between them. Thus, unsaturated fats are less dense than saturated fats and often will be liquid at room temperature. A good example of a liquid unsaturated fat at room temperature is canola oil.
A few decades ago, food scientists discovered that unsaturated fats could be resaturated or hydrogenated to behave more like saturated fats and have a longer shelf life. The process of hydrogenation–adding in hydrogens–yields trans fat. This kind of processed fat is now frowned upon and is being removed from many foods because of its associations with adverse health effects. If you check a food label and it lists among the ingredients “partially hydrogenated” oils, that can mean that the food contains trans fat.
Double X Extra: A triglyceride can have up to three different fatty acids attached to it. Canola oil, for example, consists primarily of oleic acid, linoleic acid, and linolenic acid, all of which are unsaturated fatty acids with 18 carbons in their chains.
Why do we take in fat anyway? Fat is a necessary nutrient for everything from our nervous systems to our circulatory health. It also, under appropriate conditions, is an excellent way to store up densely packaged energy for the times when stores are running low. We really can’t live very well without it.
Phospholipids: An Abundant Fat
You may have heard that oil and water don’t mix, and indeed, it is something you can observe for yourself. Drop a pat of butter–pure saturated fat–into a bowl of water and watch it just sit there. Even if you try mixing it with a spoon, it will just sit there. Now, drop a spoon of salt into the water and stir it a bit. The salt seems to vanish. You’ve just illustrated the difference between a water-fearing (hydrophobic) and a water-loving (hydrophilic) substance.
Generally speaking, compounds that have an unequal sharing of electrons (like ions or anything with a covalent bond between oxygen and hydrogen or nitrogen and hydrogen) will be hydrophilic. The reason is that a charge or an unequal electron sharing gives the molecule polarity that allows it to interact with water through hydrogen bonds. A fat, however, consists largely of hydrogen and carbon in those long chains. Carbon and hydrogen have roughly equivalent electronegativities, and their electron-sharing relationship is relatively nonpolar. Fat, lacking in polarity, doesn’t interact with water. As the butter demonstrated, it just sits there.
There is one exception to that little maxim about fat and water, and that exception is the phospholipid. This lipid has a special structure that makes it just right for the job it does: forming the membranes of cells. A phospholipid consists of a polar phosphate head–P and O don’t share equally–and a couple of nonpolar hydrocarbon tails, as the figure shows. If you look at the figure, you’ll see that one of the two tails has a little kick in it, thanks to a double bond between the two carbons there.
Phospholipids form a double layer and are the major structural components of cell membranes. Their bend, or kick, in one of the hydrocarbon tails helps ensure fluidity of the cell membrane. The molecules are bipolar, with hydrophilic heads for interacting with the internal and external watery environments of the cell and hydrophobic tails that help cell membranes behave as general security guards.
The kick and the bipolar (hydrophobic and hydrophilic) nature of the phospholipid make it the perfect molecule for building a cell membrane. A cell needs a watery outside to survive. It also needs a watery inside to survive. Thus, it must face the inside and outside worlds with something that interacts well with water. But it also must protect itself against unwanted intruders, providing a barrier that keeps unwanted things out and keeps necessary molecules in.
Phospholipids achieve it all. They assemble into a double layer around a cell but orient to allow interaction with the watery external and internal environments. On the layer facing the inside of the cell, the phospholipids orient their polar, hydrophilic heads to the watery inner environment and their tails away from it. On the layer to the outside of the cell, they do the same.
As the figure shows, the result is a double layer of phospholipids with each layer facing a polar, hydrophilic head to the watery environments. The tails of each layer face one another. They form a hydrophobic, fatty moat around a cell that serves as a general gatekeeper, much in the way that your skin does for you. Charged particles cannot simply slip across this fatty moat because they can’t interact with it. And to keep the fat fluid, one tail of each phospholipid has that little kick, giving the cell membrane a fluid, liquidy flow and keeping it from being solid and unforgiving at temperatures in which cells thrive.
Steroids: Here to Pump You Up?
Our final molecule in the lipid fatty trifecta is cholesterol. As you may have heard, there are a few different kinds of cholesterol, some of which we consider to be “good” and some of which is “bad.” The good cholesterol, high-density lipoprotein, or HDL, in part helps us out because it removes the bad cholesterol, low-density lipoprotein or LDL, from our blood. The presence of LDL is associated with inflammation of the lining of the blood vessels, which can lead to a variety of health problems.
But cholesterol has some other reasons for existing. One of its roles is in the maintenance of cell membrane fluidity. Cholesterol is inserted throughout the lipid bilayer and serves as a block to the fatty tails that might otherwise stick together and become a bit too solid.
Cholesterol’s other starring role as a lipid is as the starting molecule for a class of hormones we called steroids or steroid hormones. With a few snips here and additions there, cholesterol can be changed into the steroid hormones progesterone, testosterone, or estrogen. These molecules look quite similar, but they play very different roles in organisms. Testosterone, for example, generally masculinizes vertebrates (animals with backbones), while progesterone and estrogen play a role in regulating the ovulatory cycle.
Double X Extra: A hormone is a blood-borne signaling molecule. It can be lipid based, like testosterone, or short protein, like insulin.
As you progress through learning biology, one thing will become more and more clear: Most cells function primarily as protein factories. It may surprise you to learn that proteins, which we often talk about in terms of food intake, are the fundamental molecule of many of life’s processes. Enzymes, for example, form a single broad category of proteins, but there are millions of them, each one governing a small step in the molecular pathways that are required for living.
Levels of Structure
Amino acids are the building blocks of proteins. A few amino acids strung together is called a peptide, while many many peptides linked together form a polypeptide. When many amino acids strung together interact with each other to form a properly folded molecule, we call that molecule a protein.
For a string of amino acids to ultimately fold up into an active protein, they must first be assembled in the correct order. The code for their assembly lies in the DNA, but once that code has been read and the amino acid chain built, we call that simple, unfolded chain the primary structure of the protein.
This chain can consist of hundreds of amino acids that interact all along the sequence. Some amino acids are hydrophobic and some are hydrophilic. In this context, like interacts best with like, so the hydrophobic amino acids will interact with one another, and the hydrophilic amino acids will interact together. As these contacts occur along the string of molecules, different conformations will arise in different parts of the chain. We call these different conformations along the amino acid chain the protein’s secondary structure.
Once those interactions have occurred, the protein can fold into its final, or tertiary structure and be ready to serve as an active participant in cellular processes. To achieve the tertiary structure, the amino acid chain’s secondary interactions must usually be ongoing, and the pH, temperature, and salt balance must be just right to facilitate the folding. This tertiary folding takes place through interactions of the secondary structures along the different parts of the amino acid chain.
The final product is a properly folded protein. If we could see it with the naked eye, it might look a lot like a wadded up string of pearls, but that “wadded up” look is misleading. Protein folding is a carefully regulated process that is determined at its core by the amino acids in the chain: their hydrophobicity and hydrophilicity and how they interact together.
In many instances, however, a complete protein consists of more than one amino acid chain, and the complete protein has two or more interacting strings of amino acids. A good example is hemoglobin in red blood cells. Its job is to grab oxygen and deliver it to the body’s tissues. A complete hemoglobin protein consists of four separate amino acid chains all properly folded into their tertiary structures and interacting as a single unit. In cases like this involving two or more interacting amino acid chains, we say that the final protein has a quaternary structure. Some proteins can consist of as many as a dozen interacting chains, behaving as a single protein unit.
A Plethora of Purposes
What does a protein do? Let us count the ways. Really, that’s almost impossible because proteins do just about everything. Some of them tag things. Some of them destroy things. Some of them protect. Some mark cells as “self.” Some serve as structural materials, while others are highways or motors. They aid in communication, they operate as signaling molecules, they transfer molecules and cut them up, they interact with each other in complex, interrelated pathways to build things up and break things down. They regulate genes and package DNA, and they regulate and package each other.
As described above, proteins are the final folded arrangement of a string of amino acids. One way we obtain these building blocks for the millions of proteins our bodies make is through our diet. You may hear about foods that are high in protein or people eating high-protein diets to build muscle. When we take in those proteins, we can break them apart and use the amino acids that make them up to build proteins of our own.
How does a cell know which proteins to make? It has a code for building them, one that is especially guarded in a cellular vault in our cells called the nucleus. This code is deoxyribonucleic acid, or DNA. The cell makes a copy of this code and send it out to specialized structures that read it and build proteins based on what they read. As with any code, a typo–a mutation–can result in a message that doesn’t make as much sense. When the code gets changed, sometimes, the protein that the cell builds using that code will be changed, too.
Biohazard!The names associated with nucleic acids can be confusing because they all start with nucle-. It may seem obvious or easy now, but a brain freeze on a test could mix you up. You need to fix in your mind that the shorter term (10 letters, four syllables), nucleotide, refers to the smaller molecule, the three-part building block. The longer term (12 characters, including the space, and five syllables), nucleic acid, which is inherent in the names DNA and RNA, designates the big, long molecule.
DNA vs. RNA: A Matter of Structure
DNA and its nucleic acid cousin, ribonucleic acid, or RNA, are both made of the same kinds of building blocks. These building blocks are called nucleotides. Each nucleotide consists of three parts: a sugar (ribose for RNA and deoxyribose for DNA), a phosphate, and a nitrogenous base. In DNA, every nucleotide has identical sugars and phosphates, and in RNA, the sugar and phosphate are also the same for every nucleotide.
So what’s different? The nitrogenous bases. DNA has a set of four to use as its coding alphabet. These are the purines, adenine and guanine, and the pyrimidines, thymine and cytosine. The nucleotides are abbreviated by their initial letters as A, G, T, and C. From variations in the arrangement and number of these four molecules, all of the diversity of life arises. Just four different types of the nucleotide building blocks, and we have you, bacteria, wombats, and blue whales.
RNA is also basic at its core, consisting of only four different nucleotides. In fact, it uses three of the same nitrogenous bases as DNA–A, G, and C–but it substitutes a base called uracil (U) where DNA uses thymine. Uracil is a pyrimidine.
DNA vs. RNA: Function Wars
An interesting thing about the nitrogenous bases of the nucleotides is that they pair with each other, using hydrogen bonds, in a predictable way. An adenine will almost always bond with a thymine in DNA or a uracil in RNA, and cytosine and guanine will almost always bond with each other. This pairing capacity allows the cell to use a sequence of DNA and build either a new DNA sequence, using the old one as a template, or build an RNA sequence to make a copy of the DNA.
These two different uses of A-T/U and C-G base pairing serve two different purposes. DNA is copied into DNA usually when a cell is preparing to divide and needs two complete sets of DNA for the new cells. DNA is copied into RNA when the cell needs to send the code out of the vault so proteins can be built. The DNA stays safely where it belongs.
RNA is really a nucleic acid jack-of-all-trades. It not only serves as the copy of the DNA but also is the main component of the two types of cellular workers that read that copy and build proteins from it. At one point in this process, the three types of RNA come together in protein assembly to make sure the job is done right.
It’s tempting to cast the role of women in STEM (Science, Technology, Engineering, and Math) as one of struggles and battles because of their sex, rather than as one of contributions because of their minds. But for Women’s History Month and this Diversity in Science Carnival #14, our focus is the role of women in the enterprise of STEM. There’s more to a woman than her sex and her struggles in science–there is, after all, the enormous body of work women have contributed to science.
Our history is ongoing, but we can start with a look back. Thanks to the efforts of the Smithsonian Institution Archives, we can put faces to the names of some of the female STEMmers of history. In a presentation of photographs in an 8 by 9 space, we can see the images of 72 women who contributed to the enterprise of STEM, many of them involved with the Smithsonian in some capacity. As their clothes and the dates on the photos tell us, these women were doing their work in a time when most women didn’t even wear pants.
Some are Big Names–you’ve probably heard of Marie Curie. But others are like many of us, women working in the trenches of science, contributing to the enterprise of STEM in ways big and small. Women like Arlene Frances Fung, whose bio tells us she was born in Trinidad, went to medical school in Ireland, and by 1968 was engaged in chromosome research at a cancer institute in Philadelphia. From Trinidad to cancer research, her story is one of the millions we could tell about women’s historical contributions to science, if only we could find them all. But here there are 72, and we encourage you to click on each image, look at their direct gazes, ponder how their interest in science and knowledge trumped the heavy pressures of social mores, and discover the contributions these 72 women made, each on her own “little two inches wide of ivory.”
For more on historical and current women in science, you can also see Double X Science’s “Notable Women in Science” series, curated by Adrienne Roehrich.
And then there are the women STEMmers of today, who likely are, according to blogger Emma Leedham writing at her blog Pipettes and Paintbrushes, still underpaid. Leedham also mulls here what constitutes a role model for women–does it require being both a woman and a scientist, or one or the other?
Laurel L. James
Laurel L. James, writing at the University of Washington blog for the school’s SACNAS student chapter, answers with her post, “To identify my role as a woman in science: I must first honor my mother, my family and my past.” Her mother was the first “Miss Indian America,” and Laurel is a self-described non-traditional student at the school, where she is a graduate student in forest resources. She traces her journey to science, one that involved role models who were not scientists but who, as she writes, showed her “how to hang onto the things that are important with the expectation of getting something in return all the while, persevering and knowing who you are; while walking with grace and dignity.” I’d hazard that these words describe many a woman who has moved against the currents of her society to contribute something to the sciences.
A great site, Steminist.com, which features the “voices of women in science, tech, engineering, and math,” runs a series of interviews with modern-day STEMmers, including Double X Science’s own Jeanne Garbarino, and Naadiya Moosajee, an engineer and cofounder of South African Women in Engineering. You can follow Naadiya on Twitter here. Steminist is also running their version of March Madness, except that in honor of Women’s History Month, we can choose “Which historical women in STEM rock (our) world.” The 64 historical STEMinists in the tourney are listed here and include Emily Warren Robling (left), who took over completion of the Brooklyn Bridge when her husband’s health prevented his doing so; she is known as the first woman field engineer. Double X Science also has a series about today’s women in science, Double Xpression, which you can find here.
Today, you can find a woman–or many women–in STEM just about anywhere you look, whether it is as a government scientist at NOAA like Melanie Harrison, PhD, or at NASA. It hasn’t always been that way, and it can still be better. But women have always been a presence in STEM. In the 18thand 19th centuries, astronomer Caroline Herschellabored away through the dark hours of just about every night of her adult life, tracking the night sky. Today, women continue these labors, and STEM wouldn’t be what it is today without women like Herschel willing to stay up all night with the skies or spend days on end in the field or lean over a microscope for hours just to add a tiny bit more to what we know about our world and our universe.
For women in science, we’re there–at night, in the lab, in the field–because we love science. But as the non-science role models seem to tell us, we stick to it–and can stick with it–because we had role models in and out of science who showed us that regardless of our goals, our attitudes and willingness to move forward in spite of obstacles are really what drive us to success in STEM careers. Among the links I received for this carnival was one to Science Club for Girls, which is sponsoring a “Letter to My Young Self” roundup for Women’s History Month. The letters I’ve read invariably have that “stick with it” message, but one stood out for me, and I close with a quote from it.
It’s a letter by Chitra Thakur-Mahadik, who earned her PhD in biochemistry and hemoglobinopathy from the University of Mumbai and served as staff scientist a Mumbai children’s hospital for 25 years. She wrote to her younger, “partially sighted” self that, “The future is ahead and it is not bad!” She goes on to say, “Be fearless but be compassionate to yourself and others… be brave, keep your eyes and ears open and face the world happily. What if there are limitations? Work through them with awareness. —Yours, Chitra”
Links and resources for women in STEM, courtesy of D.N. Lee
Stay tuned for the April Diversity in Science Carnival #15: Confronting the Imposter Syndrome. This topic promises to resonate for many groups in science. I’m pretty sure we’ve all felt at least of twinge of imposter syndrome at some point in our education and careers. Your editor for this carnival will be the inimitable Scicurious, who blogs at Scientific American and Scientopia.
UPDATE: Carnival #15 is now available! Go read about imposter syndrome, why it happens, who has it, and what you can do about it.
Today, Carolyn S. Miles, president and CEO at Save the Children writes at the Huffington Post (ducks) about the latest findings regarding our ability to stop a preterm birth from happening. As anyone who’s given birth knows, it’s not easy to stop that process once it starts, and that persistent inability means devastating outcomes for some families. As Miles writes:
Our mothers were nothing like either of these people. (Source)
(Warning: We are having some fun, so what you are about to read does not explicitly contain science but does reference soy, onanism, tubed meats, and vacuums. In keeping with the DXS mission, however, we have embedded a little science here and there in the links. )
While the celebration of mothers is not a new concept, the modern version of Mother’s Day is a far cry from the ancient festivals that honoredCybele. However, in 1907, whenAnna Jarvis invented the modern Mother’s Day as a means to pay homage to her own mother, it was not her intention to use moms for profit.
But, alas, by the 1920s, this well-intended national holiday quickly morphed into the cash cow we see today. Sure, it is nice to receive a gift, but perhaps capitalism has since stripped Mother’s Day of its original meaning, and for the first week or two in May, we are bombarded with advertisements that claim to know what item every mother must have. From this, many sites have done us all the great favor of curating these cannot-live-without gifts into a single, easy to navigate list (financial kickbacks notwithstanding), often broken down into natural June Cleaveresque categories like “kitchen” and “for the home” (read: how to cook for everyone and keep shit clean).Besides the fact that these lists can be generalized to every gift-giving holiday for the lovely lady in your life, even Don Draper himself would scoff at many of these suggestions. Because we at DXS wish to ensure that your Mother’s Day experience is the best it can possibly be, we present you with a different kind of list – one that provides the most valuable unsolicited advice you will ever receive when it comes to choosing for dear old mom. Here, you will be schooled on what not to get for the woman that gave you life.
Flowers. One of the most suggested gifts for Mother’s Day is flowers. What woman doesn’t love flowers? Well, one who does not need one more thing to water, or sees her own mortality in each dried up petal aimlessly floating down onto the floor that had just been cleaned. Oh, and those tears you see building up in our eyes? Not tears of joy. You better back up or you might get caught in a sneezing fit of fury because, frankly, the last thing we want to do on “our” day is pretend that we like feeling like our heads will explode. And let us not forget how those flowers came to be available in your local flower shop or supermarket in the first place… from Colombia?
Soy Candles. Soy. For the last decade or two, we have seen the magical benefits of this plantproduct popping up in pseudoscientific “reports” in quality magazines like First for Women. And now, soy-pushers all over the intertubes will willingly exclaim that soy is the superior material for the production of candles, allegedly “soot free” (they aren’t really). Sure, anything soy-based will help the American Soy Farmer keep up with the Joneses, but a candle is a candle and unless you are also giving me a golden ticket to enjoy its inherent ambience whilst I soak in my imaginary claw-footed tub, full of bubbles and rose petals and the sultry sounds of Barry White, save it. Plus, I’d rather not burn my house down (again).
Gift Baskets! What says “I admire you like a work colleague” more than a gift basket? Sure, smoked cheeses and tubed meats taste fine after a few martinis, but when enjoying such delicacies, I prefer to do it while watching my co-workers photocopy their ass cheeks. Some things just don’t have the same effect in the home.
Teething Necklace. One website was flashing necklaces all over the place – but these weren’t just any old necklaces – they doubled as teething necklaces for the baby. Anyone who knows anything about a teething baby knows that, despite the alleged pain babies feel (hey, I don’t remember it, do you??), moms suffer the most. So instead of the necklace, why don’t you go ahead and take the dang baby for a few hours and give me a much deserved break? I’ll even sweeten the deal and throw some Tylenol in the diaper bag. And, in the strange and rare event that I might want rope burn on my neck, I’d rather get it from some fantasy role-playing in the boudoir. Take that as you will.
Vacuum Cleaner. If you really think that I want another reminder of how much I have to pick up after you and all of your friends – who regularly come over and wipe out all of the food I just deposited into my refrigerator – then yes, go ahead and buy me a vacuum. I mean, it is not like I don’t already spend all of my “free” time vacuuming the floors, so why not give me the gift that embodies what you really think of me (your maid)? Plus, Dyson has been showing commercials non-stop for a sale that runs until Mother’s Day, with the clear implication to get your mother (or if you are a mother to get yourself) a vacuum for Mother’s Day. So if you do decide to get a vacuum, make sure you have $500 for it. Remember, though, that a vacuum is really empty space, so you might want to consider getting me something more tangible–and fun.
50 Shades of Gray. Well, maybe I am not too opposed to this, but let it be known that I will probably need about ten minutes (give or take) of “alone” time after each time I pick up this series. As long as you are OK with this, I am OK with this. By the way, did you know that there are really more than 50 shades of grey?
We hope you will seriously consider this advice. After all, we really don’t need more shit to take care of, water, clean with, or… actually, we can always use some more good reads. Happy Mother’s Day!
Today’s guest post (originally posted here) is from Katie Hinde, an Assistant Professor in Human Evolutionary Biology at Harvard University. Katie studies how variation in mother’s milk influences infant development in rhesus monkeys. You can learn more about Katie and mammalian lactation by visiting her blog, Mammals Suck… Milk!. Follow Katie on Twitter @Mammals_Suck.
Milk is everywhere. From the dairy aisle at the grocery store to the explosive cover of the Mother’s Day issue of Time magazine, the ubiquity of milk makes it easy to take for granted. But surprisingly, milk synthesis is evolutionarily older than mammals. Milk is even older than dinosaurs. Moreover, milk contains constituents that infants don’t digest, namely oligosaccharides, which are the preferred diet of the neonate’s intestinal bacteria (nom nom nom!) And milk doesn’t just feed the infant, and the infant’s microbiome; the symbiotic bacteria are IN mother’s milk.
Evolutionary Origins of Lactation
The fossil record, unfortunately, leaves little direct evidence of the soft-tissue structures that first secreted milk. Despite this, paleontologists can scrutinize morphological features of fossils, such as the presence or absence of milk teeth (diphyodonty), to infer clues about the emergence of “milk.” Genome-wide surveys of the expression and function of mammary genes across divergent taxa, and experimental evo-devo manipulations of particular genes also yield critical insights. As scientists begin to integrate information from complementary approaches, a clearer understanding of the evolution of lactation emerges.
In his recent paper, leading lactation theorist Dr. Olav Oftedal discusses the ancient origins of milk secretion (2012). He contends the first milk secretions originated ~310 million years ago (MYA) in synapsids, a lineage ancestral to mammals and contemporaries with sauropsids, the ancestors of reptiles, birds, and dinosaurs. Synapsids and sauropsids produced eggs with multiple membrane layers, known as amniote eggs. Such eggs could be laid on land. However, synapsid eggs had permeable, parchment-like shells and were vulnerable to water loss. Burying these eggs in damp soil or sand near water resources- like sea turtles do- wasn’t an option, posits Oftedal. The buried temperatures would have likely been too cold for the higher metabolism of synapsids. But incubating eggs in a nest would have evaporated water from the egg. The synapsid egg was proverbially between a rock and a hard place: too warm to bury, too permeable to incubate.
Ophiacodon by Dmitri Bogdanov
Luckily for us, a mutation gave rise to secretions from glandular skin on the belly of the synapsid parent. This mechanism replenished water lost during incubation, allowing synapsids to lay eggs in a variety of terrestrial environments. As other mutations randomly arose and were favored by selection, milk composition became increasingly complex, incorporating nutritive, protective, and hormonal factors (Oftedal 2012). Some of these milk constituents are shunted into milk from maternal blood, some- although also present in the maternal blood stream- are regulated locally in the mammary gland, and some very special constituents are unique to milk. Lactose and oligosaccharides (a sugar with lactose at the reducing end) are two constituents unique to mammalian milk, but are interestingly divergent among mammals living today.
Illustration by Carl Buell
Mammalian and Primate Divergences: Milk Composition
Among all mammals studied to date, lactose and oligosaccharides are the primary sugars in milk. Lactose is synthesized in mammary glands only. Urashima and colleagues explain that lactose synthesis is contingent on the mammalian-specific protein alpha-lactalbumin (2012). Alpha-lactalbumin is very similar in amino-acid structure to C-type lysozyme, a more ancient protein found throughout vertebrates and insects. C-type lysozyme acts as an anti-bacterial agent. Oligosaccharides are predominant in the milks of marsupials and egg-laying monotremes (i.e. the platypus), but lactose is the most prevalent sugar in the milk of most placental (aka eutherian) mammals. Interestingly, the oligosaccharides in the milk of placental mammals are most similar to the oligosaccharides in the milk of monotremes. Unique oligosaccharides in marsupial milk emerged after the divergence of placental mammals.
Marsupial and monotreme young seemingly digest oligosaccharides. Among placental mammals, however, young do not have the requisite enzymes in their stomach and small intestine to utilize oligosaccharides themselves. Why do eutherian mothers synthesize oligosaccharides in milk, if infants don’t digest them?
In May, Anna Petherick’s post “Multi-tasking Milk Oligosaccharides” revealed that oligosaccharides serve a number of critical roles for supporting the healthy colonization and maintenance of the infant’s intestinal microbiome. Beneficial bacterial symbionts contribute to the digestion of nutrients from our food. Just as importantly, they are an essential component of the immune system, defending their host against many ingested pathogens. The structures of milk oligosaccharides have been described for a number of primates, including humans, and data are now available from all major primate clades; strepsirrhines (i.e. lemurs), New World monkey (i.e. capuchin), Old World monkey (i.e. rhesus), and apes (i.e. chimpanzee).
Among all non-human primates studied to date, Type II oligosaccharides are most prevalent (Type II oligosaccharides contain lacto-N-biose I). Type I oligosaccharides (containing N-acetyllactosamine) are absent, or in much lower concentrations than Type II(Taufik et al. 2012).
In human milk, there is a much greater diversity and higher abundance of milk oligosaccharides than found in the milk of other primates. Most primate taxa have between 5-30 milk oligosaccharides; humans have ~200. Even more astonishingly, humans predominantly produce Type I oligosaccharides, the preferred food of the most prevalent bacterium in the healthy human infant gut- Bifidobacteria (Urashima et al 2012, Taufik et al. 2012).
Human infants have bigger brains and an earlier age at weaning than do our closest ape relatives. Many anthropologists have hypothesized that constituents in mother’s milk, such as higher fat concentrations or unique fatty acids, underlie these differences in human development. But only oligosaccharides, a constituent that the human infant does not itself utilize, are demonstrably derived from our primate relatives (Hinde and Milligan 2011). At some point in human evolution there must have been strong selective pressure to optimize the symbiotic relationship between the infant microbiome and the milk mothers synthesize to support it. The human and Bifidobacteria genomes show signatures of co-evolution, but the selective pressures and their timing remain to be understood.
Vertical Transmission of Bacteria via Milk
In the womb, the infant is largely protected from maternal bacteria due to the placental barrier. But upon birth, the infant is confronted by a teeming microbial milieu that is both a challenge and an opportunity. The first inoculation of commensal bacteria occurs during delivery as the infant passes through the birth canal and is exposed to a broad array of maternal microbes. Infants born via C-section are instead, and unfortunately, colonized by the microbes “running around” the hospital. But exposure to the mother’s microbiome continues long after birth. Evidence for vertical transmission of maternal bacteria via milk has been shown in rodents, monkeys(Jin et al. 2011), humans(Martin et al. 2012), and… insects.
A number of insects have evolved the ability to rely on nutritionally incomplete food sources. They are able to do so because bacteria that live inside their cells provide what the food does not. These bacteria are known as endosymbionts and the specialized cells the host provides for them to live in are called bacteriocytes. For example, the tsetse fly has a bacterium, Wigglesworthia glossinidia,* that provides B vitamins not available from blood meals. Um, if you are squeamish, don’t read the previous sentence.
*I submit the tsetse fly and its bacterial symbiont (Wigglesworthia glossinidia) for consideration as the number one mutualism in which the common name of the host and the Latin name of the bacteria are awesome to say out loud! Bring on your challenger teams.
Hosokawa and colleagues recently revealed the Russian nesting dolls that are bats (Miniopterus fuliginosus), bat flies (Nycteribiidae), and endosymbiotic bacteria (proposed name Aschnera chenzii)(2012). Bat flies are the obligate ectoparasites of bats (Peterson et al. 2007). They feed on the blood of their bat hosts, and for nearly their entire lifespan, bat flies live in the fur of their bat hosts. Females briefly leave their host to deposit pupae on stationary surfaces within the bat roost.
Bat flies are even more crazy amazing because they have a uterus and provide MILK internally through the uterus to larva! Male and female bat flies have endosymbiotic bacteria living in bacteriocytes along the sides of their abdominal segments (revealed by 16S rRNA). Additionally, females host bacteria inside the milk gland tubules, “indicating the presence of endosymbiont cells in milk gland secretion”.
The authors are not yet certain of the specific nutritional role that these bacterial endosymbionts play in the bat fly host. The bacteria may provide B vitamins, as other bacterial symbionts of blood-consuming insects are known to do. My main question is what is the exact role of the bacteria in the milk gland tubules? Are they there to add nutritional value to the milk for the larva, to stowaway in milk for vertical transmission to larva, or both?
The studies described above represent new frontiers in lactation research. The capacity to secrete “milk” has been evolving since before the age of dinosaurs, but we still know relatively little about the diversity of milks produced by mammals today. Even less understood are the consequences and functions of various milk constituents in the developing neonate. Despite the many unknowns, it is increasingly evident that mother’s milk cultivates the infant’s gut bacterial communities in fascinating ways. A microbiome milk-ultivation, if you will, that has far reaching implications for human development, nutrition, and health. Integrating an evolutionary perspective into these newly discovered complexities of milk dynamics allows us to reimagine the world of “dairy” science.
Hosokawa et al. 2012. Reductive genome evolution, host-symbiont co-speciation, and uterine transmission of endosymbiotic bacteria in bat flies. ISME Journal. 6: 577-587
Jin et al. 2011. Species diversity and abundance of lactic acid bacteria in the milk of rhesus monkeys (Macaca mulatta). J Med Primatol. 40: 52-58
Martin et al. 2012. Sharing of Bacterial Strains Between Breast Milk and Infant Feces. J Hum Lact. 28: 36-44
Oftedal 2012. The evolution of milk secretion and its ancient origins. Animal. 6: 355-368.
Peterson et al. 2007. The phylogeny and evolution of hostchoice in the Hippoboscoidea(Diptera) as reconstructed using fourmolecularmarkers. Mol Phylogenet Evol. 45 :111-22
Taufik et al. 2012. Structural characterization of neutral and acidic oligosaccharides in the milks of strepsirrhine primates: greater galago, aye-aye, Coquerel’s sifaka, and mongoose lemur. Glycoconj J. 29: 119-134.
Urashima, Fukuda, & Messer. 2012. Evolution of milk oligosaccharides and lactose: a hypothesis. Animal. 6: 369-374.