Evolution: the Devil is in the Details (part Two of Four)

Genetics: DNA Facts of Life

Why did you turn out to be human rather than, say, an orangutan? Let's take a look at the DNA facts of life, so you can tell your children or grandchildren. No, it won't be boring! In fact the answer is quite simple.

The genes you received from your parents were coded for a Homo sapien. Genes, of course, go a lot further in filling out all of the assorted details which you see reflected in your mirror. Skin, eyes, hair color, size, height, type of nose, ears, mouth, and numerous other features all depend on what you inherited in your genes. Every biology student knows that much. But just what are we talking about when we use the word "genes"?

Within the nucleus of each cell there exists a long two-stranded molecule forming a chain of deoxyribonucleic acid, or DNA for short. These two strains run parallel to each other and look like a long spiraling staircase. The steps are called nucleotides or bases.

When you examine the bases, you find they are made of four chemicals: thymine (T), adenine (A), cytosine (C), and guanine (G). The sides of the staircase, which hold the steps together, are composed of alternating molecules of phosphate and deoxyribose. The whole structure is referred to as the DNA helix.

Two of the above four chemicals combine for each step. And the steps are always arranged in one of four patterns. T combines with A, or vice versa, A combines with T. Likewise, C combines with G or the other way around G with C. These are called the complementary arrangements. They don't form any other pattern.

Now here is an extraordinary fact. Everything we have said so far applies to all living plants and animals. From bacteria to redwood to man, the same DNA arrangement holds true. What makes humans anatomically different from redwoods and bacteria is nothing more than the length and sequence of nucleotide bases on the DNA helix.

Each form of life has its own distinctive number and arrangements of nucleotides. Geneticists tell us that, excluding asexual organisms and identical twins, no two plants, animals, or humans have identical sequences in their DNA. How does it work? It is similar to a factory.

The "factory" changes every thymine (T) molecule into another chemical called uracil (U). The other three chemicals stay the same. The nucleotides are then bunched together in groups of three called triplets or codons. Each codon is an order or command for the cell to take an action. These codons are translated by the cell into one of twenty amino acids. A single message may well involve hundreds or even thousands of amino acids strung together in chronological order.

The precise type, order, and number of amino acids result in a specific protein. These proteins produce specific types of cells in the embryo. And these types of cells develop into different organs: arms, legs, etc. See how it builds up? Every part of your physical body can be traced back to its DNA source.

In a single-celled bacteria, around three million nucleotide bases are in the DNA helix. For humans, the number is approximately three billion. But it is not quite the same thing. All three million nucleotides in bacteria's DNA code for proteins, while only 10 percent of human nucleotides code for protein production. What do the rest do?

They are called introns, and there is no shortage of speculation about them. They may have something to do with gene regulation or chromosome structures. Then again, they could be spacers which divide the whole gene into different functional units. But no one really knows what they do. They are just simple, highly repetitive sequences which seem meaningless.

Question to Consider: Does the DNA helix show single-celled life forms to be simple structures which reasonably could have originated by accident?

We will continue our study with mutations: see Evolution:The Devil Is in the Details (Part Three of Four.)