Читать книгу Your Body - The Fish That Evolved - Dr. Keith Harrison - Страница 9

‘G ene’ is from the ancient Greek word for descent, or birth (from the same root as genealogy or genesis). Genes are inherited instructions elling the body how to build and maintain itself. These instructions may be for something internal like the production of enzymes in the intestine or some-thing obvious like height or the shape of our nose.

Each gene is a short string of molecules. These are attached end to end to form threads of DNA or deoxyribonucleic acid – so called because it’s an acidic molecule found in the cell nucleus (a ‘nucleic acid’) and includes the sugar Ribose from which an oxygen atom has been removed (‘de-oxy ribo’). These threads of DNA are found in the nucleus of most cells and the human body contains approximately one hundred million million cells (100,000,000,000,000 or 10¹⁴ cells), each of which contains the full set of genes for making and running the whole body. A cell in the eye therefore contains the genes for making a stomach or a kneecap, even though they will never be used. This is like every library in the world containing a street atlas of every town in the world, even though most people only consult maps of their local area.

In humans, each nucleus has 46 threads of DNA, together more than 2 metres in length and containing about 24,000 genes. These 46 DNA threads are arranged in pairs. This is because we all receive one of each pair from each parent; 23 from the mother’s egg and 23 from the father’s sperm. This is a bit like getting a pair of socks for your birthday but your mother gives you one sock and your father gives you the other, except in this case you get 23 odd socks from each of them. When the gifts are put together, they make 23 matched pairs.

Each thread of the pair contains genes for features of our body – hair colour, eye colour, arm length – but the other thread of that pair also contains genes for those same features. The threads are twins. This means we all inherit two genes for most features, not one, but how this works in practice is not something we need to consider in this book.

At certain times during the life of a cell, each DNA thread twists in upon itself like a tangled rope to form a shorter fatter structure which early naturalists called a chromosome (Greek for ‘coloured body’ because, in the early days of the microscope when people were staining cells with different dyes to make them visible, the chromosomes sometimes appeared as short dark ribbons. Today, we call the threads chromosomes whether they are densely twisted or not). These twisted ‘coloured bodies’ appear when the cell is about to divide during tissue growth, wound healing or cell replacement.

Most cell types are replaced continuously. Skin wears away all the time, with new skin being formed underneath it, and red blood cells (which carry oxygen around the body) only have a life of about 120 days. Each of us has billions upon billions of red blood cells (one large drop of blood contains about 500 million) and 170 thousand million (170,000,000,000) new red blood cells are created every day by your bone marrow to replace those being destroyed by – or perhaps we should say recycled by – your spleen, liver and (again) bone marrow. When a blood donor donates half a litre of blood, the body loses about two and a half million million (2,500,000,000,000) red cells and it takes about 50 days to replace them. (It doesn’t take 15 days as would be expected from the multiplication:

170,000,000,000 per day x 15 = 2,550,000,000,000

because these cells are lost to the body and cannot be recycled. Donors must get new raw materials from their food before they can make new cells.)

Most animals have a nucleus containing DNA in their red blood cells, as in the other cells of their body, but mammals, including ourselves, lose the nucleus of the red blood cell as the cell is formed. This is how forensic scientists can tell whether a bloodstain came from a person or from the preparation of someone’s chicken supper.

As cells in the body multiply to produce new skin or blood, or any other tissues, the chromosomes, and hence the genes, must be copied into each new cell. Genes are copied into an egg when it forms in a woman’s ovary, or into a spermatozoon when it’s produced in a man’s testes (testicles). They are copied when a cell divides to become two cells, as when a fertilised egg begins to grow in the womb. By the time a baby is fully formed, all its genes have been copied numerous times.