CROSSING OVER PPT

CROSSING OVER AND ITS SIGNIFICANCE

1. UNIVERSITY OF ALLAHABAD TOPIC - CROSSING OVER AND ITS SIGNIFICANCE SUBJECT-CYTOGENETICS,MOLECULAR GENETICS AND PLANT BREEDING SUBMITED TO SUBMITED BY Dr. GIRJESH KUMAR DEEPAK KUMAR MAURYA M .SC 1 ST SEM. ( Ag.Botany ) ( Department of botany ) ENROLL. NO. U1875119 DEPARTMENT OF BOTANY

2. CROSSING OVER AND ITS SIGNIFICANCE

3. Crossing-over Crossing-over takes place during pachytene sub-stage of prophase I of meiosis, but visible at diplotene sub-stage . The term crossing over was coined by Morgan . It is another name for recombination or physical exchange of equal pieces of adjacent non-sister chromatids . During the process of crossing-over one of the paired chromosome arms exchanged physically at one or more locations . The two homologous chromosomes are connected at a certain point called chiasma . When crossing-over occurs chromatids break at chiasma and reattached to a different homologous chromosome . The chromatids resulting from the interchange of segments are known as the cross over recombinants and the chromatids that remain intact are called noncrossover parental chromatids .

4. Characteristics of Crossing over Occurs at two levels, at gross chromosomal level and at DNA level. Occurs between non-sister chromatids of homologous chromosomes. Exchange is normally reciprocal but sometimes unequal. Frequency of crossing over is closely related to physical distance between genes located on chromosomes Crossing over is a crucial process that generates genetic difference within a population .

5. The major steps in meiotic crossing over 1 ) Synapsis - T he pairing phenomenon of homologous chromosomes in presence of syneptonemal complex(DNA and Protein). 2 ) D uplication of chromosome - H omologous chromosome gets duplicated to form chromatid . 3 ) Crossing over – The exchange of chromosomal segments between two non-sister chromatids 4 ) T erminalisation - S epration of non homologous chromosomes after crossing over

6. Mechanism of crossing over Crossing over, leading to recombination of linked genes, is due to interchange of sections of homologous chromosomes. At meiosis, the homologous chromosomes come together and pair, or synapse, during prophase . The pairing is remarkably precise and is evidently brought about by mutual attraction of the parts of the chromosomes that are similar or homologous because they contain allellic genes .

7. The enzymes involved in crossing over 1 ) Recombinase is the major enzyme regulating recombination event 2) Endonuclease 3) Ligase enzyme Endonuclease is responsible for breakage of 2 non-sister chromatids at corresponding sites. This is followed by the exchange of segments and finally the exchanged segments are joined or the gap is filled by ligase enzyme.

8. Types of crossing over 1. Somatic crossing over- Pairing of homologous chromosomes occurs in germinal cells but some times in somatic cells. Somatic crossing over is reported in Drosophila by Curt Stern (1935). somatic crossing over occurs rarely. 2.Meiotic crossing over or germinal crossing over - This type of crossing over takes place in germinal cells during gametogenesis . Kinds of Crossing Over Single crossing over (only one chiasmata is form) Double crossing over (two chiasmata are form) Multiple crossing over . (more than two chiasmata are formed)

9. Factors Affecting the Crossing Over 1. Distance between the genes : The frequency of crossing over between the two genes is positively associated with the distance between their location in the chromosome. Crossing over between the two genes would increase with an increase in distance between them. 2. sex: The frequency of recombination is markedly influenced by the sex of heterozygotes for linked genes. In general, the heterogametic sex shows relatively lower recombination frequencies than the homogametic sex of the same species. Eg : No crossing over occurs between linked genes in Drosophila males and females of silkworm. 3. Age of female : The frequency of crossing over shows a progressive decline with the advancing age of Drosophila females. 4. Temperature : In Drosophila, the lowest frequency of crossing over is observed when females are cultured at 220C. The frequency of recombination tends to increase both at the lower and higher temperatures than 220C. 5. Nutrition : The frequency of crossing over in Drosophila is affected by the presence of metallic ions Eg : Ca+2 and Mg+2 in its food. Higher the amount, lower will be the crossing over frequency and vice-versa .

10. 6. Chemicals : Treatment of Drosophila females with certain antibiotics like mitomycin D and actinomycin D and certain alkylating agents such as ethylmethane sulphonate promotes crossing over. 7. Radiations : An increase in frequency of crossing over is observed when Drosophila females are irradiated with x-rays and γ -rays . 8 . Plasmagenes : In some species, plasma genes reduce the frequency of crossing over. Eg : The Tifton male sterile cytoplasm reduces the frequency of crossing over in bajra . 9. Genotype : Many genes are known to affect the occurrence as well as the rate of crossing over. For example C3G gene of Drosophila located in chromosome 3 prevents crossing over when present in homozygous state while it promotes crossing over in the heterozygous state. 10.Chromosomal aberrations : In Drosophila, some chromosomal aberrations Eg : paracentric inversions, reduce recombination between the genes located with in the inverted segment. 11.Distance from centromere : Centromere tends to suppress recombination . Therefore genes located in the vicinity of centromeres show a relatively lower frequency of crossing over than those located away from them .

11. Significance of crossing over Crossing over provides a strong proof in favour of linear arrangement of genes on the chromosomes. Recombinations or new gene combinations are produced due to crossing over,which change genetic pool by changing of gene frequency and this provide a way of evolution(micro evolution) Crossing over generates genetic difference within a population . Linkage map and genetic maps are constructed on the basis of crossing over

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