NCERT Class 11-Biology: Chapter –10 Cell Cycle and Cell Division Part 4

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Question 4:

A cell has 32 chromosomes. It undergoes mitotic division. What will be the chromosome number (N) during metaphase? What would be the DNA content (C) during anaphase?

Answer:

Both N and C will be 32.

This is due the fact that DNA is replicated during S phase, after that it would remain the same during equational division.

Question 5:

While examining the mitotic stage in a tissue, one finds some cells with 16 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number. Do you think cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versa?

Answer:

• The phenomenon of existence of two types of chromosome no. in different cell of the body is called mosaicism.

• This may arise due to non-disjunction of chromosomes or lag of anaphase.

Question 6:

The following events occur during the various phases of the cell cycle, Name the phase against each of the events.

a. Disintegration of nuclear membrane _________________________

b. Appearance of nucleolus _________________________

c. Division of centromere _________________________

d. Replication of DNA _________________________

Answer:

a. Early Prophase

b. Late Telophase

c. Anaphase

d. S phase

Question 7:

Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?

a. Nuclear membrane fails to disintegrate

b. Duplication of DNA does not occur

c. Centromeres do not divide

d. Cytokinesis does not occur.

Answer:

a. Nuclear membrane fails to disintegrate; if it happens then the spindle fibre would not reach the chromosomes.

b. Duplication of DNA does not occur,

If it would happen then the whole cell cycle would get arrested and will enter an inactive stage called the G0 stage.

c. Centromeres do not divide- Sister chromatids would not split to opposite poles and one daughter cell would get two sister chromatids whereas the other will get none.

d. Cytokinesis does not occur- It will result in polyploidy.

Question 8:

Both unicellular and multicellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?

Answer:

• In prokaryotes amitosis takes place, the cell directly divides into two daughter cells.

• Whereas in multicellular organisms, proper steps of cell division occurs; first the karyokinesis takes place then cytokinesis occurs.

Question 9:

Name the pathological condition when uncontrolled cell division occurs.

Answer:

This is called malignancy caused due to cancer.

Question 10:

Two key events take place, during S phase in animal cells, DNA replication and duplication of centriole. In which parts of the cell do events occur?

Answer:

DNA replication occur in nucleus and centriole duplicates in the cytoplasm.

Question 11:

Comment on the statement – Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in reduction of chromosome number.

Answer:

During fertilisation two haploid cells called the gametes fuse to form diploid zygote.

If meiosis is absent, than two diploid gamete would be produced which will result in formation of tetraploid zygote.

Therefore, it can be said that in order to conserve the chromosome number meiosis is very important.

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