Lecture 3: Dna Recombination

DNA RECOMBINATION

DNA RECOMBINATION Any in vivo or in vitro process involving the rearrangement of sequence(s) of nucleotides in one or more molecules of nucleic acid – DNA RECOMBINATION These include events such as e.g. deletions, inversions, replacements and amalgamations. Any molecule that has undergone such a process is referred to as a recombinant.

DNA RECOMBINATION Recombination involve in crossing over in meiosis Homologous Recombination Site-Specific Recombination Transpositional Recombination

DNA RECOMBINATION Homologous Recombination A type of recombination which requires an extensive region of sequence homology between the two participating duplexes, most commonly, between two copies of the same chromosome.

DNA RECOMBINATION e.g. The exchange of sections of homologous chromosomes (homologs) in the course of meiosis, an event that occurs between chromosomes early in the development of eggs and sperm.

DNA RECOMBINATION General recombination has the effect of increasing the diversity of gene combinations on the chromosome of the mating populations. Thereby increasing the chances of survival in a changing environment.

DNA RECOMBINATION Mechanism: Two homologous DNA double helices are broken and the two broken ends are rejoined to their opposite partners to reform two intact helices, each of which contains parts of both of the initial DNA molecules.

The breakage and reunion of two homologous DNA double helices creates two chromosomes that have crossed over

DNA RECOMBINATION Mechanism: The site of exchange can occur anywhere in the homologous nucleotide sequences of the participating chromosomes.

DNA RECOMBINATION Mechanism: At the site of exchange, a strand of one helix has become base-paired to a strand of the second helix to create a staggered joint between the two different DNA helices.

A staggered joint unites two chromosomes where they have crossed over.

DNA RECOMBINATION Mechanism: No nucleotide sequences are altered at the place of exchange; the breaking and reunion process is so precise that not a single nucleotide is lost, gained or changed.

Schematic illustration. A nick in a single DNA strand frees the strand, which then invades the second helix to form a short pairing region. Only two DNA molecules that are complementary in nucleotide sequence can base-pair to initiate a recombination event

DNA RECOMBINATION Mechanism: Agents known to introduce such nicks into DNA strands, such as gamma or x-irradiation, can trigger a genetic recombination event. Special proteins enable DNA single strands to pair with a homologous region of DNA double helix.

The binding of a helix-destabilising protein prepares a DNA single strand for base-pairing interactions.

DNA renaturation occurs, helix reformation depends on the random collision of two complementary strands, leading to helix nucleation (complementary bp are formed). A rapid zippering then occurs to complete each helix. Trial-and error recognition of complementary sequences.

DNA RECOMBINATION Zippering Ongoing (progressive and rapid) hybridization of pairs of nucleotides in two complementary strands of nucleic acid after the initial phase of binding between a few pairs of bases (Kinetics of duplex formation for individual strands)

Studies in vitro have demonstrated that helix destabilising protein (SSB protein) cooperates with the Rec A protein to facilitate pairing reactions.

The isomerization of a cross-strand exchange

With isomerization, the cutting of the two crossing strands creates two chromosomes that have crossed over.

Without isomerization, the cutting of the two crossing strands terminates the exchange without crossing over.

DNA RECOMBINATION Site-specific recombination (SSR) A form of recombination between specific dsDNA sequences in the same molecule, or in different molecules, in which there is neither synthesis nor degradation of DNA. This is an ATP independent process and is mediated by a site-specific RECOMBINASE. In this case, the two sequences of dsDNA that are recognized by a given recombinase are not necessarily identical.

Site-specific recombination

Insertion of bacteriophage lambda DNA into the E.coli host chromosome. The specific sites (attP, not being identical to the bacterial site attB) recognised by the integrase are DNA sequences shown as coloured squares.

Transpositional Recombination The movement of a transposable genetic element within a chromosome. The movement occurs without displacement of the DNA sequence from its original site. Recombination enzymes act to move the DNA sequences between the ends into and out of chromosomes. It seems likely that they have been responsible for many evolutionary important changes in genomes