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

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.

In Prophase condensation of chromosome occurs. Nuclear membrane became disintegrated. When we viewed the cells at the end of the prophase, Golgi apparatus, mitochondria, endoplasmic reticulum do not show, they disappears. In Telophase, at the beginning the chromosomes that have reached their respective poles starts decondense and lose their individuality. At the late Telophase stage the chromosomes clusters, nucleolous surrounded by the nuclear membrane and Golgi complex, endoplasmic reticulum reforms.

In this way Telophase is inverse of Prophase.    

DNA replication occurs in the nucleus and centriole duplication in the cytoplasm.

Meiosis enables the conservation of specific chromosomes number of each species. If there has been no meiosis then organism would not have been able to evolve to sexual mode of reproduction. Zygote is formed when cells fused together. The number of chromosomes in a zygote becomes double that of gametes. To ensure conservation of specific chromosomes number after fertilization, it is necessary that the gametes should have half number of chromosomes.

The oocyte of mammalian females is the cell that is found arrested in diplotene stage for months and years.

The oocyte grows and forms secondary follicle and then get arrested at the diplotene stage. As woman attains the sexual maturity first, meiosis gets completed. Meiosis occurs one by one in the egg which is released by the ovary.

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

Prophase I of the first meiotic division is more complex and typically longer than Prophase of mitotic division. It takes more than 90% of the time required for meiosis. It has been further subdivision into five phases based on chromosomal behaviour:

1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis

• Leptotene stage:

1. The chromosome become gradually visible under the light microscope.
2. Chromatin material starts condensing.
3. Nuclear membrane starts disappear.
4. Centriole reaches opposite poles.

• Zygotene stage:

1. During this stage chromosomes starts pairing with each other and this process of association is known as synapsis.
2. Such paired chromosomes are called homologous chromosomes. To hold these homologous chromosomes a protein called Synaptonemal complex develops.

• Pachytene stage:

1. The complex formed by a pair of synapsed homologous chromosomes called tetrad or bivalent, become visible.
2. Crossing over take place result in recombination of genetic material.

• Diplotene stage:

1. The beginning of diplotene is recognized by the dissociation of the synaptonemal complex.
2. Homologous chromosomes are held with X shape structures, are called Chiasmata.
3. In oocytes of some vertebrates, diplotene can lasts for months or years.

• Diakinesis stage:

1. It is the last phase of Prophase I in meiotic division.
2. Termination of Chiasmata.
3. Chromosomes are fully condensed.
4. Spindle is assembled to prepare the homologous chromosomes for separation.

The nucleolus disappears and the nucleus envelop also breaks down.

1. During zygotene stage of Prophase I chromosomes start pairing together which is called synapsis and these paired chromosomes are called homologous chromosomes. Synaptonemal complex is made up of protein formed between two homologous chromosomes. It contains a recombination nodule which consists of enzyme recombination involved in crossing over.
2. Metaphase line is considered as an imaginary line which is formed in the center of the cell during Metaphase, which is equidistant from the two opposite poles. This allows the chromosomes a place to be lined up on before they are pulled apart.

Importance of Mitosis in a multicellular organism:

The growth of multicellular organisms is due to mitosis. Mitosis occurs in somatic cells. And, also helps in the cell repair. Cells of upper layer of epidermis, blood cell and cells if the lining of the gut are being constantly replace. In plants mitosis also plays a significant role. In plants, mitosis occurs in meristematic tissue that results in continuous growth of plants throughout their life.

Importance of Meiosis:

Meiosis provides evidence of basic relationship of organisms. It also increases the genetic variation in the population of organisms from one generation to another. And, various is essential for evolution.

In the Meiosis II, the two chromatids of each chromosome separate from each other and goes to separate daughter cells. With the result, the number of chromosomes remains the same as produced by meiosis- I. Meiosis –II is known as homotypic division. The four stage of this division are-

1. PROPHASE- II: It is very short. It occurs simultaneously in both the nuclei formed by meiosis- I. centrioles, already duplicated, move apart in pairs. Each pair develops astral rays round it to form an ester. Spindle is laid down between the 2 pairs of centrioles. The chromosomes, each comprising two chromatids, become visible in the nucleus. They are set free in the cytoplasm by breaking of the nuclear envelope. Nucleoli disappear.
2. METAPHASE- II: The chromosomes take up positions at the equator of the spindle, forming a single metaphase plate. The chromatids of each chromosome are joined at their kinetochores by spindle fibres extending from the opposite poles of the spindle as in mitosis.
3. ANAPHASE- II: The two chromatids of each chromosome start moving away from each other. Finally, they reach the poles of the spindle. Here, they are called the chromosomes. Each pole has haploid number of chromosomes and haploid amount of chromosomes and haploid amount of DNA. This amount is one- fourth of the DNA present in the original cell that started meiosis.
4. TELOPHASE- II: The group of chromosomes at each pole of the spindle gets closed by a nuclear envelope. Nucleoli are laid down. Astral rays and spindle are lost.

1. If nuclear membrane fails to disintegrate then the spindle fibres will not be able to reach to the opposite poles and do not form chromosomes.
2. If duplication of DNA does not occur then no mitosis will occur.
3. If centromeres do not divide then sister chromatids would not split to opposite poles and one daughter cell would get two sister chromatids whereas the other will get none.
4. If cytokinesis does not occur then polyploidy appears.

In prokaryotes, the cell divides into two daughter cells by binary fission or amitosis whereas in multicellular organisms, the cell division takes place through the mitosis and then duplication occurs in the S- phase.

In pathological condition is malignancy in which cell division become uncontrolled and this cause cancer.