GENETIC RECOMBINATION.pptx

Genetic recombination (genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. The process occurs naturally and can also be carried out in the lab.

1. DR. RACHANA CHOUDHARY DEPT. OF MICROBIOLOGY SHRI SHANKARACHARYA MAHAVIDYALAYA JUNWANI BHILAI

2. SYNOPSIS GENETIC RECOMBINATION 1.Introduction 2.Biological role of recombination . 3.Mechanism of recombination. 4.Types of recombination 5.Referenc e

3. INTRODUCTION The most important features of organism ,are to adapt in the environment and to maintain their DNA sequence in the cell generation to generation with very little alternation. In long term survival of organism depend to genetic variation, a key feature through which the organism can adapt to an environment which changes with time .

4. Biological Roles for Recombination Generating new gene/allele combinations (crossing over during meiosis) Generating new genes (e.g., Immuno - globulin rearrangement) Integration of a specific DNA element DNA repair

5. MECHANISM OF RECOMBINATION i )Breakage and reunion Two homologous duplex of chromosomes laying in paired form break between the gene loci a and b, and a+ and b+. The broken segments rejoin crosswise . and yield recombinant containing a and b+ segment , and b segment. This type of recombinant does not require the synthesis of new DNA.

6. ii)Breakage and copying One helix of paired homologous chromosomes . Segment b is replaced by a newly synthesize segment copied from b+ segment and attached to a section . thus the recombination contain ab + and a+ b+.

7. iii)Complete copy choice In 1931, belling proposed this theory for recombination of chromosomes in higher animals. However , it has been questioned by several workers. Therefore, it has only historical importance . The process of copying shift to the other parental strand and some of the other strands .

8. Types of Recombination General recombination Non reciprocal recombination Site specific recombination

9. TYPE OF RECOMBINATION i ) General recombination General recombination occurs only between the complementary strands of two homologous DNA molecule. Smith (1989) reviewed the homologous recombination in prokaryotes. General recombination in E coli is guided by base pairing interactions between the complementary strands of two homologous DNA molecule.

10. . A) Holliday model for general recombination Holliday presented a model to show the general recombination. According to this model recombination occur in five step such as strand breakage, strand pairing, strand invasion/assimilation, chiasma formation, breakage and reunion and mis -match repair

11. a)Strand breakage Genetic recombination occur through crossing over by pairing between the complementary single strands of DNA duplex (a). Two homologous regions of DNA double helix undergo an exchange reaction. However, it is unknown how the homologous regions of DNA recognizes each other .

12. The Rec BCD protein act as DNA helicase because these hydrolyse ATP and travel along DNA helix. Thus, the Rec BCD protein result in formation of single stranded whisker at the recognition site which is displaced from the helix (c). This initiates a base pairing interaction between the two complementary sequence of DNA double helix . b)Strand pairing

13. Holliday Model R. Holliday (1964) Holliday Junctions form during recombination HJs can be resolved 2 ways, only one produces true recombinant molecules patch

14. EM of a Holliday Junction w/a few melted base pairs around junction

15. (c)Strand invasion/assimilation A single strand generated from one DNA double helix invades the another double helix (d). In E.coli rec A gene produces Rec protein which is important for recombination between the chromosomes. . Rec A proteins promotes rapid renaturation of complementary aaDNA hydrolysing ATP in the process .

16. (d) Branch migration The next step is the assimilation of strand and nick ligation(e). The donor strand gradually displaces the recipient strand which is called branch migration. Branch migration can take place at any point where two single strands with the sequence make attempt to pair with the same complementary strands. .

17. (e) Chiasma or crossing over formation Exchange of a single strand between two double helices in a different step in a general recombination even. at this stage possibly different pathway. However, in most of the cases an important structure called cross strand exchange ( also called holliday juncture or chi form or chiasmas , is formed by the two participating DNA helices (g).

18. Examples of Recombination

19. (f)Breakage and reunion The result in alteration of the original non crossing strand, and the crossing strand into the non crossing strand. In order to regenerate two separate DNA helices, breakage and reunion in two crossing strand are required. If breakage and reunion occur before isomerization the two crossing strand would not occur. Breakage and reunion occur either in the vertical and horizontal plane .

20. B) Mismatch repair theory for general recombination It is such a repair system which correct mismatch base pair of unpaired regions after recombination. This system recognizes mismatched function of DNA polymerase. The mechanism involve the excision of one of the other mismatch bases along with about 3000 nucleotide .

21. Nonreciprocal recombination

22. ( ii) Non reciprocal recombination The fundamental law of genetics is that the two partners contribute the equal amount of gene to the equal amount of gene to the offsprings . It mean that the offsprings inherit half complete set of gene from the male and half of female. In higher animal like man it is not possible to analyze these gene taking a single cell . .

23. Unlike general recombination, site specific recombination is guided by a recombination enzyme that recognizes specific nucleotide sequence present on one of both recombining DNA molecule. Site specific recombination alters the relatives position of nucleotide sequence in chromosome. The base pairing reaction depend on protein mediated recognition of the two DNA sequence will combine. Very long homologous sequence is not required (v) Site specific recombination

24. REFRENCES 1.A TEXTBOOK OF MICROBIOLOGY R.C. DUBEY AND D.K. MAHESHWARI 2.BIOTECHNOLOGY B.D. SINGH 3.A TEXTBOOK OF MICROBIOLOGY R.P. SINGH

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