Defining DNA, and what is a gene?

Enrico Coen is the president of the Genetics Society. He seemed the perfect person to ask: what is DNA? He spoke to Simon Bishop.

Enrico - OK, so DNA is basically a very long molecule that carries all the hereditary information - or the bulk of the hereditary information - that we transmit from parents to children for all organisms, be they humans, cows, mice, bacteria, they all have this molecule that is carrying the hereditary information.

Simon - But how can a molecule be a code? How can there be information in a chemical?

Enrico - So the molecule carries - it's a like a very long string that carries a sequence of four different types of subunits, that are like letters, so they're often referred to as A, G, C and T. And the sequence of these letters is specific to each DNA molecule, so every individual will have a particular sequence of these letters. And just as words and the sequence of letters in a book can carry information and can determine the meaning of the book, so a lot of the meaning of organisms or the information that's needed for organisms to develop is carried in this long molecule.

Simon - So, that's it. DNA is a chemical that has four different components. When these components - which are called nucleotides but we think of them as letters - are arranged in a long chain, they spell out a code. But never mind DNA, I've also heard of genetic information. So, what is a gene?

Enrico - A gene is basically a short stretch of DNA that carries information, usually to create or code for a particular product, such as a protein. So it's a sort of convenient unit of the DNA, just as a word is a convenient unit when we want to think about letters, or a sentence is another convenient unit, so when we look at DNA we think in terms of certain units that make sense in terms of their own structure, and a gene is basically a unit of that kind.