Cell division tutorial Q15b

The diagram below is the more accurate representation of crossing over as it shows the BREAKING of non-sister chromatids, followed by the EXCHANGE and then the REJOINING of the corresponding sections of genetic material.






The diagram below is the less accurate representation of crossing over as it does not show the exchange taking place. So please don't draw this in your exam scripts.

To help you understand better, below is a pictorial representation of the mechanism of crossing over.

Note that before crossing over, the sister chromatids are genetically identical, for example the chromosome coloured red is homozygous dominant for the 'c' allele (CC) and homozygous dominant for the 'e' allele (EE).

However, after crossing over the sister chromatids are no longer identical, for example the chromosome coloured red is still homozygous dominant for the 'c' allele (CC) but is now heterozygous for the 'e' allele (Ee).

Another diagram to show the products formed after crossing over in meiosis. You should be a pro by now!


If you are clear now about crossing over, please re-attempt MCQ11. Hope you will get it right this time! 


Some tips for MCQ11: 


- Identify/label the pair of homologous chromosomes, and the sister chromatids that are held at the centromere in the diagram. You can use two differently coloured highlighters to help you trace the chromatids from the paternal vs the maternal chromosomes. 


- Has crossing over taken place? How do you know? (Hint: before crossing over, the sister chromatids of a chromosome must be genetically identical/have identical alleles.)


- What are the products formed for this Q? (Note that this question is asking you about the daughter cells formed after MEIOSIS I, and not meiosis II!!! There is a difference, because meiosis I separates the homologous chromosomes while Meiosis II separates the chromatids!) By the way, "segregation of alleles Q from q" means that the separated daughter cell carries only allele Q or allele q (and not both Q and q alleles). 

Let me know if you have any problems (you can raise the Q after tutorial/ leave a comment on the blog). 

We will be working on Cell division SDL worksheet B and Enzymes tutorial/SDL (will probably start with the SDL) next week. I will be collecting the enzyme essay Q to mark as well so remember to do it on foolscap.

Study hard for your test!!! :)