New News Biology #47
DNA (Part 3): Structure & Function of DNA
DNA is a structure known as the ‘double helix’, because it has two strands that twist and wrap around each other.
If you took out one of these strands, and stretched it into a 2D diagram, you would see quite clearly that DNA is a polymer, being made up of many smaller units called monomers. In DNA, each of these units are known as a nucleotide.
A nucleotide is made up of three parts: a phosphate, a sugar, and a base. In a strand of DNA, the phosphate and sugar parts of every nucleotide are identical to every other one. However, the ‘base’ part of the nucleotide can vary, with four different types: A (adenine), T (thymine), C (cytosine), and G (guanine).
The way that these individual nucleotide work together is that the phosphate of one nucleotide bonds to the sugar on the nucleotide before it, and so on, forming a long chain known as the ‘sugar-phosphate backbone’. This backbone is the outer part of the double helix, and effectively serves a protective purpose for those bases in the middle.
So how do the two strands stick with each other? Well, if you took two strands, and made then so that the bases were facing the other, they would ‘pair up’, and that’s what hold the two strands together. However, it isn’t that easy. There’s a concept called complementary base pairing, and the rule is that A can only pair with T, and C can only pair with G. It can also help us figure out the complementary strand of a particular strand of DNA. With the information of one half of the DNA, or one strand, we can figure out the other strand as well.
The term ‘genetic code’ is just a sequence of bases. And a ‘gene’ is a particular sequence of bases that codes for a particular protein. Each group of three bases is known as a ‘triplet’, and codes for a specific amino acid.
The way that a ‘gene’ makes a protein is like this: Firstly, our cells interpret a strand of DNA, splitting it into triplets. Then it creates the amino acid corresponding to each triplet, and does this for the entire chain. Lastly, this chain of newly created amino acids folds up by itself into a protein.
The interesting part of proteins is that because every protein is made with a different sequence of amino acids, it has a unique shape, and in turn it will carry out a particular function. The main uses of proteins are enzymes (which act as biological catalysts), hormones (which carry messages around the body), and structural proteins (which add strength to cells & tissues).