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RECOMBINATION
Mary Theresa
MSc. Microbiology
• Two DNA molecules exchange genetic information, resulting in
the production of a new combination of alleles.
• This reco...
• Recombination can occur both during mitosis and meiosis
• During meiosis in eukaryotes, genetic recombination involves
t...
Cross-overs during meiosis I
Paternal
Maternal
Zygotene: Homologous chromosomes,
each with 2 sister chromatids, pair to
fo...
HOMOLOGOUS
RECOMBINATION
• It is a physical phenomenon where exchange of
sequence occur with no net gain or loss of nucleo...
• Illegitimate or nonhomologous recombination: occurs
in regions where no large scale sequence similarity is
apparent
• Si...
• RECIPROCAL RECOMBINATION: Equal exchange of
genetic information
• The process resulting in new DNA molecules that
carry ...
• GENE CONVERSION / NON-RECIPROCAL
RECOMBINATION: one way transfer of genetic
information, resulting in an allele of a gen...
Models For Homologous
Recombination
THE CLASSICAL ANALYSIS OF
RECOMBINATION
• In each chromosome the genes are arranged in
linear series, and corresponding gr...
The Holliday Model
• In 1964, Robin Holliday proposed a model that
accounted for heteroduplex formation and gene
conversio...
Resolution of joint molecules
• Can occur in one of two ways
• The Holliday junction can be nicked in the
same strands tha...
The steps in the Holliday Model illustrated are
(1) Two homologous chromosomes, each composed of duplex
DNA, are paired wi...
Limitation of Holliday Model
•Although the original Holliday model accounted for many important aspects of
recombination (...
MESELSON & RADDING MODEL
The Double-Strand Break Repair Model
• Given by Jack Szostak and colleagues in 1983.
• New features in this model (contras...
DSBs probably most severe form of DNA
damage, can cause loss of genes or even
cell death (apoptosis)
DSBs caused by:
- ion...
Enzymes involved are
• DNA ligase
• Rec A
• Ruv A
• Ruv
Rec BCD
• A Complex Enzyme complex
with endonuclease and helicase
activity.
MAIN FUNCTIONS
• Essential for 99% of recombin...
RecA
• 38 kDa protein
• Catalyzes strand exchange, also an ATPase
• Also binds DS DNA, but not as strongly as
SS
• Catalys...
• DNA recombination studies used to map
genes on chromosomes. recombination
frequency proportional to distance between
gen...
REFERENCES
• Modern Genetic Analysis- Antony J.F.Griffths,
William.M.Gelbart & Jefrey.H.Miler
• An Introduction to Genetic...
Recombination
Recombination
Recombination
Recombination
Recombination
Recombination
Recombination
Recombination
Recombination
Recombination
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Recombination

recombination
types of recombination
models of recombination
biological roles of recombination

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Recombination

  1. 1. RECOMBINATION Mary Theresa MSc. Microbiology
  2. 2. • Two DNA molecules exchange genetic information, resulting in the production of a new combination of alleles. • This recombination process creates genetic diversity at the level of genes that reflects differences in the DNA sequences of different organisms. • In eukaryotic cells, which are cells with a nucleus and organelles, recombination typically occurs during meiosis. During the alignment, the arms of the chromosomes can overlap and temporarily fuse, causing a crossover. • Crossovers result in recombination and the exchange of genetic material between the maternal and paternal chromosomes. • As a result, offspring can have different combinations of genes than their parents. RECOMBINATION
  3. 3. • Recombination can occur both during mitosis and meiosis • During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes. This may be followed by information exchange between the chromosomes. • Recombination may also occur during mitosis in eukaryotes where it ordinarily involves the two sister chromosomes formed after chromosomal replication.
  4. 4. Cross-overs during meiosis I Paternal Maternal Zygotene: Homologous chromosomes, each with 2 sister chromatids, pair to form bivalents (line=duplex DNA) Pachytene: Cross-overs between homologous chromosomes Diplotene: homologous chromosomes separate partially but are held together at cross-overs Metaphase I Anaphase I Anaphase I: Cross-overs resolve to allow homologous chromosomes to separate into separate cells Meiosis II
  5. 5. HOMOLOGOUS RECOMBINATION • It is a physical phenomenon where exchange of sequence occur with no net gain or loss of nucleotides • It is based on sequence complementarity. • Occurs between sequences that are nearly identical (e.g., during meiosis) • Homologous recombination is extensively studied in E.coli. • At least 25 proteins are involved in recombination in E.coli.
  6. 6. • Illegitimate or nonhomologous recombination: occurs in regions where no large scale sequence similarity is apparent • Site-specific recombination: occurs between particular short sequences (about 12 to 24 bp) present on otherwise dissimilar parental molecules. Good examples are integration of some bacteriophage, such as λ, into a bacterial chromosome and the rearrangement of immunoglobulin genes in vertebrate animals. Integration of bacterial, viral or plasmid DNA takes place. • Replicative recombination: which generates a new copy of a segment of DNA. Many transposable elements use the process of replicative recombination
  7. 7. • RECIPROCAL RECOMBINATION: Equal exchange of genetic information • The process resulting in new DNA molecules that carry genetic information derived from both parental DNA molecule • The number of alleles of each gene remains the same in this products of recombination, only their arrangement has changed.
  8. 8. • GENE CONVERSION / NON-RECIPROCAL RECOMBINATION: one way transfer of genetic information, resulting in an allele of a gene on one chromosome being changed to the allele on the homologous chromosome. • The number of types of alleles has changed in the products of this recombination. • transfer of genetic material from a ‘donor’ sequence to a highly homologous ‘acceptor’ sequence
  9. 9. Models For Homologous Recombination
  10. 10. THE CLASSICAL ANALYSIS OF RECOMBINATION • In each chromosome the genes are arranged in linear series, and corresponding groups of genes are exchanged in crossing over. • Exchanges are complementary and involve the physical exchange of material. • Each exchange event involves only two chromatids, one from each chromosome. • Meiotic recombination does not occur between sister chromatids; or if it does, it does not interfere with recombination between homologs.
  11. 11. The Holliday Model • In 1964, Robin Holliday proposed a model that accounted for heteroduplex formation and gene conversion during recombination. • Single strand invasion: • Endonuclease nicks at corresponding regions of the same strands of homologous chromosomes • Ends generated by the nicks invade the other, homologous duplex • Ligase seals nicks to form a joint molecule. • (“Holliday intermediate” or “Chi structure”) • Branch migration expands heteroduplex region.
  12. 12. Resolution of joint molecules • Can occur in one of two ways • The Holliday junction can be nicked in the same strands that were initially nicked = “horizontal resolution.” This results in NO recombination of flanking markers. • The Holliday junction can be nicked in the strands that were not initially nicked = “vertical resolution.” This results in RECOMBINATION of flanking markers
  13. 13. The steps in the Holliday Model illustrated are (1) Two homologous chromosomes, each composed of duplex DNA, are paired with similar sequences adjacent to each other. (2) An endonuclease nicks at corresponding regions of homologous strands of the paired duplexes. (3) The nicked ends dissociate from their complementary strands and each single strand invades the other duplex. This occurs in a reciprocal manner to produce a heteroduplex region derived from one strand from each parental duplex. (4) DNA ligase seals the nicks. The result is a stable joint molecule, in which one strand of each parental duplex crosses over into the other duplex. This X-shaped joint is called a Holliday intermediate or Chi structure. (5) Branch migration then expands the region of heteroduplex. The stable joint can move along the paired duplexes, feeding in more of each invading strand and extending the region of heteroduplex. (6) The recombination intermediate is then resolved by nicking a strand in each duplex and ligation.
  14. 14. Limitation of Holliday Model •Although the original Holliday model accounted for many important aspects of recombination (all that were known at the time), some additional information requires changes to the model. •For instance, the Holliday model treats both duplexes equally; both are the invader and the target of the strand invasion. Also, no new DNA synthesis is required in the Holliday model. •However, subsequent work showed that one of the duplex molecules is the used preferentially as the donor of genetic information. Hence additional models, such as one from Meselson and Radding, incorporated new DNA synthesis at the site of the nick to make and degradation of a strand of the other duplex to generate asymmetry into the two duplexes, with one the donor the other the recipient of DNA. •These ideas and others have been incorporated into a new model of recombination involving double strand breaks in the DNAs
  15. 15. MESELSON & RADDING MODEL
  16. 16. The Double-Strand Break Repair Model • Given by Jack Szostak and colleagues in 1983. • New features in this model (contrasting with the Holliday model) are initiation at double-strand breaks, nuclease digestion of the aggressor duplex, new synthesis and gap repair.
  17. 17. DSBs probably most severe form of DNA damage, can cause loss of genes or even cell death (apoptosis) DSBs caused by: - ionizing radiation - certain chemicals - some enzymes (topoisomerases, endonucleases) - torsional stress
  18. 18. Enzymes involved are • DNA ligase • Rec A • Ruv A • Ruv
  19. 19. Rec BCD • A Complex Enzyme complex with endonuclease and helicase activity. MAIN FUNCTIONS • Essential for 99% of recombination events occurring at double- stranded breaks in bacteria. • Binds double stranded break • Unwinds and degrades DNA
  20. 20. RecA • 38 kDa protein • Catalyzes strand exchange, also an ATPase • Also binds DS DNA, but not as strongly as SS • Catalyses in strand transfer • Eukaryotes have multiple homologs of RecA • RecA can generate Holliday junction • By its strand transfer &displacement reactions.
  21. 21. • DNA recombination studies used to map genes on chromosomes. recombination frequency proportional to distance between genes • Making transgenic cells and organisms
  22. 22. REFERENCES • Modern Genetic Analysis- Antony J.F.Griffths, William.M.Gelbart & Jefrey.H.Miler • An Introduction to Genetic Analysis- David.T.Suzuki & Antony.J.F.Griffths • Genetics- Benjamin Pierce • Essential Cell Biology- Hopkins & Johnson • Advanced Genetic Analysis- R.Scott Hawley & Michelle.Y.Walker

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