The document discusses DNA recombination, describing its historical context and the three main types: homologous recombination, site-specific recombination, and transposition. It outlines the processes involved in each type, including the specific mechanisms and outcomes of homologous recombination and site-specific recombination, as well as the nature of transposition. Additionally, it touches on the implications of transposition in relation to mutations and the effects on genes.

1. DNA RECOMBINATION AND
ITS TYPES:
Homologous, Site-Specific,
and Transposition

2. introduction
• Recombination: originally used by geneticists to
describe the outcome of crossing over between
two homologous chromosomes during meiosis.
Skipping effects on daughter chromosomes with
a different combination of alleles compared to
their parent chromosomes. By the 1960s, it was
understood that the main component of cell
regeneration was the subsequent breakdown and
subsequent reunification of DNA molecules. Now,
recombination refers to a variety of processes
that involve the separation and synthesis of
polynucleotides.

3. Types
Three different types of recombination events:
• Homologous recombination
• Site-specific recombination
• Transposition

4. Steps of Homologous Recombination
• Two homologous double helices are aligned
• Then a nuclease cleaves the two strands that have the
same polarity
• The free ends leave their original complementary
strands and undergo hydrogen bonding with the
complementary strands in the homologous double
helix
• Ligation produces a heteroduplex double helix a crucial
intermediate in recombination termed the Holliday
structure.
• The Holliday structure can be resolved by cutting and
ligating either

6. Site-specific recombination
• It is a recombination between two DNA
molecules that have very short sequences of
similarities.
• The two DNA partner molecules are rearranged
by specific enzymes i.e. a site-
specific recombinase, that acts alone or requires
accessory factors to shape the DNA target
performing recombination at their similar pairs of
sequences or target sites.
• The outcome of the exchange may be excision,
integration or inversion of DNA sequences.

8. Transposition
• Transposition means the removal and transfer of a
portion of DNA from one site to another of the same
or different chromosome.
• The transferred part is accompanied by a short direct
repetition performed during the conversion process.
• Modification causes anti-retroviral drugs to fight germs
and to transmit viruses to certain germs. • The cell
portion of DNA associated with mutation is called a
transposon or jumping gene.
• Not a form of reunification but a process that uses
renaming.

9. Types of mutations based on their
mutation mechanism:
• DNA transposons that transpose
replicatively: the original transposon
remaining in place and a new copy appearing
elsewhere in the genome
• DNA transposons that transpose
conservatively: the original transposon
moving to a new site by a cut-and-paste
process