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The relationship between genes and development is one of the hottest topics in biology today. In 2001 the Human Genome Project delivered the first draft of the promised sequence of human DNA, and revealed that we have about 35,000 genes. This was far fewer than most geneticists had anticipated, and recent estimates suggest the figure may be even lower—as low as 25,000.1 The big question now is how these relatively few genes can be the basis of all the exotic and intricate events that occur during embryonic and postembryonic development. Finding out exactly what genes do has acquired a new urgency. Will it be possible to work out what each gene does? And if it is, what will it tell us about the inherited differences between us? To answer these questions, we first need to say something about the molecular nature of the gene, and how it works as a unit of function and inheritance. In doing so, we shall be stressing what we see as the major properties of the genetic system, and try to explain why biologists think that this DNA-based inheritance system is so special. DNA is not the only thing that we inherit from our parents, of course. We inherit the other materials that are present in the egg, and also things such as our parents ’ food preferences, their ideas, and their real estate. Obviously, there are several routes through which materials and information can be transferred from parents to their young, and in later chapters we shall argue that all are potentially important in determining what happens in evolution . However, inheritance systems differ in what kind of information is transmitted, how it is transmitted, how much and how faithfully it is transmitted, and in the relationship between what is transmitted and the effects it has. This is why we are going to focus on these aspects of the genetic system. 2 From Genes to Characters 48 Chapter 2 From DNA to Proteins We have already mentioned the discovery of the structure of DNA by Watson and Crick in 1953. One of the fascinating and surprising things about the structure they revealed was how it reflects in a relatively straightforward way the properties that are required by the genetic inheritance system. The way that DNA is organized immediately suggests how replication may occur, and hints at how the molecule might carry information for making proteins. Both are possible because DNA is a linear molecule, made up of strings of a small number of different components.2 Figure 2.1 shows the DNA double helix, one of the icons of the twentieth century. The two strands of the helix are wound around the same axis and are held together by weak chemical bonds. Each strand is made of four different units, the nucleotides, which are joined together to form a long chain. A nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base. The differences between the four nucleotides lie in their nitrogenous bases, which can be thymine, adenine, guanine, or cytosine, and the different nucleotides are always known by an abbreviation of the names of these bases—T, A, G, and C, respectively. The weak bonds between the two nucleotide chains are formed because a base in one strand is paired with a base in the other. The association is such that A is always paired with T, and C is always paired with G. There are good chemical reasons why the pairs are always A—T and C—G, but we need not go into these. What is important for us here is that the two strands are complementary: if you know the sequence of nucleotides in one strand, you can work out the sequence in the other. Apart from the fact that the two strands must Figure 2.1 The structure and replication of DNA. From Genes to Characters 49 be complementary, the sequence of nucleotides in DNA is not restricted in any way. It was clear from the outset what the significance of the nucleotide pairs might be. In a calculated understatement at the end of their famous paper, Watson and Crick wrote, “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” It was the complementary relationship between the nucleotides in the two strands that hinted at the nature of the copying process. If the strands separate, each single strand can...

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Additional Information

ISBN
9780262322676
Related ISBN
9780262525848
MARC Record
OCLC
878130644
Pages
576
Launched on MUSE
2014-04-24
Language
English
Open Access
No
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