What is the cell's instruction manual?Nearly all your cells have a complete copy of your genes. Each gene, with a few exceptions, is the instruction for making just one particular protein. All genes are pieces of DNA - a code that sets down the order of the amino acids in a protein. Although all cells have the same genes, they only use the instructions they need, for example only a muscle cell makes muscle proteins.
What is DNA?
Strands of DNA are made up of four different chemical building blocks, or bases - adenine, cytosine, thymine and guanine. These are the 'letters' of the 'digital' DNA code, usually labelled A, C, T and G. The bases are attached to a chemical backbone to make a strand of DNA. Two strands wind around each other to make a double helix, which looks like a spiral staircase. Each 'step' is a base pair: every A is paired with a T, and every C with a G.
Why a double helix?
DNA is your cells' store of coded 'digital' information, which cells copy very nearly perfectly every time they divide. The pairing of the DNA bases in the middle of the two strands of the double helix (A with T, C with G) helps to keep this precious code intact. Each strand of DNA itself provides the pattern for building a new strand. When the cell needs to use or copy any of the information, the double helix 'unzips' to reveal this template.
How is DNA bundled up?
Your cells are all invisible to the naked eye. Yet if the DNA in just one of them were to be stretched out, it would be about two metres long. Instead, it is all bundled into the cell nucleus, which has a diameter of just five-thousandths of a millimetre. This is like trying to cram 2000 kilometres of wool into an average-sized room. All your DNA is tightly coiled up around proteins, then wound up again until it is 'supercoiled'.