Question

1) Explain how this cellular mechanism affects variation: Crossing over

Briefly explain how the mechanism (crossing over) could contribute to genetic variation in offspring. Be specific how this mechanism gives this result, when it occurs during the cell cycle, and be specific as to the type of genetic variation produced (from base-pair sequences to whole genomes).

Answer 1

Before the cell divides, DNA must replicate, as described above. It goes from 2C to 4C while the number of chromosomes remain same. The replicated segment is present as an extra arm called a chromatid. Hence one chromosome has two chromatids.

Meiosis is reduction division- it produces four cells with half the number of chromosomes that of the original. It is carried in two phases Meiosis I and Meiosis II. Meiosis I is reductional, it reduces the chromosome number to half while Meiosis II is equational, the reduced number remains same. The cells with half the number of chromosomes are called haploid. They contain only one version of the chromosomes as single copy.

As Meiosis I sets in, 2C has already gone to 4C- AA, A’A’, BB, B’B’, CC, C’C’, DD, D’D’. While A’ is the maternal version of A, A is an exact copy of A and so on. A paternal version and its copy, together as a chromosome comes together with the maternal version and its copy. So AA comes together with A’A’. This group is called bivalent. So out virtual 8 chromosome cell might be like this during Meiosis I

AA    B’B’   C’C’   DD

A’A’ BB     CC    D’D’

You can see that paternal and maternal versions sort themselves randomly. Now during the Metaphase I, Microtubules will pull one set of chromosome version and its copy to one pole and other to the opposite.During meiosis, a crucial event takes place that exchanges DNA between the non-sister chromatids of homologous chromosomes.

As we can see there is an exchange of DNA segment between A and A’. Hence genes thus will also be switched. Remember DNA segment is made up of phosphate-sugar and bases. This sequence of bases has in them stored the code that transcribes and translates into proteins. And a segment of DNA that codes for a character is called a gene. So as DNA segment gets exchanged so will be the chance of genes getting exchanged and variation be brought into.