Please help me with this i dont get it AT ALL!!?

Question

during which stage of meiosis is the diploid number of chromosomes reduced to the haploid number of chromosomes?
& also...explain the role of crossing-over in ensuring genetic variation!

Answer 1

Anaphase (sometimes called anaphase I).

Though it depends on exactly where you want to draw the line. In anaphase, the chromosomes are physically pulled apart, and in telophase I they are separated into two daughter cells, each with two haploid sets of chromosomes.

It is those daughters that divide again to produce the germ cells themselves which are haploid and have only one set of chromosomes.

As to the importance of crossing-over, think about what would happen if crossing-over never occurred: all the genes on one chromosome would always be stuck together. ALWAYS. Every germ cell would get one chromosome or the other and that would be it - two possibilities only.

But since crossing-over does occur, there are millions of possible combinations of genes in each chromosome of a germ cell instead of just two. Simply put, it allows variation on a gene level instead of a chromosome level. And since you have thousands of genes on each chromosome, that's thousands of times more variation!