Nondisjunction in meiosis

http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html

This is a very good video on nondisjunction in meiosis. Due to time constraints I don't think i'll be able to play the entire video in class, so please take some time to view it on your own.

P.S. I have created a chatbox on the right! Feel free to chat there. :)

Cell division - terminologies

TYPES OF CHROMOSOME

1) Autosome = a chromosome that is not a sex chromosome. For example, in humans there are 22 pairs of autosomes and 1 pair of sex chromosomes (XX in females or XY in males).

NUMBER OF CHROMOSOMES

2) Aneuploidy = a chromosome abnormality where there is an abnormal number (extra or missing chromosome(s) / +1, +2...etc or -1,-2...etc chromosomes) of chromosomes, due to nondisjunction in mitosis/meiosis I/meiosis II. Eg. Monosomy = presence of only one chromosome (instead of the typical two in humans) from a homologous pair.

3) Polysomy = when a diploid organism has at least one or more chromosome(s) than the normal (diploid) number, due to nondisjunction. Eg. Trisomy = 3 copies instead of 2 copies of a particular chromosome (eg. Trisomy 21 in down syndrome).

NUMBER OF SETS OF CHROMOSOMES

4) Polyploid = cells with more than two paired homologous sets of chromosomes (diploid, 2n). For example, triploid (3n/ cells with 3 times the haploid number of chromosomes/ 3 sets of chromosomes) and tetraploid (4n/ cells with 4 times the haploid number of chromosomes/ 4 sets of chromosomes).

Hope that this is less confusing for you guys now. Since I've already covered most of the terminologies of your SDL on nondisjunction here, i expect all of you to be able to answer most of my questions next week during the tutorial! ;)

Chromosome number and cancer (Cell division tutorial Q8c)

http://www.reuters.com/article/2009/05/20/us-cancer-downs-idUSTRE54J5IN20090520

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!!! :)

Answers to Cell division tutorial - factual Q

(14di) How does A lead to genetic stability? [2m]

- A (mitosis) sepArates (note the spelling) sister chromatids of a duplicated chromosome to yield two daughter cells with exactly the same number of chromosomes and is genetically identical to the parent cell by having the same alleles/nucleotide sequence.

- No chiasma is formed and no crossing over occurs during A (mitosis), retaining genetic fidelity.


(14dii) How does B lead to genetic variation? [3m]

- During prophase I of B (meiosis), crossing over occurs between homologous chromosomes, resulting in new combinations of alleles on the chromosomes of the gametes.

[independent assortment]
- The arrangement and subsequent sepAration of the homologous chromosomes of each tetrad/bivalent in metaphase I and anaphase I of B (meiosis) respectively is completely independent of the other tetrad/bivalent, producing new combinations of chromosomes in gametes.

[independent assortment]
- The arrangement and subsequent sepAration of the chromatids of each chromosome in metaphase II and anaphase II of B (meiosis) respectively is completely independent of the orientation of other chromosomes, producing new combinations of chromosomes in gametes.


(Q15c) How does meiosis lead to the formation of four haploid nuclei? [2m]

- During anaphase I, homologous chromosomes separate, resulting in two daughter cells, each with a haploid nucleus, at the end of telophase I and cytokinesis.

- During anaphase II, the sister chromatids separate, resulting in four daughter cells altogether, each with a haploid nucleus, at the end of telophase II and cytokinesis.

What is ploidy?

Some of you seem to be confused about what ploidy means. Hope the following helps:

Ploidy = no. of single sets of chromosomes in a cell
- Haploid cells (gametes, for example either the sperm or the egg cell) --> one set of chromosomes (either from father/mother)
- Diploid cells (somatic cells) --> two sets of chromosomes (one set from father/paternal side, one set from mother/maternal side)

KARYOTYPE (number of chromosomes, and what they look like under a light microscope) OF A DIPLOID CELL (below)

P.S.

Please don't forget to refer to the learning outcomes for this topic when you study. Note that when you're describing the behaviour of the chromosomes during mitosis/meiosis, you have to mention the KEY WORDS that have been BOLDED (e.g. "centromere divides") in your lecture notes. :)

Cell division tutorial Q7aiii and Q7b

Q7aiii) How non-kinetochore microtubules help in mitosis (their function had been asked in the A levels bef0re):

The non-kinetochore microtubules from opposite poles slide past and push apart from each other, causing the distance between the poles to increase and hence, elongating the cell.

Animation: http://faculty.plattsburgh.edu/donald.slish/Polar.html

Q7b) Behaviour of chromosomes during mitosis vs meiosis.

Meiosis animation

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

Mitosis and Meiosis Lecture Notes Page 1 (Terminologies)

How DNA is packaged
http://www.youtube.com/watch?v=9kQpYdCnU14