Genetic recombination mechanism

Department of Biochemistry, Nobel College, Nepal

Sunday, June 12,

2016Rajesh Chaudhary

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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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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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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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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.

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.

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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Recombination during meiosis occurs

with double stranded break

Rajesh Chaudhary

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DNA homologous recombination

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DNA homologous chromosome

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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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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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Nuclease and helicase activity of

RecBCD enzyme

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2016

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DNA strand invasion catalyzed by RecA

protein

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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.

Model for Rec A-mediated DNA strand

exchange

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Model for recombinational DNA repair of

stalled replication fork

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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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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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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

Branch migration

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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.

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.

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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DNA hybridization

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DNA hybridization

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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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Science

Genetic recombination mechanism