DNA/RNA Metabolism Blackburn & Gait, Ch. 5 Neidle, Ch. 4 Recombination understand basics of the reaction know advantages/disadvantages of recombination understand how recombinase protein works know structure of Holliday junction (X-shaped) know different types of recombination

Recombination Enables the limited exchange of genetic material between paired chromosomes Occurs between intact double helices Damage to DNA can stimulate recombination Enables the immune system to generate a diversity of protein antibodies from a limited set of genes Enables viruses to integrate their genetic material into a host’s genome Enables host organism to assort alleles (differing copies of same gene) into novel groups - favorable & unfavorable alleles can be shuffled randomly Enables repair of a damaged gene in an otherwise favorable chromosome Enables regulation of gene expression Enables rearrangement of antibody genes Can lead to cancers - example: Burkitt’s lymphoma caused by translocation between ch. 8 and 14

Recombination THREE main categories 1. Homologous recombination takes place between similar sequences 2.Site-specific recombination Limited sequence similarity between recombining DNAs 3. Transposition movement of a DNA element from one position to another, little sequence similarity needed called “illegitimate recombination”

1. Homologous Recombination

Recombinase protein One example is that of Cre recombinase from a bacteriophage (virus that infects bacteria) The enzyme mediates strand cleavage and exchange between two pieces of DNA An intermediate (half reacted) configuration of the DNA is called a Holliday junction (X-shaped)

1. Homologous Recombination Recombinase protein

1. Homologous Recombination

2. Site-specific Recombination Lambda phage integration and excision

2. Site-specific Recombination VDJ rearrangement in antibody production

3. Transposition - Recombination Bacterial antibiotic resistance

3. Transposition - Recombination Eukaryotic transposable elements Barbara McClintock - Noble prize Normal maize Active C causes synthesis of purple pigment Mutant maize Mutant C Ds insertion inactivates C and prevents pigment synthesis Mutant maize Ac allows Ds to transpose out of C in many cells, results in purple pigmented spots on kernels