NCERT Class 11-Biology: Chapter –10 Cell Cycle and Cell Division Part 5

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Question 12:

Name a cell that is found arrested in diplotene stage for months and years. Comment in 2-3 lines how it completes cell cycle?

Answer:

The oocyte of mammalian females get arrested in diplotene stage.

Process of oogenesis in females of mammal starts with their own embryogenesis.

The oocyte grows and forms secondary follicle and then it gets arrested at the diplotene stage.

As the woman attains the sexual maturity first meiosis gets completed.

Meiosis occurs one by one in the egg, which is released from the ovary.

Question 13:

How does cytokinesis in plant cells differ from that in animal cells?

Answer:

Cytokinesis in plants occurs by cell plate formation.

Cell plate formation starts from the centre and grows to the periphery.

In animals furrow is formed, from the periphery of the cell to the centre.

Long Answer Type Questions

Question 1:

Comment on the statement – Telophase is reverse of prophase.

Answer:

Prophase-

Chromosome condenses.

Nuclear membrane disintegrates

Organelles like mitochondria, endoplasmic reticulum, golgi apparatus disappears.

Telophase-

Chromosome decondenses

Nuclear membrane again gets formed

Organelles reappear.

Question 2:

What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?

Answer:

Prophase I-

a. Leptotene stage:

Nuclear membrane starts to disappear.

Chromatin material starts condensing.

Centriole reaches opposite poles.

In some members of lileaceae chromosomes remain attached to the nuclear membrane – bouquet stage.

b. Zygotene:

Pairing of homologous chromosomes start.

To hold these homologous chromosomes a protein called Synoptenemal complex develops.

c. Pachytene

Biavalent [homologous chromosomes] or tetrad becomes clearly visible.

Crossing over takes place resulting in genetic recombination.

The enzyme recombinase, for crossing over comes from the recombination nodule of the Synoptenemal complex.

d. Diplotene

Synoptenemal complex dissolves

But homologous chromosomes are still held by a X like structure called the chiasma.

Chiasmata develops at the point of crossing over.

e. Diakinesis

Terminalisation of Chiasmata: Chiasmata starts opening in a zipper like fashion towards the periphery.

Nuclear membrane and nucleolus nearly disappears.

Question 3:

Differentiate between the events of mitosis and meiosis

Answer: