The Polarity of DNA

In 1953 James Watson and Francis Crick succeeded in working out the structure of Deoxyribonucleic Acid (DNA), the molecule, which carries all our genetic information. They won a shared Nobel Prize in Biology in the same year.

Each functional portion of this molecule is referred to as a gene. Each gene is transcribed into a short template molecule of the related polymer ribonucleic acid (RNA), which is better suited for protein synthesis. This in turn is translated, by mediation of the machinery consisting of ribosomes and a set of transfer RNAs and associated enzymes into an amino acid chain (polypeptide), which is then folded into protein.

If we look at DNA under microscope, we can see these bundles of protein and DNA arranged in strips called chromosomes. The number of chromosomes in a cell varies according to the animal species. A human has 46 chromosomes (23 pairs) but a horse has 64 chromosomes (32 pairs), a fruit fly has 8 chromosomes (4 pairs), and a dog has 78 chromosomes (39 pairs).


The gene sequence inscribed in DNA, and thus in RNA, is composed of units called “codons,” each coding for a single amino acid (building block of life). Both DNA and RNA are comprised of 4 nucleotide bases. DNA comprises of adenine (A), guanine (G), cytosine (C) and thymine (T). RNA is identical with the exception of thymine (T) being substituted with uracil (U). Codons are non-overlapping groups of the three bases (3 out of 4 bases).

There are 43 = 64 codons. For example, the RNA sequence AUAAUUGGC contains the codons AUA, AUU, GGC, each of which specifies one amino acid. So, this RNA sequence represents a protein sequence, three amino acids long. The interesting portion of these codons is they contain start codon (AUG) and stop codon (UAG, UGA, and UAA). The actual frame in which a protein sequence (polypeptide) is translated is defined by a start codon, usually the first occurrence of AUG in the RNA sequence, and ends with a stop codon (UAG, UGA, UAA). The 64 codons are coded into only 20 standard amino acids. Marshall W. Nirenberg and his lab at the National Institutes of health performed the experiments, which first elucidated the correspondence between the codons and the amino acids for which they code. Har Gobind Khorana expanded on Nirenberg’s work and found the codes for the amino acids that Nirenberg’s methods failed to obtain. Khorana and Nirenberg shared the 1968 Nobel Prize in Physiology or Medicine for this work. My question is:

Why 64 codons? Why does this process of differentiation stop at 20 amino acids?
Why does the ratio of amino acids to codons approaches 1:3?

One possible answer is that the code contains Yin and Yang and we just haven’t introduced it into the 20 amino acids. The combination of amino acids and eight natures’ Yin and Yang triplets produces 32 kinds of amino acids.