PowerPoint Presentation On Genetic Recombination

PPT On Genetic Recombination

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Genetic Recombination Presentation Transcript:

1. Genetic Recombination
INTRODUCTION : Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. In eukaryotes recombination commonly occurs during meiosis as chromosomal crossover between paired chromosomes. This process leads to offspring having different combinations of genes from their parents and can produce new chimeric alleles. In evolutionary biology this shuffling of genes is thought to have many advantages

2. A recombination pathway in DNA is any way by which a broken DNA molecule is reconnected to form a whole DNA strand. In molecular biology "recombination" can also refer to artificial and deliberate recombination of disparate pieces of DNA, often from different organisms, creating what is called recombinant DNA.

3. SISTER CHROMATID EXCHANGE
When crossing over occurs between sister chromatids its called sister chromatid exchange . sister chromatid are genetically identical to each other, wit,sce does not produce a recombination of alleles. Therefore it is not considered a form of recombination .By comparison , it is also common for homologous chromosomes to cross over.

4. Perry and wolf produced harlequin chromosomes to reveal recombination between sister chromatid. In 1970s, The Russian cytogeneticist A.F ZAKHROV AND COLLEAGUES spent much more effort developing methods that inproved our ability to identify chromoes.They mode the intresting observation that chromes labled with the nucleotide analogue 5- bromo deoxyurinide become more fluorescent when stained with giemsa and then visualized microscopically.

5. Perry and wolff, grew eukaryotic cells in a laboratory and exposed them to BrdU for two rounds of DNA replication . after the second round of DNA replication , one of the sister chromatid contained one normal strand and one BrdU labled strand. The other sister chronatid have two BrdU –labled strands . when treated with two dyes , Hoechst33258 and giemsa , the sister chromatid containing two strands with BrdU stains very weakly and appears light, whereas the sister chromatid with only one strand containing BrdU stains much more strongly and appears very dark. In this way , the two sister chromatid can be distinguished microscopically . chromosomes stained in this way have been regerred to as harlequin chromosomes , because they are reminiscent of a harlequin characters costumes with its variegated pattern of light and dark patches.

6. HOMOLOGOUS RECOMBINATION
Refers to recombination between the paired chromosomes inherited from each of one's parents, generally occurring during meiosis. During prophase I the four available chromatids are in tight formation with one another. While in this formation, homologous sites on two chromatids can mesh with one another, and may exchange genetic information. Because recombination can occur with small probability at any location along chromosome, the frequency of recombination between two locations depends on their distance. Chromosomes are expected to cross over at many points along their length; independently.

7. The Holliday Model of Genetic Recombination
This model of recombination was first proposed by Robin Holliday in 1964 and re-established by David Dressler and Huntington Potter in 1976 who demonstrated that the proposed physical intermediates existed. Alignment two homologous DNA molecules. Nick the DNA at the same place on the two molecules. This must happen in strands with the same polarity.

8. Exchange strands and ligate.
The intermediate that is formed is called a Holliday intermediate or Holliday structure. The shape of this intermediate in vivo is similar to that of the greek letter chi, hence this is also called a chi form.

9. Resolution of the structure.
There are two ways in which this can happen: If the same strands are cleaved a second time then the original two DNA molecules are generated: If the other strands are cleaved, then recombinant molecules are generated:

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