The document discusses genetic recombination, detailing its necessity for altering genomes to understand diseases. It categorizes recombination into three classes: homologous, site-specific, and DNA transposition, with a focus on the mechanisms and enzymes involved in homologous recombination during meiosis. The document also explains processes like gene conversion caused by mismatch repair and the significance of mobile genetic elements.

1. Department of Biochemistry, Nobel College, Nepal
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2016
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2. Genetic recombination
Rearrangement of genetic information within and among
DNA molecules.
Why is it done and why is it necessary?
To alter the genome to understand the various disease
conditions.
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3. General classes of genetic recombination
3 general classes
1. Homologous genetic recombination / general
recombination
2. Site-specific recombination
3. DNA transposition
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4. Homologous genetic
recombination
 Genetic exchange between any
two DNA molecules or segment of
same DNA molecule.
 Its main function both in
prokaryotes and eukaryotes is to
repair the stalled damaged
replication fork.
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5. Homologous genetic recombination
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The recombination occurs with the
highest frequency during meiosis –
the process by which diploid germ-
line cells with two sets of
chromosomes divide to produce
haploid gametes (sperm and ova) in
animals.

6. Crossing over
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Genetic information is
exchanged between
closely related
homologous chromatids
by homologous genetic
recombination, a
process involving
breakage and rejoining
of the DNA.

7. So, what is the role of homologous
recombination?
Serves 3 major functions:
1. It contributes to the repair of several types of DNA damage.
2. It provides, in eukaryotic cells, a transient physical link
between chromatids that promotes the orderly segregation of
chromosomes at the first meiotic cell division
3. It enhances genetic diversity in a population.
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8. Recombination during meiosis occurs
with double stranded break
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9. DNA homologous recombination
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10. DNA homologous chromosome
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11. Recombination requires host of enzymes
and other proteins
 Enzymes that catalyze various steps of the recombination process has been
identified and isolated from both E. Coli and Eukayortes.
 In E. Coli, RecB, RecC and RecD genes encode heterotrimeric RecBCD
enzyme which has both nuclease and helicase activity.
 Rec A protein promotes all central steps in the recombination process:
 1. Pairing of two DNA molecules.
 2. formation of Holliday intermediates.
 3. Branch migration
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12. Recombination requires host of enzymes and
other proteins
Ruv A and Ruv B proteins form a complex that binds to
Holliday intermediates, displaces RecA protein, and
promote branch migration at higher rates that does RecA.
Nucleases that often cleaves Holliday intermediates, often
called resolvases, has been isolated from bacteria and
yeast.
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13. Nuclease and helicase activity of
RecBCD enzyme
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14. DNA strand invasion catalyzed by RecA
protein
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15. Rec A promoted DNA
strand exchange in vitro
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RecX, DinX,
RecF, RecO,
and RecR
regulate
assembly and
disassembly
of RecA
filament.

16. Model for Rec A-mediated DNA strand
exchange
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17. Model for recombinational DNA repair of
stalled replication fork
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18. Model for recombinational DNA repair of
stalled replication fork
Necessary enzymes for single stranded repair
RecF, RecO, RecR proteins
Necessary enzymes for double stranded repair
RecBCD
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19. Site-specific genetic recombination
Second general type of recombination.
Recombination is limited to specific sequence.
Recombination of this type occurs in virtually every cells.
Each site-specific recombination involves:
Recombinase
A short (20-200 bp) unique DNA sequence where
recombinase act
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20. Site-specific DNA
recombination
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There are two general classes of site-
specific recombination system which relies
on either Tyr or Ser residues in active site.
Step 1.
Step 2.
Step 3.
Step 4.
Holiday junction or cross-strand exchange

21. Branch migration
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22. Simplified view of branch migration
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• Catalyzed by specialized
proteins that continuously
breaks and seals the
nucleotides.
• ATP is used as the energy
source for branch migration.
• In meiosis, heteroduplex region
migrates 1000s of nucleotids
from the point of start site.

23. Gene conversion caused by mismatch
correction
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Heteroduplex DNA is formed at the sites of homologous
recombination between maternal and paternal
chromosomes. If the maternal and paternal DNA sequences
are slightly different, the heteroduplex region will include
some mismatched base pairs, which may then be corrected
by the DNA mismatch repair machinery. Such repair can
“erase” nucleotide sequences on either the paternal or the
maternal strand.
The consequence of this mismatch repair is gene conversion,
detected as a deviation from the segregation of equal copies
of maternal and paternal
alleles that normally occurs in meiosis.

24. Transposition and conservative site-specific
recombination
 Site-specific recombination do not require substantial region of
sequence homology.
 Transposition and conservative site-specific recombination largely
dedicated to moving specialized segment of DNA known as “mobile
genetic elements”.
 Virtually all cells contain mobile genetic elements commonly known as
“jumping genes”.
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25. DNA hybridization
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26. DNA hybridization
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27. DNA hybridization
DNA hybridization is a random process where
hybridization occurs through hit-and-trial.
Hybridization depends on the random collision between
two homologous DNA strand complementary to each other.
Once helix nucleation is formed, then rapid zippering
leads to complete double helix.
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