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CROSSING OVER
- 1. CROSSING OVER BY: M.JANANI III B.SC. ZOOLOGY 212018006
- 2. DEFINITION OF CROSSINGOVER • Interchange of chromosomal parts or genes between non-sister chromatids of a homologous pair of chromosomes. • Homologous pair of chromosome: Paternal chromosome + Maternal chromosome. Same size and shape. • Non-sister chromatids: The 2 chromatids from 2 different chromosomes.
- 3. SALIENT FEATURES OFCROSSING OVER • Discovered by Morgan • Crossing over results in recombination of genes , thus promoting genetic diversity. • Results in the formation of chiasmata ; the point of contact, the physical link, between two non-sister chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur. Absence of a chiasma generally results in improper segregation and aneuploidy. • It occurs during meiosis (pachytene). • It occurs only between non-sister chromatids of homologous chromosomes.
- 4. SALIENT FEATURES OFCROSSING OVER • Number/Frequency/Percentage of crossing over depends on the length of the chromosome and distance between the chromosomes • Usually crossing over is less frequent near the centromere and tip of the chromosome. • Crossing over at one point prevents another crossing over nearby.
- 5. TYPES OF CROSSINGOVER • Single Crossing Over: One chiasma is formed. Only one chromatid of each chromosome is involved. • Double Crossing Over: Two chiasmata are formed. Two , three or all four chromatids are involved. 1. Two strand 2. Three strand 3. Four strand • Multiple Crossing Over: More than two chiasmata are formed. Rare.
- 6. FACTORS AFFECTING CROSSINGOVER • Temperature • X Rays • Age • Inversion of chromosome • Location • Interference • Mutation
- 7. MECHANISMS OR THEORIESOF CROSSING OVER CHIASMA TYPE THEORY BREAKAGE FIRST THEORY
- 8. STRAIN OR TORSION THEORY •CONTACT FIRST THEORY • DIFFERENTIAL CONTRACTION THEORY
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- 10. SIGNIFICANCE • Linear arrangementof genes • Chromosome map • Recombination • Variations
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