Meiosis, Crossing Over

Mechanism of crossing over

1. Meiosis, Mechanism of Crossover, Significance in evolution. By- AMIT KUMAR SAHOO 15151

2. What is Meiosis? Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females . Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II ).

3. We Are UNIQUE! One of the reasons for this genetic mix up or uniqueness is by a process called Crossing O ver that occurs during meiosis.

4. What is Crossing over? Crossing over is the exchange of segments between the non-sister chromatids of homologous chromosome. The term crossing over was coined by Morgan.

5. Origins: There are two popular and overlapping theories explaining the origins of crossing-over, coming from the different theories on the origin of  meiosis .  The first theory rests upon the idea that meiosis evolved as another method of DNA repair, and thus crossing-over is a novel way to replace possibly damaged sections of DNA . The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity.

6. Mechanism of meiotic crossing over. The major steps in meiotic crossing over are 1 ) synapsis 2) duplication of chromosome 3 ) crossing over and 4) terminalisation .

7. -Synapsis is the intimate pairing between the two homologous chromosomes. -Synapsis is followed by duplication of chromosome (in pachytene). -Crossing over or exchange of segments between the non-sister chromatids of homologous chromosome occurs at the tetrad stage. -Crossing over can be divided into three major steps: 1 ) breakage of chromatid segments 2) their transposition (movement to the respective site) 3 ) fusion or joining.

8. The final step is terminalisation . After crossing over the non-sister chromatids starts to repel each other. Chiasma itself moves in a zipper fashion towards the end of tetrad . This movement of chiasma is known as terminalisation .

9. The Biology Underlying Mendelian Inheritance Mendel’s Laws can be derived directly from our understanding of Meiotic cell division or Meiosis. T he purpose of meiosis is to introduce further genetic diversity by creating gametes, either egg cells or sperm cells, that are genetically different from the parent cells.

10. What is the function of crossing-over? In species that reproduce sexually, offspring are genetically distinct from their parents because they inherit genetic material from both. Such genetic diversity is the product of meiosis , a type of cellular division that creates reproductive cells from a parent cell. T he paired chromosomes can exchange segments of DNA via a mechanism called crossing-over.

11. Crossing-over has two main functions. The first is to increase genetic recombination . The second is to ensure that parental chromosomes are equitably distributed among the reproductive cells produced by meiosis . Without crossing-over, the chromosomes would be distributed abruptly. Too many crossing over is also not good because could disrupt advantageous gene combinations that have established themselves over evolutionary time .

12. Crossing over in favor of plant breeding. Increasing the number of crossing-over events could lead to more genetic recombination and thus novel gene combinations, a desirable outcome in the context of plant breeding . Ex- Arabidopsis thaliana Increasing genetic recombination by inhibiting mechanisms that limit crossing-over.

13. Mechanism of Crossing Over: It occurs during Prophase I of Meiosis Genetic swapping occurs between paired homologous chromosomes in our sex cells—The Egg and Sperm Homologous Chromosomes Exchanging DNA by Crossing Over From: http://www.ultranet.com/~jkimball/BiologyPages/M/Meiosis.html#crossing_over

14. Its Why You and I Don’t Look Alike Crossing Over ensures a combination of the maternal and paternal genes we inherited BOTTOM LINE

15. So, when chromosomes separated during meiosis II, some of the daughter cell receive daughter chromosome with recombined alleles. Due to this genetic recombination offspring have a different set of genes and alleles than there parents

16. Crossing over and Chiasmata Chiasmata is the point where two homologous non-sister chromatids exchange genetic material during crossing over during meiosis. Chiasmata becomes visible during diplotene stage of prophase I during meiosis. But actual crossing over occcur during previous pachytene stage. When each tetrad which is composed of two pairs of sister chromatids begins to split. Only point of contact is chaismata .

17. Crossing over and chaismata

18. Types of Crossing over Single crossing over : Chromosomal crossover (or  crossing over ) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis (pachytene) during a process called synapsis.

19. single crossing over

20. Double crossing over It refers to formation of two chiasmata between non-sister chromatids of homologous chromosomes.  Two simultaneous reciprocal breakage and reunion events between the same two chromatids .

21. The biological significance of meiosis. The conventional view that it generates by recombination and sexual reproduction the genetic diversity on which natural selection can act . That recombination at meiosis plays an important role in the repair of genetic defects in germ line cells. That it is essential, at least in animals, for the reprogramming of gametes which give rise to the fertilized egg . That it helps maintain the immortality of the germ line

22. Significance of crossing over Crossing over is universal in occurrence, occurs in plants, animals, bacteria,  viruses and moulds . Meiotic crossing over allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between sister chromatids Helps in proving  linear  arrangement of genes. Recombination does not have any influence on the  statistical probability  that another offspring will have the same combination. This theory of “independent assortment” of alleles is  fundamental  to genetic  inheritance . Origin of  new character. Necessary for  natural selection , as it increases chances of  variation. Ex- Selection of useful recombination by geneticists has brought about green revolution in our country.

23. Role o f Crossing over in Evolution Crossing over allows genetic variants on the same chromosome to evolve independently, which greatly increases an organism's evolutionary potential . (Explanation) If there were no crossing over, all genetic variants on a chromosome would be inherited as a block. Image a chromosome copy which contains a good variant--let's say, flu resistance--at one gene, and a bad variant--let's say, tapeworm susceptibility--at a different gene. Without crossing over, the population has to choose between flu and tapeworms. Crossing over can produce a chromosome with the good variant and without the bad one, allowing the population to move toward a better solution. This speeds up the rate of adaptation.

24. References: www.nature.com/scitable/topic/chromosomes-and-cytogenetics-7 www.plantphysiol.org/content/171/4/2294.full journals.plos.org/ plosbiology / article?id =10.1371/journal.pbio.1000501 bmcgenomics.biomedcentral.com/articles/10.1186/1471-2164-13-53 https://www.ncbi.nlm.nih.gov/books/NBK21870 /

25. “Sai Ram”