Generation of Genetic Diversity

This models the relationship between repertoire size and repertoire completeness, and shows that at least 108-1010 different antibodies are needed in the repertoire if a threshold Kd of 10 8-10 10 is required, depending of the probability selected (Fig. 8.3).3 Therefore, for one initial or starting sequence, the first step in the directed evolution cycle must be capable of generating 108-1010 nucleic acid sequences that code for different proteins. This library or repertoire size is needed for directed evolution according to this model. However, good results have been obtained experimentally (mainly with enzymes) using smaller libraries, of 104 members.6 7 

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The homologous recombination illustrated in Figure 25-31 is a very elaborate process with subtle molecular consequences for the generation of genetic diversity. To understand how this process contributes to diversity, we should keep in mind that the two homologous chromo-... 

It may now be time to address the question what is directed molecular evolution The word directed implies some purpose - in our case the purpose is of relevance to mankind and not the organism from which the enzyme was obtained. The term evolution usually implies Darwinian evolution - the generation of genetic diversity followed by selection to produce a superior product. The process, at the molecular level, is summarized in Figure 2. 

If one considers combinations of possible modification of the chemical structure, it may be assumed that there are more than 1000 individual structures which may occur in planta (Dinan et al, 1999). Many of the modifications found in phyfoecdysferoids are also found in ofher classes of plant triterpenoids (e.g. BRs, sterols). This fact raises the possibility that enzymes of the biosynthetic pathways may be common to plants producing these various classes of mefabolifes. A lack of specificify of fhese enzymes may cause the generation of many diverse mefabolifes or biosynthetic products without massive genetic redundancy of fhe corresponding enzymes involved. 

The major theories describing mechanisms for the generation of antibody diversity, i.e., for V region variability, have themselves undergone evolution as more information on the structure and genetics of antibodies has become available. We will summarize several proposals in their current form and, in the next section, present arguments that have been made for and against these theories. 

Mutations affecting Ig production opens a new era of mammalian somatic cell genetics which could lead to clarification of the arrangement of Ig genes, the mechanisms responsible for generation of antibody diversity, and the suspected translocation of V and C genes. 

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