Crossing Over: Mechanisms and Types PDF

## Crossing Over - **Definition:** Interchange of chromosomal parts between non-sister chromatids of homologous chromosomes resulting in recombination of genes. - **Discovery:** Discovered by Morgan. - **Location:** Occurs during Meiosis or gametogenesis. - **Mechanism:** Interchange of chromosomal...

## Crossing Over - **Definition:** Interchange of chromosomal parts between non-sister chromatids of homologous chromosomes resulting in recombination of genes. - **Discovery:** Discovered by Morgan. - **Location:** Occurs during Meiosis or gametogenesis. - **Mechanism:** Interchange of chromosomal segments between non-sister chromatids of the homologous chromosomes. - **Factors affecting crossing over:** - **Length of chromosome:** Longer the chromosome, higher the percentage of crossing over. - **Gene Distance:** Further the genes are located, higher the chance of crossing over. - **Centromere and Tip:** Less frequent near the centromere and the tip of the chromosome - **Multiple Crossing Over Inhibition:** Crossing over at one point prevents another crossing over nearby. - **Types of Crossing Over:** - **Single Crossing Over:** Single crossing over in a homologous chromosome. - **Double Crossing Over:** Two cross overs in a homologous chromosome. - **Multiple Crossing Over:** Many cross overs in a homologous chromosome. - **Percentage of Crossing Over:** - Represents the number of recombinations in percentage to the total number of offspring. - Directly proportional to the distance between two genes. - Also called frequency of crossing over, which is equal to percentage of recombination. - **Significance of Crossing Over:** - **Linear Arrangement of Genes:** Supports the linear arrangement of genes. - **Chromosome Maps:** Frequency of crossing over helps in constructing chromosome maps. - **Recombination:** Produces new combination of genes. - **Genetic Variation:** Basis for evolution. #### Mechanism of Crossing Over - **Homologous chromosome pairing:** Homologous chromosomes move towards each other and come to lie side by side. This is called synapsis. - **Split of homologous chromosomes:** Each chromosome splits into two chromatids (tetrad stage). - **Non-sister chromatids twist:** They twist over each other and connect at the non-sister chromatids at various points called chiasmata. - **Breakage at chiasmata:** Chromatids break at chiasmata due to enzyme called endonuclease. - **Ligation:** Broken segments fuse with other chromatids by enzyme called ligase. - **Separation:** Non-sister chromatids repel each other and separate from each other. - **Terminalization:** Starts from the centromere towards the end, and results in the separation of homologous chromosomes. #### Theories of Crossing Over 1. **Chiasma Type Theory (Janssens):** - Chromosomes duplicate and form a tetrad. - Crossing over occurs only between the non-sister chromatids. - Non-sister chromatids overlap and form chiasmata, breaking and rejoining there. 2. **Breakage First Theory (Muller):** - Non-sister chromatids break first, without crossing over. - Broken segments rejoin to form new combinations. 3. **Contact First Theory (Serebrovsky):** - Non-sister chromatids first touch and cross each other. - Breakage occurs at the points of contact. - Broken segments rejoin to form new combinations. 4. **Strain or Torsion Theory (Darlington):** - Homologous chromosomes coil around each other, creating strain. - Strain leads to breakage of non-sister chromatids. - Broken ends rejoin to form chiasmata. 5. **Differential Contraction Theory (Huskin):** - Tension occurs when the homologous chromosomes contract at different rates. - Breakage occurs at the overlapping points. - Separation starts terminally and leads to separation of homologous chromosomes. #### Types of Crossing Over 1. **Single Crossing Over:** - Only one chiasma formed. - Only one chromatid of each chromosome is involved. 2. **Double Crossing Over:** - Two chiasmata formed. - Can occur between the same or different chromatids. - Can involve two, three, or all four chromatids. - Types: - Two strand double crossing over: Two non-sister chromatids are involved. Two chromatids remain parental combinations; two remain non-parental combinations. - Three strand double crossing over: Three chromatids are involved; one chromatid remains parental, two remain non-parental, - Four strand double crossing over: All four chromatids are involved. 3. **Multiple Crossing Over:** - More than two chiasmata are formed. - Very rare because of interference in crossing over. #### Factors Affecting Crossing Over - High temperature increases the crossing over frequency. - X-rays increase the crossing over frequency. - The frequency of crossing over decreases with increasing age of female Drosophila. - Some genic mutations decrease the frequency of crossing over. - Crossing over is less frequent near centromeres and the tips of the chromosomes. - Inversion of chromosome segments suppresses the crossing over. #### Tetrad Analysis 1. **Purpose:** Used to study how many strands are involved in crossing over during Meiosis. 2. **Method:** - Use individual ascospores of tetrad ascus to study crossing over. - Spores from a single diploid nucleus go through two meiotic divisions and one mitotic division, producing 8 spores arranged in a serial order in the ascus. 3. **Neurospora Crassa:** - A bread mold where each ascus contains 8 haploid ascospores that can be dissected out and grown separately. 4. **Crossing Over in Neurospora:** - Crossing over can occur in the bivalent or tetravalent stage. - If it occurs in the bivalent stage, the ascospores will be arranged in a 4:4 ratio. **Diagram of the Crossing Over Process** [A diagram of the process of crossing over is missing. I can’t provide an image without access to the internet.]