Cell cycle and Cell Division Question Answers: NCERT Class 11 Biology

Between a prokaryotic and an eukaryotic cell, prokaryotic cell has shorter cell division time than the eukaryotes because prokaryotic cells like bacteria undergoes simple form of cell division which occurs in every 20 minutes called binary fission whereas eukaryotic cells divide by the process mitosis and take 24 hours to get completed.

DNA synthesis takes place at the S- phase or synthesis phase. Interphase involves the S- phase in which replication of each chromosome occurs by the synthesis of a new DNA molecule on the template of the existing DNA. 

The Interphase is the phase that lasts for more than 95% of the duration or entire generation time of the cell cycle. It takes 10- 20 hours out of 24 hours to get completed.

It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit from G₁ phase to enter an inactive stage called G₀ phase (Quiescent phase) of the cell cycle.

a. Synaptonemal complex: Zygotene (A fibrillary, ladder like organelle is known as Synaptonemal complex which develops between the synapsed homologous chromosomes.)

b. Recombination nodules: Pachytene (Prophase- I)

c. Appearance/ activation of enzyme recombinase: Pachytene (Prophase- I)

d. Termination of Chiasmata: Diakinesis (Prophase- I)

e. Interkinesis: After Telophase I/ before meiosis II

f. Formation of dyad of cells: Telophase I/ after meiosis I

Interphase is the longest phase of the cell cycle which lasts for 10 to 20 hours and takes 75 to 90 % of the entire generation time. It involves a series of changing events and further divided into three phases: G₁ phase, S phase and G₂ phase.

Basic Fuchsin, Acetocarmine etc.

In animals, reproductive cells or germ cells of the body exhibits meiosis. In plants, Androecium and Gynoecium parts of the plant exhibit meiosis.

For the formation of 32 cells, two E. coli cells take 4 cycles. So total time will be 4 x 20 = 80 minutes.

In humans, the cells of the skin, hair follicles, nail base or any somatic cells are used to demonstrate stage in mitosis. It occurs in the gonads also for the multiplication of undifferentiated germ cells.  

There are two sister chromatids attached by the centromere at the center is to be classifies as chromosome. When the anaphase ends and all the chromatids reach the opposite poles. These chromatids are now referred to as chromosomes.

Non- sister chromatids of homologous chromosomes can cross over. Two homologous chromosomes (bivalent) are termed as non- sister chromatids. Crossing over involves mutual exchange of the corresponding segments of non- sister chromatids of homologous chromosomes.

The average cell cycle span for a mammalian cell is approximately 24 hours.

Meiosis is the process involving the reduction in the amount of genetic material. It comprises two successive nuclear and cell divisions, with a single cycle of DNA replication. As a result, at the end of meiosis II, four haploid cells are formed.

Significance of meiosis

1. Meiosis maintains the chromosome number from generation to generation. It reduces the chromosome number to half so that the process of fertilisation restores the original number in the zygote.

2. Variations are caused by the cross-over and the random distribution of homologous chromosomes between daughter cells. Variations play an important role in evolution.

3. Chromosomal mutations are brought about by the introduction of certain abnormalities. These chromosomal mutations may be advantageous for an individual.

(i) In some insects and lower plants, fertilization is immediately followed by zygotic meiosis, which leads to the production of haploid organisms. This type of life cycle is known as haplontic life cycle.

(ii) The phenomenon of polyploidy can be observed in some haploid cells in higher plants in which cell division does not occur. Polyploidy is a state in which cells contain multiple pairs of chromosomes than the basic set. Polyploidy can be artificially induced in plants by applying colichine to cell culture.

Mitotic cell division cannot take place without DNA replication in S phase. Two important events take place during S phase – one is the synthesis or duplication of DNA and the other is the duplication of the centriole. DNA duplication is important as it maintains the chromosome number in the daughter cells and hence Mitosis is an equational division. Therefore, the duplication of DNA is an essential step and without it, no mitosis can take place.

There can be DNA replication without cell division. During cell division, the parent cell gets divided into two daughter cells. However, if there is a repeated replication of DNA without any cell division, then this DNA will keep accumulating inside the cell. This would increase the volume of the cell nucleus, thereby causing cell expansion. An example of DNA duplication without cell division is commonly observed in the salivary glands of Drosophila. The chromosome undergoing repeated DNA duplication is known as polytene chromosome.

During meiosis, the number of chromosomes and the amount of DNA in a cell change.

(i) Number of chromosomes (N) per cell

During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.

(ii) Amount of DNA content (C) per cell

During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present. During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.

Interphase involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication in an orderly manner. Interphase is divided into three phases.

(i) G₁ phase

(ii) S phase

(iii) G₂ phase

G₁ phase – It is the stage during which the cell grows and prepares its DNA for replication. In this phase, the cell is metabolically active.

S phase – It is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same.

G₂ phase – In this phase, the cell continues to grow and prepares itself for division. The proteins and RNA required for mitosis are synthesised during this stage.

G0 or quiescent phase is the stage where cells remain metabolically active, but do not proliferate unless called to do so. Such cells are used for replacing the cells lost during injury.

Mitosis is the process of cell division wherein the chromosomes replicate and get equally distributed into two daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell, i.e., diploid. Hence, mitosis is known as equational division.

 (i) Metaphase

(ii) Anaphase

(iii) Zygotene of meiosis I

(iv) Pachytene of meiosis I

(a) Synapsis

The pairing of homologous chromosomes is called synapsis. This occurs during the second stage of prophase I or zygotene.

(b) Bivalent

Bivalent or tetrad is a pair of synapsed homologous chromosomes. They are formed during the zygotene stage of prophase I of meiosis.

(c) Chiasmata

Chiasmata is the site where two non sister chromatids have crossed over. It represents the site of cross-over. It is formed during the diplotene stage of prophase I of meiosis.

(a) Spermatogenesis or the formation of sperms in human beings occurs by the process of meiosis. It results in the formation of four equal-sized daughter cells.

(b) Oogenesis or the formation of ovum in human beings occurs by the process of meiosis. It results in the formation of four daughter cells which are unequal in size i.e. are big mature ovum and 3 smell polar bodies.