Read Online What a Wonderful World by Marcus Chown - Free Book Online
Authors: Marcus Chown
Ads: Link
stretch right the way from your head to your toe. Packing all that DNA into a tiny cell, invisible to the human eye, is therefore a biological challenge. A cell achieves this impressive feat by packaging the DNA into shorter stretches known as chromosomes, so called because they were first revealed with the aid of coloured, or chromatic, dyes. 10 Human cells have 46 chromosomes – two sets of essentially the same chromosomes.
Dogs have 78 chromosomes; horses 64; and cats and pigs 38. The number of chromosomes appears to bear little relation to an organism’s complexity. The Adder’s-tongue fern,
Ophioglossum vulgatum
, for instance, has a whopping 1,440 chromosomes, the largest number of any living thing. 11
Back to humans. Recall that, every day, your body creates about 300 billion new cells – more than there are stars in our Milky Way Galaxy. 12 In this process, known as mitosis, a cell first creates a copy of all 46 of its chromosomes. That makes a total of 92. Then, the cell splits into two ‘daughter’ cells, each with 46 chromosomes, exactly like the original.
Sex is the opposite process. Instead of splitting one cell into two, two cells – one from each parent – are fused into one. However, if the final cell is to have the correct complement of 46 chromosomes, the cells from each parent – known as sex cells, or gametes – must each contain only 23 chromosomes, or
half the normal number
.
The creation of sex cells by both males and females, therefore, requires a process quite distinct from mitosis. In meiosis, as in mitosis, a cell first makes a copy of all 46 chromosomes, to make a total of 92. But then it splits
not once but twice
. The end result is four gametes, each of which contains 23 chromosomes.
Incidentally, some shuffling of the genes is carried out during meiosis, so that each of the gametes is genetically different from its parent. This shuffling might once upon a time have been an accident – a result of the complex manoeuvring of chromosomes during meiosis – but it might have become frozen because creating offspring with the maximum amount of genetic novelty has survival value. And this shuffling of genes to create variety is
even before
sex cells fuse to generate yet more variety.
The gametes from each parent could, of course, be the same size. And this is true for some organisms. But very often one is far bigger than the other because it contains the fuel and protein machinery to drive development once fusion occurs. For biologists, the essential difference between the sexes lies in the gametes. Organisms that produce large gametes that cannot move about – known as eggs, or ova – are female – while organisms that produce small gametes that can move about – known as sperm – are male. All other things that are usually associated with the difference between the sexes – penises, vaginas, breasts and beards – are ultimately just consequences of the differences between sperm and egg.
Biologists believe that the first sexually reproducing organisms produced gametes of the same size. This is an interesting observation. It means that
sex came before sexes
.
Now, finally, we come to the fusion of two gametes – one from each parent – which is the central act of sex. Here, the two gametes, each with 23 chromosomes, combine to make a single cell, known as a zygote. Subsequently, the zygote will split, again and again, by normal mitosis, to make the 100 trillion or so cells that compose an adult human being.
Clearly, the zygote contains 23 chromosomes from the mother and 23 chromosomes from the father. 13 At a gross level, therefore, each one of your cells has two copies of
exactly the same genes
. After all, men and women are genetically more similar than, for instance, men and chimpanzees – and, recall, chimpanzees share 98 to 99 per cent of their DNA with humans. 14
But, although your mother and father contributed the same genes to you, they may have
Dogs have 78 chromosomes; horses 64; and cats and pigs 38. The number of chromosomes appears to bear little relation to an organism’s complexity. The Adder’s-tongue fern,
Ophioglossum vulgatum
, for instance, has a whopping 1,440 chromosomes, the largest number of any living thing. 11
Back to humans. Recall that, every day, your body creates about 300 billion new cells – more than there are stars in our Milky Way Galaxy. 12 In this process, known as mitosis, a cell first creates a copy of all 46 of its chromosomes. That makes a total of 92. Then, the cell splits into two ‘daughter’ cells, each with 46 chromosomes, exactly like the original.
Sex is the opposite process. Instead of splitting one cell into two, two cells – one from each parent – are fused into one. However, if the final cell is to have the correct complement of 46 chromosomes, the cells from each parent – known as sex cells, or gametes – must each contain only 23 chromosomes, or
half the normal number
.
The creation of sex cells by both males and females, therefore, requires a process quite distinct from mitosis. In meiosis, as in mitosis, a cell first makes a copy of all 46 chromosomes, to make a total of 92. But then it splits
not once but twice
. The end result is four gametes, each of which contains 23 chromosomes.
Incidentally, some shuffling of the genes is carried out during meiosis, so that each of the gametes is genetically different from its parent. This shuffling might once upon a time have been an accident – a result of the complex manoeuvring of chromosomes during meiosis – but it might have become frozen because creating offspring with the maximum amount of genetic novelty has survival value. And this shuffling of genes to create variety is
even before
sex cells fuse to generate yet more variety.
The gametes from each parent could, of course, be the same size. And this is true for some organisms. But very often one is far bigger than the other because it contains the fuel and protein machinery to drive development once fusion occurs. For biologists, the essential difference between the sexes lies in the gametes. Organisms that produce large gametes that cannot move about – known as eggs, or ova – are female – while organisms that produce small gametes that can move about – known as sperm – are male. All other things that are usually associated with the difference between the sexes – penises, vaginas, breasts and beards – are ultimately just consequences of the differences between sperm and egg.
Biologists believe that the first sexually reproducing organisms produced gametes of the same size. This is an interesting observation. It means that
sex came before sexes
.
Now, finally, we come to the fusion of two gametes – one from each parent – which is the central act of sex. Here, the two gametes, each with 23 chromosomes, combine to make a single cell, known as a zygote. Subsequently, the zygote will split, again and again, by normal mitosis, to make the 100 trillion or so cells that compose an adult human being.
Clearly, the zygote contains 23 chromosomes from the mother and 23 chromosomes from the father. 13 At a gross level, therefore, each one of your cells has two copies of
exactly the same genes
. After all, men and women are genetically more similar than, for instance, men and chimpanzees – and, recall, chimpanzees share 98 to 99 per cent of their DNA with humans. 14
But, although your mother and father contributed the same genes to you, they may have
Similar Books
