1. CONTENTS Genetic recombination Homologous/ Legitimate recombination
Introduction to recombination mechanism
Key steps of Homologous recombination
Holiday model of recombination
Enzymes involved in Homologous Recombination
Recombination between homologous chromosomes
Illegitimate Recombination
Possible mechanisms of Illegitimate recombination
Slipped mispairing
Strand-slippage and Joining
Advantages of Recombination
Disadvantages of Recombination
Biological roles of Recombination
References

2. GENETIC RECOMBINATION
Recombination is the process in which one or more nucleic acid molecules are rearranged or combined to produce a new nucleotide sequence.
Genetic material from two parents is combined to produce a recombinant chromosome with a new, different genotype.
Recombination results in a new arrangement of genes or parts of genes and normally is accompanied by a phenotypic change.
Recombination occurs between precisely corresponding sequences, so that a single base pair is added to or lost from the recombinant chromosomes.

3. GENETIC RECOMBINATION

4. HOMOLOGOUS(LEGITIMATE) RECOMBINATION AND NONHOMOLOGOUS(ILLEGITIMATE) RECOMBINATION

5. Homologous/ Legitimate recombination
Recombination involving reaction between homologous sequences of DNA is called Generalized or Homologous or Legitimate recombination.
Homologous recombination is an essential cellular process catalyzed by enzymes synthesized and regulated for this purpose.
It is a reaction between two duplexes of DNA.
Its critical feature is that the enzymes responsible can use any pair of homologous sequences as substrates.

6. Introduction to recombination mechanism
At least 4 types of naturally occurring recombination have been identified in living organisms.
General or Homologous recombination: occurs between DNA molecules of very similar sequence, such as homologous chromosomes.
Illegitimate or Non-homologous recombination: occurs in regions where no large sequence similarity is apparent. E.g. translocations between different chromosomes
Site-specific recombination: occurs between particular short sequences. It requires a special enzymatic machinery for each particular site.
Transposition: it may be considered as a type of site specific recombination. DNA element moves from one site to another, usually little sequence similarity is involved.

7. The following characteristics are probably common to homologous recombination in all cells
The DNA molecules can then “cross over” in a complex reaction, both strands of each double helix are broken and the broken ends are rejoined the ends of the opposite DNA molecules to re-from two intact double helices.

8. The site of exchange can occur anywhere. in. the
The site of exchange can occur anywhere in the homologous nucleotide sequences of the two participating DNA molecules.
No nucleotide sequences are altered at the site of exchange; the cleavage and rejoining events occur so precisely that not a single nucleotide is lost or gained.

9. Key steps of Homologous recombination
Strand pairing
Strand breakage
Strand invasion
Branch migration
Resolution

10. Strand pairing: Alignment of 2 homologous DNA molecules.
Strand breakage: Introduction of breaks in the DNA.
Strand invasion: Formation of initial short regions base pairing between the two recombining DNA molecules.
Branch migration: Movement of the Holliday junction.
Resolution: Cleavage of Holliday junction.

11. Enzymes involved in Homologous Recombination
RecA protein
RecBCD endonuclease (recB, recC, recD)
3 proteins in Holiday junction:
RuvA
RuvB
RuvC

12. HOLLIDAY MODEL OF RECOMBINATION

13. Holliday Junction

14. RECOMBINATION BETWEEN HOMOLOGOUS CHROMOSOMES
This type recombination produce a new combination alleles in the resulting chromosome, which refers to the shuffling of genetic material to create a genetic combination that differs from the original.

15. SISTER CHROMATID EXCHANGE
Crossing over occur between replicated chromosomes in a diploid species.
When crossing over takes place between sister chromatids, the process is called sister chromatid exchange(SCE).
SCE does not produce a new combination of alleles.

16. Sister chromatid exchange

17. Nonhomologous (Illegitimate )Recombination
Illegitimate recombination refers to any DNA rearrangement that leads to the covalent joining of non-homologous linear DNA segments.
Illegitimate recombination occurs at DNA sequences that share little or no homology.
Absence of homology is one of the hallmarks of illegitimate recombination.
An illegitimate recombination leads to the formation of novel DNA joints.
Illegitimate recombination occurs in most of the Gram positive bacteria. E.g., Kulyveromyces lactis
Micro-homology in the range of 3-25 bp can play a role to direct the reassociation of DNA ends

18. Possible mechanisms of Illegitimate recombination
Franklin (1971) proposed two fundamentally different mechanisms for illegitimate recombination.
Slipped mispairing
Cutting and rejoining

19. Slipped mispairing
Slipped mispairing involves errors in DNA replication.
The polymerase either fails to copy or repeatedly copies a given sequence to yield a deletion or tandem duplication respectively.
This model equates deletions and duplications with very large – and + frameshift mutations.
Does not involve cutting and rejoining of DNA molecules.

21. Strand-slippage and Joining
The second mechanism of illegitimate recombination involves the strand-slippage rejoining of DNA molecules.
Enzymes that catalyze DNA excission and ligation are required by this model.
The enzymes such as DNA repair enzymes, DNA gyrase and DNA ligase are involved

22. DNA ends are juxtaposed, paired at micro-homology if present, gaps filed in and nicks ligated.
This reaction does not require homology between the two DNA molecules.
The broken ends rejoin to restore the parental DNA.
In the presence of a DNA fragments this may be joined via its ends to the broken ends of the chromosomes.

23. Strand slippage can occurs during DNA replication if the newly synthesized strand dissociates from its reassociates in a new position
It is stimulated by the formation of an intra-strand secondary structure. Example hairpin DNA

24. HAIRPIN DNA
It is difficult to replicate and it brings into close proximity the newly replicated strand and its target for reassociation.
Strand-slippage events occur o the lagging-strand of the DNA replication fork.
Strand-slippage reactions are subject to mismatch repair.

27. Advantages of Recombination
It makes new combinations of alleles along chromosomes.
It restricts the effects of mutations largely to the region around a gene, not the whole chromosome
It is essential for removing deleterious mutations from the genome. If recombination did not occur, then one deleterious mutant allele would cause an entire chromosome to be eliminated from the population.
Allows beneficial mutations to be incorporated.

28. With recombination, the mutant allele can be separated from the other genes on that chromosome.
Genetic recombination shuffles together the genetic material carried by different members of a sexual species.
Recombination allows favorable mutants that arise in separate individuals to be combined within the same genome.
Genetic variation and number of mutant alleles can be increased

29. Disadvantages of Recombination
Recombination separates advantageous gene combinations.
It breaks down the linkage disequilibrium.
It separates favorable gene combinations.
All mutations could potentially be incorporated.

30. Applications of Recombination
In medical Biotechnology and Medical sector.
Used to map genes on chromosomes.
Making transgenic cells and organisms.
In agriculture.
Recombination allows us to produce large amounts of medically important proteins including
insulin (diabetes),
blood clotting factors (hemophilia),
growth hormone (dwarfism),
tissue plasminogen activator (treating blood clots).

31. Biological roles of Recombination
Generating new gene/allele combinations (crossing over during meiosis).
Generating new genes (e.g., Immunoglobulin rearrangement).
Integration of a specific DNA element ( or viruses).
DNA repair.

32. REFERENCES
Lewin’s Genes X (International Edition) by, Jocelyn E. Krebs (University of Alaska Ancharge), Elliott S. Goldstein( Arizona State University),Stephen T. Kilpatrick ( University of Pittsburghat Johnstown) P.No
Lewin’s Genes XI (Student edition) by, Jones and Bartlett P.No
Genetic analysis and principle(Third edition) By, R0bert J.Brooker. P.No
A Text book of Basic and Molecular GENETICS by, Pradeep Parihar
The Gene- Purohithar P.No
Essential cell biology(second edition) Bruce.alberts. Dennis Byr, Karen Hopkin Ale sander Johnson , Julian Lewis Martin Raff, Keith Roberts Peter Walter P.No

33. QUESTIONS ?

34. Thank you…