What is DNA? Why do we need it?

What is DNA?
Despite providing all the genetic information that constitutes any living being, DNA is remarkably simple.

On a biochemical level DNA is a linear polymer with a backbone composed of alternating sugar and phosphates, with bases attached to the sugar. Deoxyribose is the five-carbon sugar used in DNA. Four bases are used in DNA: adenine (A) and guanine (G) (purines); cytosine (C) and thymine (T) (pyrimidines).

The sequence of these bases is what provides the genetic information which codes for each individual. The structure of DNA is shown in the image below.

The two DNA strands are arranged in an anti-parallel double helix, due to laws known as Watson and Crick base pairing (named after those who discovered the structure of DNA), an adenine can only base pair with a thymine, with 2 hydrogen bonds, and a cytosine only to a guanine, with 3 hydrogen bonds (this is shown above). This strict and complementary base pairing is what governs DNA replication as well as coding for proteins. Although other forms of DNA can exist within cells the anti-parallel double helix known as the B helix is by far the most common and well documented.

Why do we need DNA?
The main function of DNA is to code for proteins. This does not just simply refer to structural proteins that constitute skin and hair, but also many other functional proteins, such as enzymes, that catalyse the chemical reactions of the body or build and break down molecules as required. It is remarkable to think that DNA holds all our genetic information, yet the only substance that it codes for is proteins. These proteins coded for by DNA form the basis of life itself, and no organism can hope to exist without enzymes (made of proteins) that catalyse reactions.

How does DNA code for proteins?
The sequence of bases (shown in the images above) is what codes for the sequence of amino acids that make up proteins. Every 3 bases of DNA codes for a specific amino acid, and this is known as a codon. The sequence of each of the codons then determines the sequence of amino acids, which ultimately leads to the specific structure of the protein. The process of converting the DNA code to a functional protein is very complex (and perhaps could be the subject of another article).

There are 20 amino acids that are coded by DNA and a specific sequence of bases will constitute a specific amino acid. However with 4 possible bases and each codon being 3 bases long there are 64 (4 x 4 x 4) possible combinations of 3 bases. This results in more than one codon coding for the same amino acid. This is known as degeneracy, which is an important quality of DNA, as it allows the protein code to remain the same despite certain mutations in DNA taking place, ultimately resulting in no mutagenic outcome within the coded protein.

DNA and disease
Broadly speaking, DNA contributes to disease in two ways.

The first through genetics, ranging from inherited diseases obeying simple Mendelian genetics (such as cystic fibrosis), to common conditions with a strong genetic component, such as hypertension, autism and multiple sclerosis, but which don’t simply follow Mendelian inheritance.

The second way in which DNA contributes to disease is through diseases of DNA itself, particularly its maintenance and/or repair. In these conditions, failure to protect and/or repair DNA leads to accumulation of changes in the DNA, ranging from small changes in sequence to full breaks in the double stranded DNA and rearrangement of whole parts of chromosomes. These changes, mutations, lead to abnormal cellular behaviours, the most common of which are cancer and ageing.