What a Wonderful World by Marcus Chown Read Free Book Online

sequence of just four molecules, or bases – adenine (A), guanine (G), cytosine (C) and thymine (T) – which are joined in pairs.
A, G, C
and
T
are the four letters of the genetic code. 6 Each triplet of bases codes for a particular amino acid. And amino acids are the building blocks of proteins, miraculous molecules that can carry out all manner of biological tasks, from speeding up the chemical reactions of life to detecting sunlight in your eye to providing the scaffolding that keeps your body rigid enough not to collapse into a puddle of jelly and water.
    A stretch of DNA that encodes a protein is called a gene. And herein lies the connection with Mendel. The traits he identified that were inherited were associated with genes. A particular gene, for instance, makes a protein that influences the development of a pea to be wrinkly or smooth.
    There are about 3 billion letters in a strand of human DNA, accounting for about 23,000 genes. This seems a woefully inadequate number to create a human being, and biologists were truly shocked that there were not more. But they have had no choice but to live with it – 23,000 genes are all there are.
    Some of the genes are involved in controlling other genes. They switch off or switch on their ability to make, or express, proteins at various times in a developing embryo. And they do this depending on factors such as the concentration of a particular chemical in the cell. 7 Such control genes cause different sections of DNA to be read in different types of cell, explaining how,despite every cell in a human being containing a copy of exactly the same DNA, some cells develop as blood cells, others as liver cells or brain cells, and so on.
    But DNA explains not only the mechanism of inheritance but the mechanism of variation too. If an offspring is to inherit traits from its parents, their DNA must be copied. With a whopping 3 billion letters to reproduce faithfully in the case of human DNA, the amazing thing is how good the copying process is. 8 But it is not perfect. A mistake is made about once every 1 billion base pairs. Sometimes a letter is not copied correctly. Or a sequence of DNA is deleted or duplicated. There are a myriad possible transcription errors. In addition, changes in genes can be caused by cancer-causing chemicals, viruses, ultraviolet light and nuclear radiation.
    The upshot is that over time
genes gradually change
.
    There is a lot of redundancy built into DNA to minimise copying mistakes, so many of the individual changes make little difference – the protein encoded by the gene still works. Some changes are harmful, causing inherited diseases such as cystic fibrosis. But, very occasionally, a change in DNA turns out to make a beneficial change to an organism – for instance, conferring on it an increased resistance to malaria. Of course, the ultimate arbiter of what is
beneficial
to an organism is its environment. A change in a gene that results in a thick, warm coat is beneficial to an animal living in a world plunging into an ice age but not to one living in a tropical world.
    It is worth pointing out that changes, or mutations, occur in the DNA of all organisms. But, whereas simple organisms such as bacteria merely create copies, or clones, of themselves when they reproduce, other creatures have sex, producing offspringwith half their genes from each parent. Such a composite of different traits passed down the maternal and paternal line greatly boosts the novel gene combinations available for natural selection. 9
    Mutations explain the existence of
species
– groups of animals, which, broadly speaking, cannot interbreed. Species can arise in many ways. For instance, a geographical barrier such as a river or mountain range might split a population in two. Or, as in the case of the Galápagos, an ocean might divide creatures from their cousins on the mainland. Separated in this way and subjected to different survival pressures, the DNA of each group