Germ-line theory

Hood and Talmage (7), Smith et al. (8), and Garver and Hilsch-mann (16) take the view that the germ line theory provides the simplest... 

This statement must be qualified because of recent data (see Note added in proof, p. 406). This weakens the argument against the germ line theory. 

The germ-line theory According to this model the primary structure of every V region an animal can produce is encoded in its DNA. Every vertebrate genome contains many thousands of different V genes with divergence due to natural selection of favourable random point mutations, duplications and recombinations. 

The end of a linear chromosome is called a telomere. Telomeres require a special mechanism, because the ends of a linear chromosome can t be replicated by the standard DNA polymerases. Replication requires both a template and a primer at whose 3 end synthesis begins. The primer can t be copied by the polymerase it primes. What copies the DNA complementary to the primer In a circular chromosome, the primer site is to the 3 direction of another polymerase, but in a linear chromosome, no place exists for that polymerase to bind. As a result, unless a special mechanism for copying the ends of chromosomes is used, there will be a progressive loss of information from the end of the linear chromosome. Two characteristics about telomeres help avoid this situation. First, they consist of a short sequence—for example, AGGGTT—repeated many times at the end of each chromosome. Telomeres, therefore, are part of the highly repetitive DNA complement of a eukaryotic cell. Secondly, a specific enzyme, telomerase, carries out the synthesis of this reiterated DNA. Telomerase contains a small RNA subunit that provides the template for the sequence of the telomeric DNA. Eukaryotic somatic cells have a lifespan of only about 50 doublings, unless they are cancerous. One theory holds that a lack of telomerase in cells outside the germ line causes this limitation. 

Pig. 9. Radii of the critical ice germs as a function of temperature. Diamonds Our measurements, Solid Line theory according to Ref. 19. 

Objections to the theory of multiple germ line genes, discussed in the previous section are, in a sense, arguments favoring its leading contenders, the somatic mutation and somatic recombination theories. 

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