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20
An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?
Explanation
Three hundred pollen mother cells have to be there to produce 1200 pollen grains. because one pollen mother cell will produce four pollen grains.
21
At what stage of cell cycle does DNA synthesis take place?
Explanation
Synthetic phase or S-phase of interphase, is the stage of cell cycle where DNA synthesis or replication takes place.
22
It is said that the one cycle of cell division in human cells (eukaryotic cells) takes 24 hours. Which phase of the cycle, do you think occupies the maximum part of cell cycle?
Explanation
If a cell takes 24 hours to divide, it will have 18-20 hours time to spend on interphase stage to prepare itself to undergo cell division.
23
It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit ... phase to enter an inactive stage called... of cell cycle. Fill in the blanks.
Explanation
It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit $G_1$ phase to enter an inactive stage called quiescent stage ( $G_0$ ) of cell cycle. Muscle cells when reach a level of maturity, no longer divide and just perform their function all through its life. Similar is the case with nerve cells too which are highly specific and lack the ability to divide once they get specialised. They remain in that state until they die.
24

In which phase of meiosis are the following formed? Choose the answers from hint points given below.

(a) Synaptonemal complex ........... .

(b) Recombination nodules ........... .

(c) Appearance/activation of enzyme recombinase ............ .

(d) Termination of chiasmata ......... .

(e) Interkinesis ............ .

(f) Formation of dyad of cells .......... .

Hints (a) Zygotene,

(b) Pachytene,

(c) Pachytene,

(d) Diakinesis,

(e) After Telophase-I/before prophase of meosis-II,

(f) Telophase-I/after meiosis-I.

Explanation

(a) Synaptonemal complex zygotene.

(b) Recombination nodules pachytene.

(c) Appearance/activation of enzyme recombinase telophase-l/after, meiosis-l ... .

(d) Termination of chiasmata diakinesis.

(e) Interkinesis after telophase-I/before prophase of meiosis-II.

(f) Formation of dyad of cells pachytene.

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