Magic words and passwords are fun – and good learning tools. We teach our children to say the “magic word” when they ask for something, and many of us teach our children a password to use with school pick-ups so they don’t go home with a stranger.
But there’s one bird species that one-ups us humans on special passwords – the superb fairy-wrens of southeastern Australia. Not only do they teach their chicks a special secret password note, but they do it before the chicks even hatch. And just like humans, they’re trying to keep the strangers away. These are the findings of Diane Colombelli-Négrel and her colleagues in a Nov. 8 study in Current Biology.
The superb fairy-wren of southeastern Australia. Photo by JJ Sullivan.
See, fairy-wrens are a bird species who are sometimes exploited by a “brood parasite.” Brood parasites are animals that use the parenting of a different species to save themselves all that energy of raising their young. It’s like having a baby and dropping it off in your neighbor’s bassinet to deal with to save you the trouble. In the case of superb fairy-wrens, the brood parasite is the Horsfield’s bronze cuckoo. Momma cuckoos lay their eggs in fairy-wren nests with the expectation that once the cuckoo chick hatches, he’ll reap the benefits of having a fairy-wren mother to feed and protect him.
But fairy-wrens have adapted to this evolutionary trick with a clever one of their own. A little more than halfway through their 14-day incubation, starting on day nine, fairy-wren mothers sing an “incubation song” while sitting on their eggs. Every four minutes, she sings a two-second tune, and the little growing chicks in her eggs are listening… and learning.
She sings daily until the eggs hatch five days later, and then the chicks do what any other newborn bird does: they beg for food. But their begging cries contain a single unique note pulled from their mother’s song. That note becomes a password letting the mother fairy-wren know that each of these chicks are really hers.
So what about the cuckoos? Well the mother cuckoos usually drop off their eggs just a few days before the fairy-wrens hatch – too late in the incubation period for the cuckoo chicks inside to learn the password note. When a cuckoo posing as a fairy-wren hatches, he doesn’t know the password of his fake step-brothers and sisters, so he doesn’t incorporate that single special note into his cries. And so the fairy-wren mother ignores him. In fact, when she notices there’s interloper in the nest, she and her mate will usually fly off and make a new nest elsewhere.
Colombelli-Negrel and her colleagues discovered this unique password adaptation through a series of cross-fostering experiments. They observed 15 fairy-wren nests during incubation periods, when they heard the mothers singing to their eggs. When they swapped eggs of fairy-wrens among the nests, the newly hatched chicks begged for food using the special note of the mother who incubated them, not the foster mother whose nest they hatched in. When the researchers played a loudspeaker under a nest with the wrong begging call, the mother fairy-wrens didn’t feed their chicks.
Fairy-wrens stick with their mates for life, so dad is involved in caring for the chicks as well. But often so are other males because even though the fairy-wrens are socially monogamous, they tend to have open marriages – both males and females will mate with others, and a clutch of eggs is often the result of different more than father.
The female fairy-wrens therefore make sure that dad and any other helpers know the secret password by singing them a “solicitation song” away from the nest. If dad or any other helpers are assisting with feeding, then, like mom, they only feed the chicks who sing the secret password.
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 not easy being green. First, you have decide which green to be. (Source)
[We at Double X Science had been considering a “toxins” post but then found the following post by Jennifer Mo, a happily childfree vegetarian who lives in California with a cat named Brie but variously nicknamed Walnut “for her brain capacity” or Toxokitty for her history of toxoplasmosis–which, as it turns out, turns up in Jennifer’s guest post, below. This post first appeared at Jennifer’s blog, It’s Not Easy to Be Green, where she writes about environmental issues as a “rationalist and a pragmatist.” You can also follow Jennifer on Twitter @noteasy2begreen. We appreciated the pragmatics of this particular post quite a bit and thank Jennifer for allowing us to host it at Double X Science. We are particularly taken with the fact that she asks if we’d ever wanted to call our own brain a troglodyte.]
It’s a common demand from the public to scientists: prove to us something is safe before unleashing your monster on the world. And on one hand, it’s a totally fair, reasonable request to not be treated as lab rats. I get that. I hate the idea of having big chemical corporations profiting off their creations that create long term problems for ordinary people and the environment. On the other, whether you’re talking about GMOs or synthetic chemicals, it’s a problematic request for a couple of key reasons:
It assumes a binary between safe and unsafe without regard to exposure level or other circumstances. Just about everything can be harmful under the right (or perhaps I should say wrong?) conditions. Take water, for example.TonsContinue reading →
It could be Andrew Wakefield or a brain-hijacking microbe.
by Meredith Swett Walker
I’m a scientist, but I’ve learned that when we become parents, paranoia can trump the powers of rational analysis I’ve so carefully nurtured and developed. For some parents, media-whipped fears about vaccines take front and center in the anxiety lineup. For me, a brain-infecting microbe that makes mice hang around cats is at the top of my parenting paranoia list.
Parenting requires making many, many choices. Some seem inconsequential, like whether your child will wear overalls or sweatpants, pigtails or a pixie cut. But other choices have to do with health issues such as circumcision, immunization, and breast milk vs. formula – just a few in an endless list. For geeks like me, the first impulse is to research each issue, make a choice, and prepare an argument for anyone who questions the decision (and believe me, someone will.) My response usually goes something like this: “Well, recent studies have shown that yada yada yada…” Then I pat myself on the back for being so informed and making such a well-reasoned decision.
My process ran into trouble, though, when my relationship with a university and its online library access ended. What happens when you can’t get your hands on peer-reviewed scientific journal articles? One consolation should be that we live in the “Information Age.” Surely, Google, a fast internet connection, and an overwhelming flood of information should lead to what we need to make well-reasoned, science-based parenting choices. Surely.
Maybe not. A friend recently shared with me an article from the open-access (i.e., free) online journal PLoS: “Why Most Biomedical Findings Echoed by Newspapers Turn Out to be False: The Case of Attention Deficit Hyperactivity Disorder.” The gist is that the news media preferentially cover initial findings described in the most prominent scientific journals. The key word there is initial. No initial result is going to be the final word in science, and all results require confirmation from other researchers repeating or extending the experiments. Sadly, in practice, many of the follow-up studies don’t get published in the most prominent journals because they are not “a big scoop.” Yet they often show that the initial, Big Headline Finding was overblown or even incorrect.
That brings me to an example that really pushes my buttons — childhood immunizations. In 1998, Andrew Wakefield and colleagues published a study in the prominent British medical journal the Lancet. The paper examined a hypothesized association between the MMR (measles, mumps, rubella) vaccine and autism, but the authors used fairly moderate language in their conclusions. But then, Wakefield participated in a press conference about the paper and asserted in much stronger language that the MMR combined vaccine and autism were linked and that parents should turn to single shots for measles, mumps, and rubella. The news media ate it up.
The scientific community immediately pointed out a number of glaring flaws in the study, and subsequent investigations over the next decade failed to reproduce or confirm the results. But it was too late. The popular media and celebrities like Jenny McCarthy had already done the damage. Parents were terrified, vaccination rates dropped, and deadly measles and whooping cough outbreaks starting cropping up.
Yes, the news media covered subsequent studies reporting no link between vaccines and autism, but let’s face it: Science is slow, and news is fast. In the interval, scary information takes root. The Lancet retracted the article 12 years after its publication, and in 2011, British investigative journalist Brian Deer demonstrated that Wakefield actively falsified data. Still, to this day, vaccination rates have not fully recovered, and many parents remain misinformed and concerned about vaccinating their children. Indeed, the Wakefield debacle has been directly blamed for a huge and ongoing measles outbreak in Wales.
I could haz Toxoplasmodium in my poop, so be careful.
Admittedly, the MMR case is an extreme example but also a good one of how a single initial study and the ensuing media hysteria can have a huge effect on parents — and on children’s health.
And we all have our trigger points for fear. One (of the many) things in our family tree is schizophrenia. A member of our extended family developed schizophrenia as an adolescent and has never recovered. Schizophrenia can run in families, so my two children have up to a 4% chance of developing this disorder compared to the 1.1% chance of someone without close relatives who have it.
So along comes my March 2012 issue of The Atlantic featuring “How Your Cat Is Making You Crazy” by Kathleen MacAuliffe. I would have found this article fascinating even if schizophrenia weren’t a concern. Its subject is a parasite called Toxoplasmosis gondii, which usually cycles through two hosts: cats and rodents. Toxo, as I’ll call this beast, starts life as an egg in a cat, is pooped out, and then gets picked up by a new cat. How does it get into a new cat? Cats, unlike dogs, are pretty fastidious and don’t tend to eat or otherwise mess around with cat poop. So Toxo gets itself into a less fastidious but tasty morsel like a mouse, instead, making its way into the cat when the mouse becomes dinner.
That seems simple enough, but there’s more. Toxo infection ups the odds of a mouse–cat encounter by hijacking the mouse’s brain and changing its behavior. The mouse’s activity level increases (cats love to chase fast-moving objects), and the rodent might become less wary in exposed areas and even attracted to the smell of cats. Watch these videos, and you’ll see how the infected mice move faster and wander into unknown spaces, seemingly without fear, as you can see in this video and this one.
The trouble for humans is that we also canpick up Toxo through contact with cat poop or eating undercooked meat or unwashed veggies from a garden where cats poop. Becoming infected with Toxo during pregnancy can be very harmful to a fetus, so pregnant women have long been warned off cleaning kitty litter boxes. But healthy, non-pregnant adults infected with Toxo weren’t thought to experience any detrimental effects — until recently. According to MacAuliffe’s article, which focuses on the work of Czech biologist Jaroslav Flegr, Toxo might alter human behavior, too, in mouse-like ways, such as reducing fearfulness. In most people, these purported behavioral shifts are probably very subtle and unremarkable. But Flegr suggests that in some people, Toxo infection serves as the trigger for mental illness, including schizophrenia.
Schizophrenia likely develops because of interactions between genes and the environment. Having risk gene variants isn’t a guarantee a person will develop schizophrenia, and it can arise in people without those risk variants. The list of potential environmental triggers is long and includes childhood stress, prenatal undernutrition, drug abuse, and … infections with microbes like Toxo.
Reading this article set me off on a tear of worrying. We have a cat, but I wasn’t worried about her. She is an indoor cat (we love birds), and there is a very low incidence of Toxo infections in indoor cats. But we have outdoor cats and feral cats in our neighborhood. They sometimes hang out in our yard, where my kids like to play in the dirt and eat things out of the garden, including the dirt itself. Oh, poop.
I took to Google and researched cat traps and repellents and how to get kids to wash their hands. I laid awake at night for hours strategizing about how to keep my home and yard Toxo free. And then I realized, even if I managed to exclude all cats from my yard and the totally impossible feat of getting my children (ages 1 and 2) to wash their hands before they touched their faces or food every time, I was still doomed to failure. My kids would go to friend’s houses and play in their Toxo-infested yards. Or they might already have encountered Toxo anyway.
Toxo was something I couldn’t control, and I needed to let it go. At our next check-up, I talked to our pediatrician about it, who had never heard about the potential Toxo–schizophrenia link. She graciously concealed her “Oh, Lord, another parent with a loony theory” reaction and calmed me down. As she put it, my only real option to prevent Toxo infection was to never allow my children to play outdoors or in the dirt, and the detrimental effects of that were likely far greater than the risk of schizophrenia, Toxo or no Toxo.
And she also reminded me of what I already knew and should have remembered: These findings about Toxo are initial findings.
As a scientist, I know that the schizophrenia–Toxo link needs more study. A lot more study. As a parent, well … yeah. I still worry, and no lack of replication or confirmation is likely to stop me.