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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
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
Ac allows Ds to
transpose out of C in
many cells, results in
purple pigmented
spots on kernels
Mutant maize
Mutant C
Ds insertion inactivates C
and prevents pigment
synthesis