Cell Cycle and Cell Division | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 10, Cell Cycle and Cell Division: the phases of the cell cycle, the events of interphase, the four stages of mitosis and cytokinesis, the significance of mitosis, the two divisions of meiosis with the five stages of prophase I, and the significance of meiosis. The stages are set out in order, with a table of how the chromosome number and DNA content change through the cycle.

What is the cell cycle?

All organisms, even the largest, start their life from a single cell. All cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time it divides. Cycles of growth and division allow a single cell to form a structure consisting of millions of cells.

Definition: The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells.

Cell division, DNA replication and cell growth have to take place in a coordinated way, to ensure correct division and the formation of progeny cells containing intact genomes. Two points about timing matter here.

• Cell growth, in terms of cytoplasmic increase, is a continuous process.
• DNA synthesis occurs only during one specific stage in the cell cycle.

The replicated chromosomes (DNA) are then distributed to the daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.

What are the phases of the cell cycle?

A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide once in approximately every 24 hours. The duration can vary from organism to organism and from cell type to cell type. Yeast, for example, can progress through the cell cycle in only about 90 minutes.

The cycle is drawn as a circle. Most of it is interphase, divided into three sectors marked G₁, S and G₂ in that order. A short stretch on the left, labelled M phase, is divided into five sectors marked prophase, metaphase, anaphase, telophase and cytokinesis. At the end of the M phase two small cells are drawn outside the circle, to show the formation of two cells from one cell, and a third cell is drawn above G₁ beside a curved arrow marked G₀.

See Fig. 10.1 in your NCERT textbook

Note: "Resting phase" is a misleading name for interphase. The cell is not resting. It is growing and replicating its DNA, and it spends more than 95 per cent of the cycle doing so.

What happens during interphase?

Interphase is divided into three further phases.

DNA content and chromosome number

During the S phase the amount of DNA per cell doubles: if the initial amount is denoted 2C, it increases to 4C. There is no increase in the chromosome number. If the cell had the diploid or 2n number of chromosomes at G₁, the number remains 2n even after the S phase.

Worked example: the onion root tip cell

1. An onion root tip cell has 16 chromosomes. The DNA content after the M phase is given as 2C. Find the chromosome number at G₁, after S and after M, and the DNA content at G₁, after S and at G₂.
2. Chromosome number does not change in the S phase or in mitosis, which is an equational division. So the number is 16 at G₁, 16 after the S phase and 16 in each cell after the M phase.
3. DNA content at G₁ is the same as after the M phase: 2C.
4. DNA doubles in the S phase, so it is 4C after S, and it stays 4C through G₂ until the cell divides.

The quiescent stage

Some cells in adult animals do not appear to divide, for example heart cells. Many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. Cells that do not divide further exit the G₁ phase to enter an inactive stage called the quiescent stage (G₀). Cells in this stage remain metabolically active but no longer proliferate unless called on to do so, depending on the requirement of the organism.

What is the M phase, and in which cells does mitosis occur?

The M phase is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same, mitosis is also called the equational division.

For convenience, the nuclear division (karyokinesis) is divided into four stages. Cell division is a progressive process, and very clear-cut lines cannot be drawn between the stages.

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Karyokinesis is followed by cytokinesis, the division of the cytoplasm.

What happens in prophase and metaphase?

Prophase

Prophase, the first stage of karyokinesis, follows the S and G₂ phases of interphase. In the S and G₂ phases the new DNA molecules formed are not distinct but intertwined. Prophase is marked by the initiation of condensation of chromosomal material, and the chromosomal material becomes untangled during chromatin condensation.

The completion of prophase is marked by these characteristic events.

• Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
• The centrosome, which had undergone duplication during interphase, begins to move towards opposite poles of the cell.
• Each centrosome radiates out microtubules called asters. The two asters together with the spindle fibres form the mitotic apparatus.

Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.

Metaphase

The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, so the chromosomes are spread through the cytoplasm of the cell. By this stage condensation of the chromosomes is completed, and they can be observed clearly under the microscope. This is the stage at which the morphology of chromosomes is most easily studied.

• A metaphase chromosome is made up of two sister chromatids, held together by the centromere.
• Small disc-shaped structures at the surface of the centromeres are called kinetochores. They serve as the sites of attachment of spindle fibres to the chromosomes.
• All the chromosomes come to lie at the equator. One chromatid of each chromosome is connected by its kinetochore to spindle fibres from one pole, and its sister chromatid is connected by its kinetochore to spindle fibres from the opposite pole.
• The plane of alignment of the chromosomes at metaphase is called the metaphase plate.

Four cells are drawn one below the other. In early prophase the chromosomes appear as long thin threads inside the nucleus. In late prophase they are shorter and thicker. In the transition to metaphase the nuclear envelope is gone, the chromosomes lie scattered and spindle fibres run from two poles. In metaphase all the chromosomes are lined up across the middle of the cell, attached to spindle fibres from both poles.

See Fig. 10.2 a and b in your NCERT textbook

What happens in anaphase and telophase?

Anaphase

At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously. The two daughter chromatids, now referred to as the daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, its centromere remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.

Telophase

At the beginning of telophase, the final stage of karyokinesis, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen, and each set of chromatin material tends to collect at each of the two poles.

Note: Telophase reverses prophase. In prophase the chromosomes condense and the nucleolus, nuclear envelope, golgi complexes and ER disappear. In telophase the chromosomes decondense and all four reappear.

How does cytokinesis differ in animal and plant cells?

Mitosis accomplishes the segregation of duplicated chromosomes into daughter nuclei (karyokinesis). The cell itself is then divided into two daughter cells by the separation of cytoplasm, called cytokinesis, at the end of which cell division is complete.

At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.

In some organisms karyokinesis is not followed by cytokinesis. A multinucleate condition then arises, leading to the formation of a syncytium, for example the liquid endosperm in coconut.

Note: The two mechanisms run in opposite directions. In an animal cell the furrow moves from the outside towards the centre. In a plant cell the cell-plate grows from the centre towards the outside.

What is the significance of mitosis?

Mitosis, the equational division, is usually restricted to diploid cells. In some lower plants and in some social insects, haploid cells also divide by mitosis.

• Identical cells: mitosis usually results in the production of diploid daughter cells with an identical genetic complement.
• Growth: the growth of multicellular organisms is due to mitosis.
• Nucleo-cytoplasmic ratio: cell growth disturbs the ratio between the nucleus and the cytoplasm, so it becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
• Cell repair: a very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, the cells of the lining of the gut, and blood cells are being constantly replaced.
• Continuous growth of plants: mitotic divisions in the meristematic tissues, the apical and the lateral cambium, result in a continuous growth of plants throughout their life.

What is meiosis, and what are its key features?

The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells.

Definition: Meiosis is the specialised kind of cell division that reduces the chromosome number by half and results in the production of haploid daughter cells.

Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms, whereas fertilisation restores the diploid phase. Meiosis is seen during gametogenesis in plants and animals. Its key features are as follows.

• Meiosis involves two sequential cycles of nuclear and cell division, called meiosis I and meiosis II, but only a single cycle of DNA replication.
• Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
• Meiosis involves pairing of homologous chromosomes, and recombination between non-sister chromatids of homologous chromosomes.
• Four haploid cells are formed at the end of meiosis II.

What happens in prophase I?

Prophase of the first meiotic division is typically longer and more complex than the prophase of mitosis. It is subdivided into five phases based on chromosomal behaviour.

1. Leptotene: the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
2. Zygotene: chromosomes start pairing together. This process of association is called synapsis, and such paired chromosomes are called homologous chromosomes. Synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
3. Pachytene: the four chromatids of each bivalent become distinct and clearly appear as tetrads. Recombination nodules appear; they are the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.
4. Diplotene: the synaptonemal complex dissolves, and the recombined homologous chromosomes of the bivalents tend to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata.
5. Diakinesis: marked by terminalisation of chiasmata. The chromosomes are fully condensed, and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis the nucleolus disappears and the nuclear envelope breaks down.

Definition: Crossing over is the exchange of genetic material between two homologous chromosomes. It is an enzyme-mediated process, and the enzyme involved is called recombinase.

Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.

What happens in the rest of meiosis I and in meiosis II?

In telophase I the chromosomes in many cases undergo some dispersion, but they do not reach the extremely extended state of the interphase nucleus. In contrast to meiosis I, meiosis II resembles a normal mitosis.

Figure 10.3 shows four stages from left to right. In prophase I the homologous chromosomes lie paired. In metaphase I the pairs are lined up at the equator, attached to spindle fibres. In anaphase I whole chromosomes, each still with two chromatids, move to opposite poles. In telophase I there are two cells. Figure 10.4 follows both of these cells through prophase II, metaphase II and anaphase II, in which the sister chromatids separate, to telophase II, which shows four cells.

See Figs. 10.3 and 10.4 in your NCERT textbook

Anaphase of mitosis and anaphase I of meiosis

What is the significance of meiosis, and how does it differ from mitosis?

• Meiosis is the mechanism by which the conservation of the specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process itself, paradoxically, results in a reduction of chromosome number by half.
• It increases the genetic variability in the population of organisms from one generation to the next.
• Variations are very important for the process of evolution.

How do you answer the NCERT exercise questions?

Number of chromosomes and amount of DNA through the cycle

The last exercise asks how the chromosome number and the DNA content per cell change. For a diploid cell that starts with 2n chromosomes and 2C of DNA:

Equal and unequal daughter cells in meiosis

The four daughter cells are equal in size when sperms are formed. They are unequal when an egg is formed: one large egg cell and small polar bodies result.

Glossary

• Cell cycle — The sequence of events by which a cell duplicates its genome, synthesises its other constituents and divides into two daughter cells.
• Interphase — The phase between two successive M phases, in which the cell grows and replicates its DNA.
• Karyokinesis — The division of the nucleus, corresponding to the separation of daughter chromosomes.
• Cytokinesis — The division of the cytoplasm, by which one cell becomes two daughter cells.
• Quiescent stage (G₀) — An inactive stage entered from G₁ by cells that do not divide further but remain metabolically active.
• Kinetochore — A small disc-shaped structure at the surface of the centromere where spindle fibres attach to the chromosome.
• Metaphase plate — The plane of alignment of the chromosomes at metaphase, at the equator of the spindle.
• Cell-plate — The simple precursor of the new cell wall in a dividing plant cell; it represents the middle lamella.
• Syncytium — A multinucleate condition that arises when karyokinesis is not followed by cytokinesis, as in the liquid endosperm of coconut.
• Synapsis — The pairing of homologous chromosomes during the zygotene stage of prophase I.
• Bivalent — The complex formed by a pair of synapsed homologous chromosomes; also called a tetrad.
• Crossing over — The exchange of genetic material between two homologous chromosomes, mediated by the enzyme recombinase.
• Chiasmata — The X-shaped structures seen in diplotene where homologous chromosomes remain attached at the sites of crossovers.
• Interkinesis — The generally short-lived stage between the two meiotic divisions, with no replication of DNA.

Common errors and misconceptions

• Misconception: The chromosome number doubles in the S phase. Correct: Only the amount of DNA doubles, from 2C to 4C. The chromosome number stays 2n.
• Misconception: Interphase is a resting phase in which nothing happens. Correct: The cell grows and replicates its DNA in interphase, which lasts more than 95 per cent of the cell cycle.
• Misconception: The nuclear envelope is present in metaphase. Correct: The complete disintegration of the nuclear envelope marks the start of metaphase.
• Misconception: In a plant cell cytokinesis takes place by a furrow. Correct: A furrow forms in animal cells. In plant cells a cell-plate forms in the centre and grows outward.
• Misconception: Crossing over takes place in zygotene. Correct: Pairing (synapsis) takes place in zygotene. Crossing over takes place in pachytene.
• Misconception: Sister chromatids separate in anaphase I. Correct: Homologous chromosomes separate in anaphase I. Sister chromatids separate in anaphase II.
• Misconception: DNA replicates again during interkinesis. Correct: There is no replication of DNA during interkinesis. Meiosis has only a single cycle of DNA replication.
• Misconception: Meiosis produces two haploid cells. Correct: Meiosis I produces a dyad of cells, and meiosis II produces a tetrad of four haploid cells.

Exam-style questions with model answers

Q1. What is the quiescent stage (G₀) of the cell cycle? [1 mark]

1. G₀ is an inactive stage that cells enter from the G₁ phase when they do not divide further. Such cells remain metabolically active but do not proliferate unless called on to do so.

Q2. Distinguish cytokinesis from karyokinesis. [2 marks]

1. Karyokinesis is the division of the nucleus, in which the daughter chromosomes separate into two daughter nuclei. It has four stages: prophase, metaphase, anaphase and telophase.
2. Cytokinesis is the division of the cytoplasm that follows, by which the cell itself is divided into two daughter cells.

Q3. Why is mitosis called equational division, and meiosis reduction division? [2 marks]

1. In mitosis the number of chromosomes in the parent and progeny cells is the same, so it is called equational division.
2. Meiosis reduces the chromosome number by half, producing haploid daughter cells from a diploid cell, so it is called reduction division.

Q4. Describe the events taking place during interphase. [3 marks]

1. G₁ phase: the interval between mitosis and the initiation of DNA replication. The cell is metabolically active and grows continuously but does not replicate its DNA.
2. S phase: DNA synthesis or replication takes place, and the amount of DNA per cell doubles from 2C to 4C, while the chromosome number stays the same. In animal cells the centriole duplicates in the cytoplasm.
3. G₂ phase: proteins are synthesised in preparation for mitosis, while cell growth continues.

Q5. How does cytokinesis in plant cells differ from that in animal cells? [3 marks]

1. In an animal cell a furrow appears in the plasma membrane, gradually deepens and ultimately joins in the centre, dividing the cytoplasm into two.
2. Plant cells are enclosed by a relatively inextensible cell wall, so wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls.
3. The new wall begins as a simple precursor called the cell-plate, which represents the middle lamella between the walls of two adjacent cells.

Q6. Describe synapsis, bivalent and chiasmata. [3 marks]

1. Synapsis is the pairing of homologous chromosomes during zygotene of prophase I. It is accompanied by the formation of the synaptonemal complex.
2. A bivalent, or tetrad, is the complex formed by a pair of synapsed homologous chromosomes. Its four chromatids become distinct at pachytene.
3. Chiasmata are the X-shaped structures seen at diplotene, when the recombined homologous chromosomes separate from each other except at the sites of crossovers.

Q7. Describe the four stages of karyokinesis in mitosis. [5 marks]

1. Prophase: chromosomal material condenses to form compact chromosomes, each of two chromatids attached at the centromere. The centrosomes move towards opposite poles and radiate asters, which with the spindle fibres form the mitotic apparatus.
2. By the end of prophase the golgi complexes, endoplasmic reticulum, nucleolus and nuclear envelope are no longer seen.
3. Metaphase: spindle fibres attach to the kinetochores of the chromosomes, and the chromosomes are moved to the spindle equator and aligned along the metaphase plate.
4. Anaphase: the centromeres split and the chromatids separate and move to opposite poles, with the centromere leading and the arms trailing.
5. Telophase: the chromosomes cluster at opposite poles and lose their identity as discrete elements. A nuclear envelope develops around each cluster, forming two daughter nuclei, and the nucleolus, golgi complex and ER reform.

Q8. Describe the five stages of prophase I of meiosis. [5 marks]

1. Leptotene: the chromosomes become gradually visible under the light microscope, and their compaction continues.
2. Zygotene: homologous chromosomes start pairing, a process called synapsis, accompanied by the formation of the synaptonemal complex. Each pair of synapsed chromosomes is a bivalent or tetrad.
3. Pachytene: the four chromatids of each bivalent become distinct. Recombination nodules appear, and crossing over occurs between non-sister chromatids of homologous chromosomes, mediated by the enzyme recombinase.
4. Diplotene: the synaptonemal complex dissolves and the homologous chromosomes separate except at the sites of crossovers, forming X-shaped chiasmata.
5. Diakinesis: the chiasmata terminalise, the chromosomes are fully condensed and the meiotic spindle is assembled. By its end the nucleolus disappears and the nuclear envelope breaks down.

Key takeaways

• The cell cycle has an interphase of G₁, S and G₂ and an M phase; in a 24-hour human cell cycle, division proper lasts only about an hour.
• In the S phase the DNA content doubles from 2C to 4C, but the chromosome number remains 2n.
• Cells that stop dividing leave G₁ for the quiescent stage G₀, where they stay metabolically active but do not proliferate.
• Mitosis has four stages of karyokinesis, prophase, metaphase, anaphase and telophase, followed by cytokinesis.
• Animal cells divide their cytoplasm by a furrow from the outside; plant cells form a cell-plate that grows outward from the centre.
• Meiosis has two divisions but a single DNA replication, and gives four haploid cells from one diploid cell.
• In prophase I, synapsis occurs in zygotene, crossing over in pachytene and chiasmata appear in diplotene.
• Homologous chromosomes separate in anaphase I, and sister chromatids separate in anaphase II.

Test yourself

How long does a yeast cell take to progress through the cell cycle?

Yeast can progress through the cell cycle in only about 90 minutes, compared with about 24 hours for human cells in culture.

In which phase does the centriole duplicate in an animal cell?

In animal cells the centriole duplicates in the cytoplasm during the S phase, when DNA replication begins in the nucleus.

At which stage is the morphology of chromosomes most easily studied?

The morphology of chromosomes is most easily studied at metaphase, when condensation of the chromosomes is completed.

What are asters?

Asters are the microtubules that radiate out from each centrosome; the two asters together with the spindle fibres form the mitotic apparatus.

Give one example of a syncytium.

The liquid endosperm in coconut is a syncytium, formed because karyokinesis is not followed by cytokinesis.

Which enzyme mediates crossing over?

Crossing over is an enzyme-mediated process, and the enzyme involved is called recombinase.

In which stage of prophase I does the synaptonemal complex dissolve?

The synaptonemal complex dissolves at the beginning of diplotene, when the chiasmata become visible.

What is formed at the end of telophase I and of telophase II?

A dyad of cells is formed at the end of telophase I, and a tetrad of four haploid cells at the end of telophase II.