DNA is an alphabet soup of four amino acids Adenine (A), Guanine (G), Cytosine (C), and Thymine (T), seemingly strung out in a sequence millions of nucleotides long (AGGCGGAAGGTTCTATCG…..). DNA is also referred to as the “blue print of life,” inferring that all these letters have something to do with anatomical structure and the metabolic processes representative of life. But how do all the A’s, G’s, C’s, and T’s do that? In the 1950’s and 60’s a group of pretty intuitive scientists began to unravel the secrets of DNA to find the answer to this question.
In 1953, James D. Watson and Francis Crick, based on work done by crystallographer Rosalind Franklin, revealed the double helix structure of DNA and discover that its nucleotides were arranged in groups of three. They further learned that these groupings contained a code representing the twenty different amino acids. For instance, the group of letters “AAA” or “AAG,” code for the amino acid Lysine. These groupings of three nucleotides are called “codons.” If you take the sequences of codons and look at the amino acids represented by them, you will find that such sequences define or “code” for proteins. Proteins are the chemical workhorse of living organisms, and they are all produced from copies made from the DNA molecule.
DNA can’t do it alone. Another DNA like substance called RNA, is involved in the process of reading the DNA codons and translating them into proteins. This process is called gene expression. In 1956, Arthur Kornberg discovered an enzyme called “polymerase,” produced by RNA. Polymerase is involved in the initial stage of gene expression, performing a process called transcription which involves reading and copying segments of DNA. The polymerase transcription process produces a segment or polypeptide chain of amino acids called messenger RNA (mRNA). The messenger RNA permeates the membrane of the cell nucleus into the cytoplasm surrounding it. Here the second phase of gene expression takes place involving the translation of mRNA into proteins.
In the cytoplasm, mRNA is acted on by little chemical factories called “ribosomes.” Ribosomes are actually a conglomeration of RNA and about sixty other proteins. The ribosomes build proteins by translating the coded mRNA and assembling the actual polypeptide chains that will become a protein. When the translation is complete, the resulting protein drifts off to perform its intended function which may take place inside or outside of the cell membrane.
This is of course just an overview of the transcription and translation processes which facilitate gene expression. There are a lot of other chemical reactions taking place within the cell to support the profusion of chemistry going on during transcription and translation, as well as other functions to maintain each cell as an independent living organism. But everything going on in a cell begins with the expression of the genes of the DNA molecule.