DNA/RNA Metabolism

Blackburn & Gait, Ch. 5Neidle, 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

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 elementsBarbara McClintock - Noble prize

Normal maizeActive C causes

synthesis of purple pigment

Mutant maizeMutant C

Ds insertion inactivates C and prevents pigment

synthesis

Mutant maizeAc allows Ds to

transpose out of C in many cells, results in

purple pigmented spots on kernels