4/07/2017

IIT JAM BIOTECH COACHING IN DELHI

Q27. Which of the following statements about meiosis is NOT ture?
1. Kinetochores of sister chromatids attach to opposite poles in Meiosis l.
2. Kinetochores  of sister chromatids attach to opposite poles in Meiosis ll.
3. Chiasma is formed in prophase l.
4. Homologous chromosomes are segregated in Meiosis l.

Ans (1) We study Cell division from standard 8th onwards, but still it is pretty tricky and one could faulter if one does not understand the fundamentals. Infact when I was in Dept. of Genetics, Delhi University, pursuing my Post Graduation we were asked to explain Mendal laws in relation to cell division by Prof. Burma from IISc.,.........so this topics holds a lot of relevance.

Infact I find it surprising that students cannot differentiate between Law of Segregation & Law of Independent Assortment even after completing their PG......because they dont relate it to meiosis. 
 (Teachers are you listening pleaseeeee...)

Now it may take many pages if i was to discuss Cell cycle and its regulation but for the sake of answer 
always remember that in Meiosis-1, its the homologous chromosomes which separate & in Meiosis-II, its the sister chromatids which separate, so choice 2 & 4 are correct and choice 1, which says that it is the  "Kinetochores of sister chromatids attach to opposite poles in Meiosis l" is a wrong option.

Further Chiasma is the site where crossing over takes place during meiosis prophase-1. Infact prophase-1 is further divided into Leptonene, Zygotene, Pachytene, Diplotene & Diakinesis. 

In many books they have mentioned that crossing over i.e chiasma formation takes place in pachytene stage but in reality it occurs in diplotene where non- sister chromatids of homologous pair start repelling each other and form points of cross-over. In Pachytene it is the pairing between homologous pair which is completed.

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