Of peas and mice and men

(This one is a longer post than usual. Apologies)

“Do you believe in God?” asked the smiling theologist sitting next to me in the plane.

“Sure, I have nothing against the existence (or lack of) a greater power”, I replied.

He scowled. Clearly, I must be a part of some Dark side. An ad showing some animals and people was playing on the little TV screen. He jumped at me and thundered, “How can you say that people came from animals. Look at them, they are so different. This evolution theory nonsense cannot be true. I took a class on genetics once and they said something about some DNA that I didn’t understand. It’s all wrong.” I shuddered, and ate my dinner.

I have nothing against people who don’t’ believe in evolution. Half this country does not, and they are entitled to do so. But assuming something is false because it is “just a theory” is wrong, especially if you have not tried to understand it. A scientific hypothesis is proposed, then rigorously tested to be true or not. If true, it need NOT be declared a law. Gravity remains a theory, but the apple still fell on Newton. Newton did not float up to the apple and bump his head.

So I thought primer on the molecular basis of evolution (that a freshman might get in college) wouldn't be out of place. So, here’s my “Molecular evolution 101”, incase you were curious about it, but never had a chance to learn more.

Darwin proposed his theory (Origin of species) about 150 years ago, based on his observations mainly in South America. Different pressures had forced organisms to acquire certain properties over time, and this trait (phenotype) was incorporated and passed on over generations. That was what became called evolution. Earlier, Mendel (a monk) did genetic studies with the humble sweet pea. He crossed flowers of one color with flowers of another color, and got offspring flowers of one, the other, or a mixture of both colors. He found that traits pass on as unmodified “units” from parents to offspring. We call these “units” genes. So if a white flower and a red flower cross, the offspring is pink. Doing simple math, if two alleles make up each pea, and each pea has alleles WW and RR (for white or red), WW * RR= Pink (WR). The pink is a heterozygous plant, carrying 1W+1R allele. Now if pink and pink cross, then WR * WR can give WW, WR, RW and RR (1 white, 2 pink, 1 red). This is the classic law of genetics. Now this W or R thingy (the allele) makes the “gene.” It has a specific trait/function, and is passed on from parent to offspring.

A “gene” is actually a piece of DNA that encodes a PROTEIN, which has the function. DNA is made up of four nucleic acids, forming double-helical chains. These can be used as a template (like a printing press) replicate itself, i.e. propagate (this we understand thanks to the Watson and Crick’s DNA double-helix structure). This DNA template is also used to make mRNA. RNA, like DNA, is also made up of four nucleic acids, but differs by using a different nucleic acid (Uracil) as opposed to Thymine in DNA. mRNAs can be 100’s-1000’s of nucleic acids long, each coding for A SINGLE PROTEIN. Crick cracked this code, and found that every three nucleic acids encoded for only one amino acid. Since there are 4 nucleic acids, 4^3 gives 64 combinations. But 3 of these combinations code for a “STOP” sign (codon), and with some redundancy in the codon usage, you end up with 60/3 = 20 amino acids.

Amino acids are made from this mRNA template as a chain that ends when the “STOP” sign is encountered. This chain forms what is called a protein. This PROTEIN is thus the product of the GENE.

DNA --> RNA --> protein (with some specific exceptions).

This protein has a specific function associated with it. Groups of proteins come together for more complex functions and form complex organelles. A single cell has thousands of proteins, and in multicellular organisms (like ourselves) there is a huge permutation of proteins in different cells, resulting in increased complexity.

Yeast have only around 5000 genes, but mammals (and humans) have over 20000 genes. The permutations of protein expression patterns are many times more complex. This is why a mammal is more complex than yeast. But how did this complexity come about? Because, a protein can “evolve” function, change its function. This happens by “mutations”. If the DNA template is changed (by external or internal stress, like UV radiation), say by a couple of nucleic acids, the RNA changes, and thus the protein. Now this new protein may be useless or it might have obtained a slightly different function. So, thousands of random mutations will eventually create one mutant that has some function, or even improved function. This is how from one protein, more proteins can evolve. So, it is possible that from one gene, many different proteins emerge over millions of years, with permutations and combinations. This is called DIVERGENT EVOLUTION. If however, by mutagenesis, two different genes come up with two different protein products that are similar in function, this becomes CONVERGENT EVOLUTION. These “genes” are passed on from parent to offspring, more mutations are acquired on the way, and things “evolve”.

Can we do this in a test tube? Easily. Random mutagenesis (with chemical agents) results in such slow changes. But you can just as easily chop up pieces of DNA, stick them together randomly, and screen the products (this is Gene Shuffling), and find proteins of improved function, or completely different function from the starting material. Over millions of years, evolution can thus result in thousands of different products and different organisms.

Here we have it, a 101 of molecular evolution. Does this exclude the presence of God? No. Which came first, DNA or RNA? Not sure (but there are THEORIES). Who created all of this? That is up to your beliefs. Every one can be happy.