Chromosome Structure

Chromosome Structurespliced together to produce antibodies. The mammalianimmune system can make antibodies to a diverse array ofdifferent structures by cutting and pasting genes together inan immense number ofdifferent ways. The antibodysystem is essential for life, because it protects an organismfrom invasion by bacterial and viral pathogens. Thus, sitespecificrecombination has evolved as one key mechanismthat changes the genome in a small subset ofdifferentiatedcells, and this ultimately allows an organism to survive andstay the same.With multiple transposons, efficient homologous andsite-specific recombination systems, the occurrence offrequent chromosome breaks, and the presence of multiplelong inverted and direct repeats, it would seem impossiblefor chromosomes to remain constant through time.Surprisingly, the genetic maps of E. coli and S. typhimuriumare very similar after 140 million years of separationfrom a common ancestor. Thus, the efficiency of enzymesystems designed to promote chromosome stability isnearly equal to the forces that promote change.Further ReadingCasjens S (1998) Bacterial genome structure. Annual Review ofGenetics32: 339–377.Charlebois RL (ed) (1999) Organization ofthe Prokaryotic Genome.Washington DC: ASM Press.Cozzarelli NR and Wang JC (1990) DNA Topology and its BiologicalEffects. Cold Spring Harbor, New York: Cold Spring Harbor Press.Kornberg A and Baker T (1991) DNA Replication, 2nd edn. New York:WH Freeman.Kuzminov A (1999) Recombinational repair ofDNA damage inEscherichia coli and bacteriophage l. Microbiology and MolecularBiology Reviews 63: 751–813.Neidhardt FC (ed.) (1996) Escherichia coli and Salmonella. WashingtonDC: ASM Press.10 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net