Is crossing over an advantage to an organism?

Answer 1

This is done by one of Mum's chromosomes swapping a segment of DNA with one of Dad's chromosomes.

In the first prophase of meiosis (prophase I) each chromosome contains two molecules of DNA. You can see these as the two sister-chromatids of each chromosome. During this prophase, each chromosome pairs with the corresponding chromosome from the other set. In other words, the chromosome 1 from the male parent aligns itself beside chromosome 1 from the female parent, the two copies of chromosome 2 do likewise, and so on. The two copies of one chromosome are called homologous chromosomes.

Each pair of chromosomes is called a bivalent, and consists of four chromatids twisted around each other. At various points along this bivalent, two non-sister chromatids will break at the same locus (point along the DNA) and recombine, so that now a stretch of DNA from Mum has attached to a stretch from Dad, and vice versa.

On average, this crossing-over happens three or four times along the length of a chromosome, so that one DNA molecule now consists of alternating segments from each parent.

Because no two parents are genetically identical, the recombined DNA has a base sequence that is different from that supplied by either parent. It is like a linear mosaic.

The positions of crossing-over are pretty well random, so every time meiosis takes place in one individual novel base sequences are generated.