Mitochondrial DNA is entirely inherited from an organism’s mother. The number varies in other animals: fruit flies have only eight and the black mulberry plant has 308, for example. Humans usually have 46 of these in each cell, 23 from each parent. The coiled strands of your DNA are thus organised into chromosomes. So your three billion bases, which are more than 99 per cent the same as everyone else’s, need to be packaged up neatly. If you stretched it out, it would be almost two metres long. There is a lot of DNA packed in to every human cell. In this way, each gene tells the cell’s machinery how to make a vast array of proteins. ACT, for instance, tells cells to make an amino acid called threonine. Three bases in a row together code for a specific amino acid, the basic building blocks of proteins. The bases always pair up with the same complementary compound on the other strand of DNA: A with T, and C with G. These come in four types: adenine (A), guanine (G), cytosine (C) and thymine (T). The order of DNA’s chemical bases form the genetic code. But it was the work of many researchers throughout the decades that followed that determined what DNA codes for, how it is read, and how it is copied and passed on to new cells and future generations. It is rather like a spiral staircase or twisted ladder in which every rung is a bond between matching “bases” on its two strands. The 1953 discovery of the shape of DNA, known as a double helix, is mainly credited to Francis Crick, James Watson, Rosalind Franklin and Maurice Wilkins. We also have some DNA inside the mitochondria that power our cells, while plants have extra DNA within the chloroplasts that enable them to photosynthesise. DNA, which stands for deoxyribonucleic acid, is curled up and stored as chromosomes in the nucleus of every one of our cells. You get half of your DNA from each of your biological parents, and you will pass on a selection of half of it to any child you might conceive. DNA is the code of life, the means by which every living organism on Earth stores its genetic information.
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