![]() The sister chromatids of a specific chromosome in the bivalent act as a single unit. Spindle fibers originate and the kinetochore fibers connect to the kinetochore of the homologous chromosome from each pole and organize bivalents at the equator. The nuclear membrane disorganizes at the beginning of this stage. But still, they are united at one point at the end not by chiasmata. At the same time separation of the homologous chromosomes is completed. The condensation of chromosomes reaches its maximum point during this phase. Otherwise, the chromatids can separate from each other. Each bivalent has at least one chiasmata. ![]() The homologous chromosomes remain united by chiasmata. So, they begin to separate from each other. Pachytene may last for days, weeks, or even years but leptotene and zygotene can last only for a few hours. Crossing over produces new recombination. Thus, the reshuffling of genetic material takes place. The exchange of segment of the non-sister chromatids of homologous chromosomes is called crossing over. Non-sister chromatids of homologous chromosomes exchange their segments by chiasmata formation during crossing over. The pairing of homologous chromosomes is completed. Each paired but not fused complex structure is called as bivalent or tetrad. But this pairing has no definite starting point. Exact point to point pairing takes place. It is the first vital phenomenon of meiosis. The pairing of homologous chromosomes starts during zygotene. The size of the nucleus increases and homologous chromosomes start getting closer to each other. The chromosomes become shorten and thick so they become visible. Prophase I further consist of the following stages. Meiosis was first discovered and described by German biologist OscarHertwig in sea urchin eggs in 1876. The word meiosis originates from the Greek word ‘meioun’, meaning “to make small,” as it results in a reduction in chromosome number. If there is no meiosis the chromosome number will become double after every generation. Secondly, maintains the number of chromosomes constant in species. Meiosis is significant for variation produces by crossing over and a random assortment of chromosomes. The completion result is the production of four haploid cells. Mechanically, the process resembles mitosis, though its outcomes are basically different. Meiosis II is the second meiotic division, that involves equational partition, or separation of sister chromatids. In meiosis I there is segregation of paired chromosomes and at the end, each daughter cell contains one chromosome (with two chromatids). It occurs in animals at the time of gamete formation and in plants at the time of spore formation. Crossing over – the reshuffling of genetic material (chromosomes) takes place which produces new recombinants and causes genetic diversity. The prophase of meiosis is also significant because it has some important events. Homologous chromosomes can exchange parts in a process called "crossing over.The new daughter cells formed are haploid. In Metaphase I, homologous chromosome pairs line up. This shuffling process is known as recombination or "crossing over" and occurs while the chromome pairs are lined up in Metaphase I. Each sibling is 50% mom and 50% dad, but which 50% of each can vary in the siblings. But this happens independently for each trait, so just because you got your dad's brown eyes doesn't mean you'll get his blond hair too. Each sperm and egg will end up with either B or b from mom and either B or b from dad. This leads to four possibilities: You could get B from mom and B from dad, or B from mom and b from dad, or b from mom and B from dad, or b from mom and b from dad. Imagine, for example, that eye color was controlled by a single gene, and that mom could have B, the allele for brown eyes or b, the allele for blue eyes, and dad could also have B or b. ![]() But each non-identical-twin child of these parents ends up with a different combination. You ended up with half of mom's paired genes and half of dad's paired genes. Your parents each have at least one pair of alleles (versions of a gene) for every trait (and many pairs of alleles for each polygenic trait).
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