Five years of heavy meticulous biology courses, and it all boils down to one single idea:
DNA -> RNA -> Protein
This is known as the central dogma of Biology.
Really, that’s all there is to it. Every single life-form on Earth follows this universal norm (except for retroviruses- these are the viruses that have an RNA rather than a DNA genome and so they go the other way around, hence the “retro” prefix. That group includes the notorious HIV).
Retroviruses aside, all organisms are made up of the same genetic material. So an ant, a clam, an elephant, a bacterium, and us humans, are all made up of the same molecules. What molecules are we talking about? You guessed it, DNA.
DNA stands for deoxyribonucleic acid. Thanks to Watson & Crick in the early ‘50s, we now know that DNA is a double helix that has sugar (deoxyribose) phosphates in the backbone and nitrogenous bases in the middle, connected by hydrogen bonds.The sugar, phosphate, and nitrogenous base moieties are collectively known as a nucleotide.
There are 4 different nucleotides: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C).
The whole of life is based on these 4 “letters”
So if all organisms carry the same genetic material, how come we’re all different? The difference lies not in the molecule itself, but rather in the order of these molecules, in their sequence.
Alright, so we have a differential DNA sequence, what does this mean? Let’s take a look at the second component of our central dogma, RNA.
RNA stands for Ribonucleic acid (notice the “deoxy” prefix is missing. This is because DNA and RNA differ by one oxygen molecule in the sugar moiety, it is missing in DNA).
In a process termed transcription DNA is transcribed into RNA, particularly messenger RNA or mRNA. As its name indicates, it carries a message. This message will subsequently be translated into proteins in a process known as translation.
There is a specific genetic code that translation follows: The nucleotides are read in groups of 3’s known as codons. A codon will be translated into an amino acid (building blocks of proteins), and hence a whole sequence will eventually give a protein (the final component of the central dogma).
The following table shows the different codons and their corresponding amino acid.
Notice the redundancy in the code: multiple codons can give the same amino acid (this is logical because we have 64 possible codon combinations and only 20 amino acids). Keep this idea in mind, I shall return to it later.
So to sum up: A sequence of DNA will be transcribed into an mRNA which in turn will generate a sequence of amino acids through translation. A few modifications later, a fully-functional protein is produced.
Voila! The crash course on basic genetics is over. Now the fun part (I have an unconventional definition of fun):
We are all familiar with the fact that our cells divide. And for the genetic material to be passed on equally to the daughter cells, it has to be duplicated before it is divided. This duplication process is known as DNA replication.
The enzyme responsible for this process is prone to error and, consequently, a replicated sequence can lack a 100% identity to the original.
Let’s take an example.
In this case, we have a substitution of the A nucleotide by the T nucleotide. This changes the GAG codon to a GTG codon. This substitution eventually generated another amino acid,Valine instead of Glutamic acid.
The gene in question is the Hemoglobin gene. The hemoglobin protein is found in your red blood cells where it takes on the chore of carrying oxygen from your lungs to the rest of your body. The substituted amino acid has different physio-chemical properties than the original one and subsequently leads to deformed red blood cells. (The picture below depicts the regular red blood cells which are round biconcave disks next to the malformed sickle cells)
But mutations do not always end up with deleterious effects. Which reminds me, sickle cell disease actually has an advantage when found in a heterozygous state- that is when only one allele is mutated rather than both. The advantage is a higher resistance to Malaria. How? The agent of Malaria is an intracellular parasite called Plasmodium falciparum. The target cells of this parasite are usually red blood cells. Once inside, the parasite feeds on hemoglobin. The mutated hemoglobin in sickle cell disease is more difficult to digest. (Think of this: What if sickle cell was brought about as a defense to Malaria, and it just so happened to have a different side-effect when found in a homozygous state?)
Pardon the digression, back to our topic.
Remember the redundancy of the genetic code I mentioned earlier? Consider this: what if a mutation occurred in such a way as to generate a different codon, but one that codes for the same amino acid? Well in that case, we have no problem, and what would’ve taken place is termed a silent mutation.
Another case would be having a mutation in a non-coding part of the DNA. Only 3% of our (humans) DNA is coding (i.e gives a protein), the rest is known as junk DNA.
This sounds like a total waste of space and energy. Why house 97% of useless DNA, better yet, why go through the trouble of replicating it at every cell division? You have to keep in mind that all cellular processes demand energy and resources. A dead-end process that aimlessly uses up these resources would surely be eliminated by evolution.
How is it then, that junk DNA was preserved?
Well, this brings us to the reason why I chose to write this article. When people hear the word “mutation”, they automatically assume it is a bad thing. It needs to be clarified that mutation is not an aberration, otherwise it would’ve been immediately (in evolutionary time) gotten rid of. Mutation is a necessity for the the perpetuity of life. It is the motor that drives the whole of evolution, by constantly supplying natural selection with a myriad of different “options” that would be then cherry-picked in accordance with the pressure in question.
We seem to have reached an obstacle. How do we allow mutation to take place while simultaneously avoiding any deleterious effects?
It seems evolution has devised an ingenious scheme yet again. Junk DNA.
With no selection pressure against it, the accumulation of mutations is unleashed, paving the way for the possible emergence of a new gene that might be favored by natural selection.
Newly-acquired genes can easily be observed in experiments performed on bacteria. This is because they have a very small generation time (optimal conditions can give you a new generation every 20 minutes) and because selective pressures can be easily applied.
First idea that should come to mind? Anti-biotics. Try growing a bacterial cell culture on a petri dish that has a certain anti-biotic embedded in it. Most colonies will probably die out. Eventually though, one colony will develop a resistance to that anti-biotic and spread like wildfire. The advantageous trait will be passed on from generation to generation until your whole petri dish is swarming with resistant bacteria.
This is why you shouldn’t pop anti-biotics like M&Ms. It’ll be like the experiment we talked about, except, you are the petri dish.
Well, to wrap this up, I hope you can now see mutation in a new light and you can better comprehend how evolution takes place.
For my next article.. no, I’ll leave you in suspense.