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From Genes to Genomes Concepts and Applications of DNA Technology von Dale, Jeremy W. (eBook)

  • Erscheinungsdatum: 28.11.2011
  • Verlag: Wiley
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From Genes to Genomes

The latest edition of this highly successful textbook introduces the key techniques and concepts involved in cloning genes and in studying their expression and variation. The new edition features: Increased coverage of whole-genome sequencing technologies and enhanced treatment of bioinformatics. Clear, two-colour diagrams throughout. A dedicated website including all figures. Noted for its outstanding balance between clarity of coverage and level of detail, this book provides an excellent introduction to the fast moving world of molecular genetics.

Produktinformationen

    Format: ePUB
    Kopierschutz: AdobeDRM
    Seitenzahl: 408
    Erscheinungsdatum: 28.11.2011
    Sprache: Englisch
    ISBN: 9781119954279
    Verlag: Wiley
    Größe: 5707 kBytes
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From Genes to Genomes

1

From Genes to Genomes

1.1 Introduction

The classical approach to genetics starts with the identification of variants that have a specific phenotype , i.e., they differ from the wildtype in some way that can be seen (or detected in other ways) and defined. For Gregor Mendel, the father of modern genetics, this was the appearance of his peas (e.g., green versus yellow, or round versus wrinkled). One of the postulates he arrived at was that these characteristics assorted independently of one another. For example, when crossing one type of pea that produces yellow, wrinkled peas with another that produces green, round peas, the first generation (F 1 ) are all round and yellow (because round is dominant over wrinkled, and yellow is dominant over green). In the second (F 2 ) generation, there is a 3 : 1 mixture of round versus wrinkled peas, and independently a 3 : 1 mixture of yellow to green peas.

Of course Mendel did not know why this happened. We now know that if two genes are located on different chromosomes, which will segregate independently during meiosis, the genes will be distributed independently amongst the progeny. Independent assortment can also happen if the two genes are on the same chromosome, but only if they are so far apart that any recombination between the homologous chromosomes will be sufficient to reassort them independently. However, if they are quite close together, recombination is less likely, and they will therefore tend to remain associated during meiosis. They will therefore be inherited together. We refer to genes that do not segregate independently as linked ; the closer they are, the greater the degree of linkage, i.e., the more likely they are to stay together during meiosis. Measuring the degree of linkage ( linkage analysis ) is a central tool in classical genetics, in that it provides a way of mapping genes, i.e., determining their relative position on the chromosome.

Bacteria and yeasts provide much more convenient systems for genetic analysis, because they grow quickly, as unicellular organisms, on defined media. You can therefore use chemical or physical mutagens (such as ultraviolet irradiation) to produce a wide range of mutations, and can select specific mutations from very large pools of organisms – remembering that an overnight culture of Escherichia coli will contain some 10 9 bacteria per millilitre. So we can use genetic techniques to investigate detailed aspects of the physiology of such cells, including identifying the relevant genes by mapping the position of the mutations.

For multicellular organisms, the range of phenotypes is even greater, as there are then questions concerning the development of different parts of the organism, and how each individual part influences the development of others. However, animals have much longer generation times than bacteria, and using millions of animals (especially mammals) to identify the mutations you are interested in is logistically impossible, and ethically indefensible. Human genetics is even more difficult as you cannot use selected breeding to map genes; you have to rely on the analysis of real families, who have chosen to breed with no consideration for the needs of science. Nevertheless, classical genetics has contributed extensively to the study of developmental processes, notably in the fruit fly Drosophila melanogaster , where it is possible to study quite large numbers of animals, due to their relative ease of housing and short generation times, and to use mutagenic agents to enhance the rate of variation.

However, these methods suffered from a number of limitations. In particular, they could only be applied, in general, to mutations that gave rise to a phenotype that could be defined in some way, including shape, physio

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