One of the most powerful pieces of evidence for evolution, I believe, is the universality of DNA. It is common to all forms of known life. Even viruses, which cellular biologists argue are non-living (incorrectly IMHO) are, at their most basic, either DNA or RNA and a protein wrapping. All living things pass their information from one generation to the next coded in DNA. All living things. Not all living things except for the Asian Pygmy Elephant (which I have seen in the wild) and tarantulas (which I haven’t). ALL living things pass their genetic information to their children coded in DNA. Some, like bacteria, undergo binary fission, doubling their looped chromosome and dividing in two. They pass their DNA on in entirety and the daughter cells produced are clones, identical to the parent cell in every way. For other sexually reproducing organisms, it’s a case of pros and cons. Pros: your genetic information is passed on and you’re having sex. Cons: Only half of your information is passed on, there is no guarantee that this information will be expressed and you’re having sex. Sex is a pro and con? Just think of that male praying mantis, having its head eaten off in the act of passing on half its genetic information.
So what is this DNA/RNA stuff? And how does it make me me, you you, us people and not a cactus? As you should have learned, DNA and RNA are polymers, the subunits are comprised of a sugar- this is the deoxyribose in DNA and ribose in RNA. DNA also has a phosphate attached to the sugar. So far these components are just structural. What makes these molecules so special are the nucleic acids attached to the sugars: guanine, cytosine, adenine and thymine. RNA uses uracil instead of thymine, chemically a closely related molecule. Thymine is formed by the methylation of uracil– ask your chemistry teacher. So you know that guanine and cytosine join via three hydrogen bonds, adenine and thymine (or uracil) via two hydrogren bonds. You also know that DNA is a double-stranded molecule that is often represented as a twisted ladder. The rungs of the ladder are the paired nucleic acids and the up–righty bits (what are the vertical parts of a ladder called?) are formed from the sugar phosphates. It is then twisted to form the helix, making a complete rotation every ten or so base pairs. RNA is single stranded. What you might not yet know about RNA is this single-strandedness (just invented a word!) is vital in its role and function. As it is a single strand, and as it has nucleic acids that will readily bond with their complementary partner base, they can be folded on themselves and form a three-dimensional shape that, in a similar way to a protein, defines what and how it can then do. Just like proteins, shape is important. Ribosomes are comprised of r-RNA, ribosomal RNA. It is the shape of them that allows for amino acids to be arranged in the primary protein structure. t-RNA, or transfer RNA, facilitate amino acids moving from the cytoplasm to the ribosome and being placed in the correct position in the growing protein. Again, the shape of the molecule determines its role and function.
Another difference between RNA and DNA is size. RNA are much shorter molecules than DNA which, depending on the text you read, could be between 1.6 – 2 metres in length. In the nucleus of every diploid cell in your body is nearly 2 metres of DNA. This makes packaging a bit of an issue. How small are cells? Really, really small. We measure them in microns. And this length of DNA is found in the nucleus of each and every one of these cells. So how does this length of DNA manage to fit in the nucleus? The first stage of this process is the twisting which I’ve already mentioned. This shortens the entire length. Next, the DNA is tightly wrapped around a protein called histone at intervals along its length, into units called nucleosomes , again shortening the molecule. Experiments have shown that these nucleosomes are on average 200 base pairs in length. This is the basic structure of chromatin and is how DNA is packaged within the nucleus unless it is going through mitotic or meiotic division. When new somatic or gametic cells are produced this chromatin is condensed into chromosomes. The molecule is coiled to roughly look like a telephone cord (for those of you who can’t remember these speak to your history teacher) and coiled again. This supercoiling is done with the aid of further protein molecules and results in 23 pairs of short chromosomes forming in a human. Chromosomes, genes and the relationship between genotype and phenotype will be the subject of my next rave.
I want to step back from the structure and packaging of DNA at this stage and have a look at chromosomes and genes. DNA packaging is pretty straight forward and your teacher or textbook can paint a pretty good picture. Even Wikipedia doesn’t stuff this up too much. If you are struggling conceptually, have a look at this link.
To talk about chromosomes and genes, first you must have a clear understanding of what DNA does. DNA codes for protein, or more simply, your DNA dictates what proteins are made in what quantities in what cells throughout your body. It tells your cells to make protein. That is all. Protein. Not carbohydrates or lipids. Proteins. And this is what is passed from your parents to you and from you to your children: a list of proteins to be made and a biological timeline for this production. You need to think of your DNA as a novel. It is a book all about you. And the language or code that this is written in is the nucleic acids. It is a four letter alphabet (well, five if you include uracil). Our alphabet uses twenty six letters, the language of DNA uses four. And the words these spell are only three letters long. The three letter DNA words are called codons and each of these biological words represents an amino acid. Like our language, the letters are arranged into words, but these words need to be in sentences to make sense. A DNA sentence is made of a string of codons and this sentence will say “Histone” or “Collagen”. The codons spell out the sentences that are the primary protein structure. We call these codon sentences genes.
A gene is a discrete section of DNA that provides the instructions to construct a protein. Of the 20,000 to 25,000 genes found in the human genome, an incredible percentage of this genetic information is shared with a vast number of animals. We share genes with other forms of life. Our closest genetic relative is the chimpanzee. Around 6 million years ago there were no humans or chimps but an ancestor that contributed genetic information to both lineages, and still today 98% of our genes are IDENTICAL to those of a chimp. But it is not just our close relatives, we share genes with organisms as morphological and phylogentically distant as earthworms. For example, humans have different versions of the gene to produce haemoglobin. These different versions are called alleles. We can have either A, B, or O haemoglobin. Chimpanzees also have the same ABO blood group system with the same genes. Now, if you have the haemoglobin genotype AA and a chimp has a haemoglobin genotype of AA you are more closely related to the chimp for that gene than you are with another human with a blood group of BO, for example. But only for that gene. And maybe for the gene for arse-scratching.
While I was joking about the arse-scratching gene, let’s think about innate behaviour for a second. Innate, not learned behaviour. Your DNA instructs your body to make proteins. All other compounds and structures results from the interactions of these proteins. But what about innate behaviour? Things an organism does but is never taught. Sex for example. Or nest building in birds. Or walking for a new born foal. Birds do not pass a series of genetic instructions from one generation to the next on how to build a nest. Or instructions for the rifle bird of Papua New Guinea on how to perform an intricate mating dance. This knowledge is innate. They are born with it. And, to the best of our knowledge results from the interactions of different proteins found wit in the animal’s genome. This blows me away. Excuse me while I stop to ponder...
Now, not all DNA codes for proteins. The length of your DNA is much greater than the sum of the genes it contains. In fact only about 5% of your DNA codes for protein. To return to the analogy of our DNA being like a novel, there are spaces between the gene words and sentences. Within genes are areas of non-coding (not involved in protein synthesis) regions called introns. And between the genes are sections of non-coding DNA called Spacer DNA. This Spacer DNA was once commonly called Junk DNA because it was thought that this DNA was useless. It was named ‘junk’ in 1972 and for nearly 30 years was thought of as junk. However, this non-coding DNA maybe extremely important in the regulation of gene expression (when, where and for how long a gene is turned on) and may also be a significant player in diseases such as breast cancer, HIV, Crohn’s disease, Alzheimer’s, heart disease, ovarian and skin cancer. Our understanding of the relationship between coding and non-coding regions of DNA is at its infancy.
All life shares the same code. This is powerful evidence for evolution from a common ancestor and allows us to transplant a human gene for insulin production and place it successfully into a bacterium alongside the bacteria’s genes. I will get to this later when I talk about genetic modification.