Genetic recombination involves the exchange of genetic material between chromosomes or DNA molecules. It occurs through two main types - homologous recombination, which exchanges DNA between similar sequences, and non-homologous recombination between dissimilar sequences. Recombination is important for genetic diversity, DNA repair, and proper chromosome segregation during cell division. It can happen during both mitosis and meiosis, but only meiotic recombination shuffles genes from parents to offspring. There are also different mechanisms of recombination, including site-specific, transposition, and various DNA repair pathways that facilitate genetic exchange.

2. Genetic Recombination
 Genetic recombination is the reassortment of genes or nucleotide sequences to
produce non parental gene combinations or new linkage relationship of genes.
 It plays a central role in the evolution of gene diversification.
 Genetic recombination requires the intimate association of the whole or a part of
the genetic material of two or more different genotypes.

3. Why do chromosomes undergo
recombination?
• Specialized DNA repair systems,
• Specialized activities in DNA replication,
• Regulation of expression of certain genes,
• Facilitation of proper chromosome segregation during eukaryotic cell
division,
• Maintenance of genetic diversity,
• Implementation of programmed genetic rearrangements during
embryonic development.

4. Mitotic and meiotic recombination
Recombination can occur both during mitosis and meiosis
Only meiotic recombination serves the important role of reassorting genes
Mitotic recombination may be important for repair of mutations in one of
a pair of sister chromatids

5. Genetic
Recombination
Interchromosomal
recombination
Intrachromosomal
recombination
Intergenic Intragenic

6.  Genetic recombination
1.Intrachromosomal recombination
2.Interchromosomal recombination
 Intrachromosomal :
take place within one DNA molecule to produce deletions,duplications,inversions
& transpositions.
 Interchromosomal :
occurs between two parental DNA molcules,linkage groups or chromosomes to
produce a recombinant DNA or recombinant chromosome.
# is restricted to Eukaryotes ---- mitotic & meiotic cross over

7. Intrachromosomal intergenic recombination
# Occurs at different loci on the same DNA or chromosome.
# It common among Viruses, Prokaryotes & Eukaryotes.
# It occurs by mitotic & meiotic crossing over , both these mechanisms are
reciprocal.
1.Intergenic 2.Intragenic
Intrachromosomal
recombination

8.  Intrachromosomal intragenic recombination
# It occurs between the mutational sites of genes & its rare phenomenon.
# It may take place at many sites called Recons.
# It may be reciprocal and non-reciprocal

9. Based on the genetic mechanism
1. Homologous / generalized recombination
having perfect sequence similarities
2. Non-homologous recombination
3. Site-specific recombination
4. Transposition

11.  Homologous recombination : is a type of genetic recombination in which
nucleotide sequences are exchanged between two similar or identical molecules of
DNA.
 Recombination can occur between similar molecules of DNA, as in homologous
recombination, or dissimilar molecules, as in non-homologous end joining
(NHEJ).

12. Recombination between homologous DNA
sites
Homologous recombination allows for the exchange of blocks of genes
between homologous chromosomes and thereby is a mechanism for
generating genetic diversity
Recombination occurs randomly between two homologous sequences and
the frequency of recombination between two sites is proportional to the
distance between the sites

16.  Non homologous recombination: When the fragments of DNA are exchanged
between two non homologous chromosomes ( dissimilar molecules ).
 Translocations between different chromosomes or deletions that remove several
genes along a chromosome. However, when the DNA sequence at the breakpoints
for these events is analyzed, short regions of sequence similarity are found in
some cases. For instance, recombination between two similar genes that are
several million bp apart can lead to deletion of the intervening genes in somatic
cells.

18.  Eg: Translocation mutation
# Chronic Myelogenous leukaemia ( CML ) : Translocation of long arm of
chromosome 22 usually on chromosome 9.
Translocation of chromosome generate Philadelphia chromosome is
Chromosome 22 & 9

20. Recombination repair DNA breaks
Homologous recombination
Resection
Rad50, Mre11,
Xrs2 complex
Strand invasion
Rad52
Rad51;
BRCA2
DNA synthesis
  
Ligation, branch migration,
Holliday junction resolution
DSB
Non-homologous end-joining
Ku70, Ku80
Ligation
DSB
DNA-PKcs
Rad50, Mre11,
Xrs2 complex “Cleaning up” of
ends
XRCC4/Li
gase IV

22.  Specialized recombination called Site-specific recombination, first
characterized in prokaryotes SSR is responsible for integration of phage
genomes into the bacterial chromosomes.
 Enzymes that catalyze site specific recombination are generally called
Recombinase & more than 100 of them are known.
 Phage integration or related to these enzymes are also known as the
Integrase family
 Classical model of site specific recombination is illustrated by Phage
lambda…
in lysogenic cycle-------- Prophage formation

23. SSR can generate three different types of DNA rearrangements:
1.Insertion
is the addition of one or more nucleotide base pairs into a DNA sequence
2.Deletion
(also called deficiency) is a mutation in which a part of a chromosome or a
sequence of DNA is missing.
3.Inversion
is a chromosome rearrangement in which a segment of a chromosome is
reversed end to end, shortly reversing the orientation of a chromosomal segment.

26. Some genetic elements move to new chromosomal locations by
transposition
 Transposition is a specific form of genetic recombination that moves certain
genetic elements from one DNA site to another within the genome.
 These mobile genetic elements are called transposable elements or
transposons or jumping gene or mobile gene.

28. Types of Transposition
 Cut and pasteTransposition(Conservative)
 Replicative transposition
 Retrotransposition

29. Cut-and-Paste transposition (conservative)
In cut-and-paste transposition, an element is cut out of one
site in a chromosome and pasted into a new site.
• The movement is double stranded piece of DNA.

30. Replicative transposition
In replicative transposition, an element is replicated, and one
copy is inserted at a new site; one copy also remains at the original
site.
• Single Strand is moved

31. Retrotransposition
In retrotransposition, the element makes an RNA copy of itself
which is reversed-transcribed into a DNA copy which is then
inserted (cDNA)

32. Types of recombination
A+ B+ C+
A- B- C-
A+
B+ C+A-
B- C-
Homologous
or general
A B C
D E F
A B
CD E
F
Site-specific
att
att l
att att
l integrase
Replicative
recombination,
transposition
A B C A B C
transposase
l
E. coli
Non homologous
recombination

33. Refferences
 Lewin’s Genes X1
Jocelyn E Krebs
Elliotts Goldstein
Stephent T Kilpatrick
 Cell and molecular biology
P K Gupta
 Cell and molecular biology
Gerald Karp
 Lewin’s essential
 Cell & molecular biology,Genetics & human genetics
K K Bhaskaran