NCERT Class 11 Biology Cell Cycle and Cell Division Important Notes

Phases of Cell Cycle

The cell cycle is divided into two basic phases:

a. Interphase: The phase between subsequent cell divisions is called the interphase. The interphase lasts for more than 95% of the cell cycle.

b. M Phase (Mitosis phase): The actual cell division takes place in the M phase. The M phase lasts for less than 5% of the cell cycle. The M phase is composed of two major steps, viz. karyokinesis and cytokinesis. Division of nucleus happens during karyokinesis. Division of cytoplasm happens during cytokinesis. 

The interphase is further divided into three phases, which are as follows:

a. G1 phase (Gap 1): During this phase, the cell is metabolically active and continuously grows.

b. S phase (Synthesis): During this phase, DNA synthesis or replication takes place. The amount of DNA becomes double during this phase, but the number of chromosomes remains the same.

c. G2 phase (Gap 2): During this phase, protein synthesis takes place. 

Quiescent Stage (G0): Cells which do not divide further, exit G1 phase to enter an inactive stage. This stage is called quiescent stage (G0) of the cell cycle. The cells in this stage remain metabolically active but do not undergo division. But these cells can resume division as and when required.

M PHASE

Mitosis is divided into four stages, viz. Prophase, Metaphase, Anaphase and Telophase

Prophase

• Condensation of chromosomal material takes place. A chromosome is seen to be composed of two chromatids. The chromatids are attached together at the centromere.

• Spindle fibres are formed.

• Various cell organelles; like golgi bodies and ER cannot be seen during this staged. Nucleolus and nuclear envelope also disappear.

 Metaphase

• All the chromosomes come to lie at the equator.

• In each chromosome, one chromatid is connected to the spindle fibre from one pole and another chromatid is connected to the spindle fibre from another pole.

• The plane of alignment of chromosomes during this phase is called metaphase plate. 

Anaphase

• Centromeres split which results in separation of chromatids.

• After that, chromatids move to opposite poles.

 Telophase

• The chromosomes form clusters at opposite poles. They become inconspicuous.

Cytokinesis

Division of cytoplasm is achieved by cytokinesis. In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. Thus, the cytoplasm is divided into two parts. In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.

Significance of Mitosis

• Mitosis results in the formation of new cells which are required for growth and repair.

• Mitosis results in the formation of two daughter cells; which have identical genetic makeup, similar to the mother cell. 

MEIOSIS

• Meiosis involves two sequential cycles of nuclear and cell division, but only a single cycle of DNA replication. Meiosis is divided into meiosis I and meiosis II.

• Meiosis I begins after the S phase, and meiosis II follows meiosis I.

• Pairing of homologous chromosomes happens during meiosis which results in recombination of genes.

• Four haploid daughter cells are formed at the end of meiosis. 

MEIOSIS I

Prophase I:

• Prophase in meiosis I is typically longer and more complex than the prophase in meiosis II. Prophase I is subdivided into five phases, viz. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Leptotene:

• During this stage, the chromosomes become gradually visible under light microscope. Compaction of chromosomes continues throughout this phase. 

Zygotene:

• Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes.

• Formation of synapsis is accompanied by the formation of synaptonemal complex.

•The synaptonemal complex by a pair of homologous chromosomes is called a bivalent or a tetrad.

Pachytene:

• Bivalent chromosomes clearly appear as tetrads, at this stage.

• Recombination nodules appear. These nodules are the sites at which crossing over takes place between non-sister chromatids of the homologous chromosomes.

• Exchange of genetic materials between two homologous chromosomes takes place during crossing over. This leads to recombination of genetic materials on the two chromosomes. 

Diplotene:

• Synapotnemal complex is dissolved at this stage.

• The recombined homologous chromosomes of the bivalent separate from each other; except at the site of crossing over.

• The X-shaped structures; thus formed; are called chiasmata. 

Diakinesis:

• Chiasmata is terminated at this stage.

• Meiotic spindles are formed to prepare the homologous chromosomes for separation.

• Nucleolus disappears and nuclear envelope breaks down by the end of diakinesis. 

Metaphase I:

• The bivalent chromosomes are aligned on the equatorial plate.

• Spindle fibres from opposite poles attach to the pair of homologous chromosomes. 

Anaphase I:

• Homologous chromosomes separate, but sister chromatids remain attached at their centromeres. 

Telophase I:

• Nuclear membrane and nucleolus reappear.

• This is followed by cytokinesis and this stage is called the diad of cells.

• The stage between the two meiosis divisions is called interkinesis. Interkinesis is usually short lived. 

MEIOSIS II

Prophase II: Meiosis II resembles the mitotic cell division. It begins immediately after cytokinesis. Nuclear membrane disappears. Chromosomes again become compact.

Metaphase II: The chromosomes align at the equator. Spindle fibres from the opposite poles get attached to the kinetochores of sister chromatids.

Anaphase II: Centromeres split and sister chromatids move towards the opposite poles.                  

Telophase II: The two groups of chromosomes get enclosed by nuclear envelope. This is followed by cytokinesis; resulting in the formation of four daughter cells.

Significance of Meiosis:

• Conservation of specific chromosome number of each species is achieved across successive generations in sexually reproducing organisms through meiosis.

• Meiosis helps in increasing the genetic variations in the population of organisms from one generation to the next.

NCERT Solution for Class 11 Biology Cell Cycle and Cell Division

Question – 1- What is the average cell cycle span for a mammalian cell?

Answer: 24 hours

Question – 2 - Distinguish cytokinesis from karyokinesis.

Answer: Division of cytoplasm takes place during cytokinesis, while division of nucleus takes place during karyokinesis.

Question – 3 - Describe the events taking place during interphase.

Answer: The interphase is divided into three phases, which are as follows:

a. G1 phase (Gap 1): During this phase, the cell is metabolically active and continuously grows.

b. S phase (Synthesis): During this phase, DNA synthesis or replication takes place. The amount of DNA becomes double during this phase, but the number of chromosomes remains the same.

c. G2 phase (Gap 2): During this phase, protein synthesis takes place. 

Question – 4 - What is G0 (quiescent phase) of cell cycle?

Answer: Quiescent Stage (G0): Cells which do not divide further, exit G1 phase to enter an inactive stage. This stage is called quiescent stage (G0) of the cell cycle. The cells in this stage remain metabolically active but do not undergo division. But these cells can resume division as and when required.

Question – 5 - Why is mitosis called equational division?

Answer: The number of chromosomes in daughter cells is same as in mother cell, so mitosis is called equational division.

Question – 6 - Name the stage of cell cycle at which one of the following events occur:

(a) Chromosomes are moved to spindle equator.

Answer: Metaphase

(b)Centromere splits and chromatids separate.

Answer: Anaphase

(c)Pairing between homologous chromosomes takes place.

Answer: Zygotene

(d)Crossing over between homologous chromosomes takes place.

Answer: Pachytene

Question – 7 - Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.

Answer: The pairing of chromosomes during zygotene is called synapsis. The synaptonemal complex formed by a pair of homologous chromosomes is called bivalent. The X-shaped structure, formed during crossing over is called chiasmata.

Question – 8 - How does cytokinesis in plant cells differ from that in animal cells?

Answer: In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. Thus, the cytoplasm is divided into two parts. In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.

Question – 9 - Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Answer: The four daughter cells formed after microsporogenesis in flowering plants are equal in size. The four daughter cells formed after megasporogenesis in flowering plants are unequal in size.

Question – 10 - Distinguish anaphase of mitosis from anaphase I of meiosis.

Answer: The centromere splits during anphase of mitosis, while it does not split during anaphase I of meiosis.

Question – 11 - List the main differences between mitosis and meiosis.

Answer:                              

Question – 12 - What is the significance of meiosis?

Answer: Significance of Meiosis:

• Conservation of specific chromosome number of each species is achieved across successive generations in sexually reproducing organisms through meiosis.

• Meiosis helps in increasing the genetic variations in the population of organisms from one generation to the next. 

Question – 13 - Discuss with your teacher about

(a)Haploid insects and lower plants where cell-division occurs, and

Answer: Male bees, wasps and ants are haploid as they are produced from unfertilized eggs.

(b)Some haploid cells in higher plants where cell-division does not occur.

Answer: Cell division does not happen in synergids and antipodal cells; in the ovule.

Question – 14 - Can there be mitosis without DNA replication in ‘S’ phase?

Answer: There cannot be mitosis without DNA replication, because additional DNAs are required for the formation of new cells.

Question – 15 - Can there be DNA replication without cell division?

Answer: DNA replication can take place without cells division; as in case of formation of new mitochondria and chloroplasts.

Question – 16 - Analyse the events during every stage of cell cycle and notice how the following two parameters change

(a)Number of chromosomes (N) per cell

Answer: Number of chromosomes becomes half after meiosis.

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

Answer: Amount of DNA becomes double after S phase.