Genetic Recombination 3 by: Nouf alyami

Content I. INTRODUCTION. II. GENERAL RECOMBINATION III. SITE-SPECIFIC RECOMBINATION

I. INTRODUCTION. Although DNA replication and repair is crucial to cell survival, it does not explain human genetic variability. Some of the variability is imported by DNA rearrangements that are caused by genetic recombination, either general or site-specific

II. GENERAL RECOMBINATION. Involves single-stranded DNA and requires DNA sequence homology. A. An important example of general recombination occurs during "crossing over," when two homologous chromosomes pair during meiosis (gamete formation) B. RecBCD protein makes single-strand nicks in DNA to form single- stranded "whiskers." C. Single- strand binding (SSB) protein stabilizes the single-stranded DNA.

D. During synapsis, RecA protein allows the single strand to invade and interact with the double, helix DNA of the other chromosome. This interaction requires DNA sequence homology. E. A DNA strand on the homologous chromosome repeats this process to form an important intermediate structure (crossover exchange, or Holliday junction) that has two crossing strands and two noncrossing strands. F. The two crossing strands are cut, and DNA repair produces two homologous chromosomes with exchanged DNA segments.

III. SITE-SPECIFIC RECOMBINATION. involves the insertion of double-stranded DNA. A.An important example of site-specific recombination is the insertion of viral DNA into host DNA. B. Many DNA viruses and other transposable elements encode for a recombination enzyme called integrase or transposase, respectively. C. Integrase recognizes specific nucleotide sequences and cuts the viral DNA.

D. The cut ends of the viral DNA attack and break the host double- helix DNA. E. The viral DNA is inserted into the host DNA. F. DNA repair occurs to fill the gaps.