What is "crossing over" and when does it over occur? How does it contribute to genetic variation in offspring?

Answer 1

Maybe you can understand what crossing over is by investigating the process of meiotic cell division. There are 2 stages for meiotic c.d, namely phase I and II. Each phase is further divided into 4 stages, prophase, metaphase, anaphase and telophase. In prophase I, apart from the shortening and thickening of the chromosomes, there is also pairing up of homologous chromosomes, forming a bivalent. This process is called synapsis. Later on, each chromosome will have its chromatid replicated, producing two sister chromatids. Hence, a tetrad, consisting of four strands, becoms visible in each bivalent. NOW, here's the crossing over. In each tetra, one or ore parts of two non-sister chromatids form cross-shaped structure called chiasma. In each chiasma, exchange of corresponding segments occue between two non-sister chromatids. Ask you a simple question. Why is it not the sister chromatids to have the corresponding segment exchanged? Huh? It is because their genetic makeup is identical. So the crossing over will occur after the pairing up of homologous chromosomes. Or the non-sister chromatids will not come together. Another question. Why is it necessary for the homologous chromosomes to come together but not arbitrary chromosomes? they are of different gentic makeup in this case. Huh? This is because there is no corresponding segment. Ther carry different loci. So, the process results in recombination of genetic materials and is called crossing over. It occurs in later prophase I. For the genetic variation, just like what i have explained to you before, because of the exchange of gentic information between corresponding segment of the chromosome where chiasma is formed, the sister chromatids of a chromosome is no longer carrying the identical genetic information. You should be reminded that it is the gamete which produces offspring after fertilization. If there is no crossing over, there will be 2 paris of identical gamates produced if we simply concern 1 pair of homologous chromosome. SO, by crossing over, the genetic variation is enhanced. You can simply think by considering the case i have told you.

One more inspiring question for you, what is the biological significance of crossing over? Of course, the genetic variation is increased. But what's mroe is that they may allow organisms to adapt to a new environment. besides, they are raw materials for natural selction. And they are inheritable and can be passed from one generation to another, unlike somatic mutation, which will also increases genetic variation in that generation.

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Answer 2

Take a look at this animation and listen to the explanation. It's the best one of the four or five I investigated.

http://www.zo.utexas.edu/faculty/sjasper/images/13.10.mov