Sitespecific recombination Sitespecific recombination occurs at a specific

Site-specific recombination

• Site-specific recombination occurs at a specific DNA sequence. Site-specific recombination • The example was found in the integration between λ DNA and E. coli DNA. • Both contain a sequence, 5'-TTTATAC-3', called the attachment site, which allows the two DNA molecules to attach by base pairing.

• Once attached, the enzyme integrase catalyzes two single strand breaks as in the Holliday model. • After a short branch migration, the integrase exerts a second strand cuts on two other strands. • Resolution of two Holliday junctions completes the integration process.

Types of site-specific recombination Transpositional site-specific recombination Conservative site-specific recombination

Transpositional site-specific recombination It does not involve the formation of heteroduplex DNA between mobile DNA segment and its host, so a short homologous sequence is not required Modest target site selectivity and insert mobile DNA into many sites

Conservative sitespecific recombination • It requires the formation of heteroduplex, so a short homologous sequence is required

Consequence of site-specific recombination • Insertions: If the recombination occurs on sites that are situated on two different DNA molecules (intermolecular recombination) • Excisions: If the recombination takes place between two sites that are found on the same molecule in the same orientation • Inversions: Occur if the recombination takes place between two sites on the same molecule in which sequences are in opposite orientation (inverted repeat)

Transposition/replicative Recombination

Transposition Recombination • Transposition recombination is a process in which a mobile element is inserted into a target DNA. • It may occur by one of two mechanisms: ü Directly as DNA ü Through RNA • The mobile elements that transpose through DNA are called transposons and those via RNA are referred to as retrotransposons.

Classification of Mobile Elements

Mechanism of transposition recombination

Mechanism of Transposition Recombination

Transposons Retrotransposons • The transposon in donor DNA is cut by a special enzyme and then inserted into a target DNA. In bacteria, this enzyme is called transposase which has both nuclease and ligase activities. • The retrotransposon in donor DNA is first transcribed into RNA and then reversetranscribed into DNA, which is inserted into a target DNA by the same recombination mechanism as the DNA intermediates of transposons.

Mechanism of Transposition Recombination

The transposition may be carried by two ways

Cut and Paste Transposition

Copy and Paste Transposition

Copy and Paste Transposition

Different Transposons Predominate in different Organisms For example, most bacterial transposons are DNA types, with a few related to the nonretroviral retrotransposons also present In Drosophila, DNAbased, retroviral, and nonretroviral transposons are all found In yeast, the main mobile elements that have been observed are retrovirallike retrotransposons The human genome contains all three types of transposon

Movies • http: //www. courses. fas. harvard. edu/~biotext/animations/ General. Recombination. html • http: //www. blackwellpublishing. com/trun/artwork/Anima tions/Recombination/recombination. html

Books Recommended Lewin Watson Weaver Bruce Albert Peter Palolella Gene X Molecular Biology of the Gene Molecular Biology of the Cell Introduction to Molecular Biology