Cell Cycle and Cell Division | ISC Class 11 Biology Notes

This note covers the cell cycle, interphase and the quiescent stage, C-value and chromosome number, mitosis, cytokinesis, meiosis, the substages of prophase I, and the differences and biological significance of the two kinds of division.

What is the cell cycle, and what must a dividing cell coordinate?

A cell is the basic structural and functional unit of life. Cell division produces daughter cells from a parental cell. Repeated growth and division allow a single cell to give rise to a population of cells and contribute to the growth of multicellular organisms.

DNA, or deoxyribonucleic acid, is the genetic material. A genome is the complete genetic material of a cell. DNA replication means copying DNA. Before division, the cell must coordinate this copying with growth and the production of other cell constituents.

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

Which cell structures matter here?

A eukaryotic cell has a membrane-bound nucleus. The nucleus contains chromosomes, structures carrying genetic material. The cytoplasm is the semifluid material of the cell outside the nucleus. An organelle is a cell structure with a particular function.

Chromatin is the DNA-containing material associated with proteins in the nucleus. It becomes compact and forms clearly visible chromosomes during division. Cytoplasmic growth is continuous, whereas DNA synthesis takes place during a particular stage of the cell cycle.

What are its two main phases?

Interphase is the interval between successive divisions. The M phase, meaning mitosis phase, is the period of actual division. It begins with karyokinesis, division of the nucleus, and usually ends with cytokinesis, division of the cytoplasm.

Mitosis is nuclear division that preserves chromosome number in the daughter nuclei. The events of division distribute replicated chromosomes to those nuclei. These events are under genetic control, so the cell cycle is an organised sequence rather than simply an increase in cell size.

What happens during interphase and the quiescent stage?

Interphase is sometimes called a resting phase, but the cell is actively preparing for division. It grows and copies its DNA in an ordered manner. Its three parts are G₁, meaning Gap 1; S, meaning synthesis; and G₂, meaning Gap 2.

How do G₁, S and G₂ differ?

G₁ phase lies between mitosis and the start of DNA replication. The cell is metabolically active, meaning that it carries out the chemical reactions needed for life. It grows continuously without replicating its DNA. Most organelle duplication also occurs during this phase.

S phase is the period of DNA synthesis. The amount of DNA per cell doubles, but chromosome number does not increase. In animal cells, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm.

A centriole is a cylindrical cell structure associated with division in animal cells. The centrosome is an organelle usually containing two centrioles. During G₂ phase, proteins are synthesised in preparation for mitosis, while cell growth continues.

How long does the cycle take?

Human cells in culture divide approximately every 24 hours. Within this average cycle, division proper lasts only about an hour, while interphase occupies more than 95% of the cycle. Duration varies between organisms and between cell types; yeast can complete it in only about 90 minutes.

What does G₀ mean?

G₀, the quiescent stage, is entered when cells leave G₁ and cease further division. They remain metabolically active but no longer proliferate, meaning increase in number by division, unless called on to do so according to the organism's requirements.

Some adult animal cells, such as heart cells, do not appear to exhibit division. Many other cells divide only occasionally, when replacement is needed after injury or cell death. Quiescence therefore describes a state of division, not the disappearance of cellular metabolism.

The circular drawing contains G₁, S and G₂ sectors and a smaller M-phase region divided into prophase, metaphase, anaphase, telophase and cytokinesis. An arrow indicates progression, and small cell drawings show one cell giving rise to two.

See Fig. 10.1 in your NCERT textbook

How do C-value, DNA content and chromosome number differ?

C-value is the amount of nuclear DNA in an unreplicated haploid chromosome complement. Here C denotes that reference amount. A haploid complement contains one chromosome set, represented by n; a diploid complement contains two sets, represented by 2n.

Thus, C describes DNA quantity, whereas n describes chromosome sets. For a diploid cell entering the cycle, 2C means twice the haploid reference DNA amount. After DNA replication, 4C means four times that reference amount. These symbols are not interchangeable.

Why does replication not double chromosome number?

After replication, each chromosome consists of two sister chromatids, the copies produced from that chromosome. They remain attached at the centromere, the primary constriction of the chromosome. Copying DNA therefore doubles DNA content without immediately producing twice as many separate chromosomes.

A cell that has 2n chromosomes during G₁ still has 2n chromosomes after S phase. The difference is that the chromosomes have replicated. G₂ follows this replication and prepares the cell for mitosis without another round of DNA copying.

For example, an onion root-tip cell has 16 chromosomes. It has 16 during G₁ and still 16 after S phase. Each daughter cell after mitosis also has 16. If its DNA content after division is 2C, the corresponding G₁, end-S and G₂ contents are 2C, 4C and 4C.

Note: State whether a value refers to the whole dividing cell, a daughter nucleus or a completed daughter cell. The table gives whole-cell values before mitosis and separate daughter-cell values after division has finished.

How does mitosis begin, and what happens in prophase?

Mitosis is called equational division because the chromosome number of the parent is retained in its daughter cells. It involves extensive reorganisation of cell components. Its four nuclear stages are convenient descriptions of a progressive process; very clear-cut boundaries cannot be drawn between them.

1. Prophase: Chromatin begins to condense into compact chromosomes, and the division apparatus develops.
2. Metaphase: Chromosomes become aligned at the cell's equator, the central plane between its opposite poles.
3. Anaphase: Sister chromatids separate and move towards opposite poles, the two ends of the division apparatus.
4. Telophase: The separated chromosome groups become enclosed in daughter nuclei and lose their compact appearance.

What changes identify prophase?

Prophase follows S and G₂. The previously intertwined chromosomal material becomes untangled as it condenses. Each compact chromosome consists of two chromatids joined at its centromere. The centrosomes duplicated during interphase begin moving towards opposite poles.

Each centrosome radiates microtubules, protein-based tubular components of the cell's internal framework, in a star-like arrangement called an aster. The two asters and the spindle fibres, fibres that attach to and move chromosomes, together form the mitotic apparatus.

The nuclear envelope is the double membrane surrounding the nucleus. The nucleolus is a non-membranous body inside the nucleus associated with ribosome formation. Ribosomes are structures that synthesise proteins. Neither the nucleolus nor the nuclear envelope is visible at the end of prophase.

The Golgi complex, a membranous structure involved in packaging cell products, and the endoplasmic reticulum, a membrane network involved in synthesis and transport, are also no longer visible at this stage. Their disappearance accompanies the reorganisation needed for chromosome separation.

The drawings are labelled Early Prophase, Late Prophase, Transition to Metaphase and Metaphase. Coloured chromosomes become compact; spindle fibres extend between opposite poles, and the final drawing places the chromosomes across the middle.

See Fig. 10.2a and b in your NCERT textbook

How do metaphase, anaphase and telophase distribute chromosomes?

After prophase, mitosis places chromosomes in an organised arrangement, separates their chromatids and rebuilds nuclei around the separated groups. These stages explain why prior DNA replication can produce daughter cells with the same chromosome number and genetic complement as the parent.

What happens at metaphase?

Complete disintegration of the nuclear envelope marks the start of metaphase. Chromosome condensation is complete, so chromosome morphology, meaning form and structure, is most easily studied at this stage. Each chromosome still has two sister chromatids held together at the centromere.

Kinetochores are small disc-shaped structures on the surface of centromeres. They provide attachment sites for spindle fibres. One sister chromatid connects through its kinetochore to fibres from one pole, and the other connects to fibres from the opposite pole.

The chromosomes move to the equator of the spindle. Their plane of alignment is called the metaphase plate. Alignment and attachment prepare the chromatids for separation; they do not themselves mean that separation has already happened.

What happens at anaphase?

At the beginning of anaphase, centromeres split simultaneously. Sister chromatids separate and are now called daughter chromosomes. They move towards opposite poles. Each chromosome travels with its centromere at the leading edge and its arms trailing behind.

The important change is therefore from two joined chromatids to two separate daughter chromosomes. This is different from metaphase, when the chromatids are still associated even though their kinetochores are connected to opposite poles.

What happens at telophase?

When the chromosomes reach their respective poles, they decondense, meaning become less compact, and lose their identity as distinct structures. A nuclear envelope develops around each chromosome group, producing two daughter nuclei. The nucleolus, Golgi complex and endoplasmic reticulum reform.

Anaphase shows two groups of chromosomes moving apart. Telophase shows two forming nuclei in a constricted cell. The final pair of separate cells is labelled Interphase, illustrating the cells after division.

See Fig. 10.2c to e in your NCERT textbook

How does cytokinesis differ in animal and plant cells?

Karyokinesis separates the replicated chromosomes into daughter nuclei. Cytokinesis separates the cytoplasm and completes the formation of daughter cells. The mechanisms differ because plant cells have a relatively inextensible cell wall, a rigid covering outside the cell membrane.

What happens in an animal cell?

A cleavage furrow, an inward groove, appears in the plasma membrane, the cell's limiting membrane. The furrow gradually deepens and joins in the centre. This divides the cytoplasm into two parts, each associated with a daughter nucleus.

What happens in a plant cell?

Wall formation starts in the centre and grows outwards to meet the existing lateral walls. It begins with a cell plate, the precursor of the new partition. This represents the middle lamella, the layer between the walls of adjacent cells.

During cytoplasmic division, organelles are distributed between daughter cells. These include mitochondria, organelles involved in energy production through respiration, and plastids, plant cell organelles that include pigment-bearing and nutrient-storing forms.

Must nuclear division be followed by cytoplasmic division?

In some organisms, karyokinesis is not followed by cytokinesis. A multinucleate condition, with several nuclei in a shared cytoplasm, results. Such a structure is called a syncytium. Liquid endosperm in coconut is an example; endosperm is tissue that nourishes the developing embryo.

This exception explains why nuclear division and cell division must be distinguished. Seeing daughter nuclei does not by itself establish that separate daughter cells have formed. The description of M phase therefore says that it usually ends with cytokinesis.

Why is mitosis important for growth, replacement and repair?

Mitosis usually produces diploid daughter cells with an identical genetic complement. It increases cell number while maintaining chromosome number. This makes it important both during the growth of an organism and when cells need to be replaced during its life.

How does it support growth and cellular balance?

Growth in multicellular organisms depends on mitosis. A growing cell also changes the relationship between the nucleus and the cytoplasm. Division helps restore the nucleo-cytoplasmic ratio, meaning the ratio of nuclear volume to cytoplasmic volume.

In plants, meristematic tissues are tissues containing actively dividing cells. Division in apical meristems at growing tips and in the lateral cambium, a lateral meristem, contributes to continuous plant growth throughout life.

How does it support replacement and repair?

Mitosis contributes to cell repair and replacement. Cells in the upper layer of the epidermis, the outer covering of the body, cells lining the gut and blood cells are constantly replaced. These examples connect division with maintenance as well as growth.

Is mitosis limited to diploid cells?

Mitosis is usually restricted to diploid cells, but this is not an absolute rule. In animals it is generally associated with diploid somatic cells, meaning body cells, with exceptions such as haploid cells of male honey bees.

Plants can show mitotic division in both haploid and diploid cells. The defining feature of mitosis is preservation of chromosome number, not an unavoidable diploid starting condition. A statement that mitosis always occurs only in diploid cells would therefore be incorrect.

What makes meiosis a reduction division?

Meiosis is a specialised division that reduces chromosome number by half and produces haploid daughter cells from a diploid cell. It establishes the haploid phase in the life cycle of sexually reproducing organisms. Fertilisation, the fusion of gametes (sex cells), restores the diploid phase.

Gametes are the sex cells that fuse during sexual reproduction. Each contributes a haploid chromosome complement. Reduction followed by fusion allows the characteristic chromosome number to be retained across generations instead of being repeatedly increased.

How are replication and division arranged?

Meiosis contains two successive cycles of nuclear and cell division, meiosis I and meiosis II, but just one cycle of DNA replication. Replication occurs in S phase before meiosis I and produces sister chromatids.

Homologous chromosomes are corresponding chromosomes of a pair, carrying the same kinds of genes at corresponding positions. A gene is a unit of heredity. The two homologues pair during meiosis I, whereas sister chromatids are the copies of one chromosome.

Non-sister chromatids belong to different chromosomes of a homologous pair. Genetic material can be exchanged between them, producing recombination, a rearrangement of genetic material. Pairing and recombination make meiosis I different from an ordinary mitotic division.

What is the overall outcome?

Meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids. At the end of meiosis II, four haploid cells have formed. Each division is described through prophase, metaphase, anaphase and telophase, with the first prophase being especially complex.

What happens in the five substages of prophase I?

Prophase I is typically longer and more complex than mitotic prophase. Its five substages are distinguished by chromosome behaviour. The sequence links chromosome condensation, pairing, exchange of genetic material and preparation for the separation of homologous chromosomes.

1. Leptotene: Chromosomes gradually become visible under a light microscope. Their compaction continues throughout this substage. This is the beginning of the visible chromosome changes of prophase I, before the pairing characteristic of the next substage.
2. Zygotene: Homologous chromosomes begin pairing. This association is called synapsis and is accompanied by a specialised structure called the synaptonemal complex. A synapsed homologous pair forms a bivalent, also called a tetrad because it contains four chromatids.
3. Pachytene: The four chromatids of each bivalent become distinct. Recombination nodules appear at sites of crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recombination is completed by the end of this substage.
4. Diplotene: The synaptonemal complex dissolves. Recombined homologous chromosomes tend to separate except at crossover sites. Their X-shaped connections are called chiasmata; one connection is a chiasma. In oocytes, or developing egg cells, of some vertebrates, diplotene can last months or years. Vertebrates are animals with a vertebral column.
5. Diakinesis: Chiasmata undergo terminalisation, their apparent movement towards chromosome ends. Chromosomes are fully condensed and the meiotic spindle assembles. By the end of this substage, the nucleolus disappears and the nuclear envelope breaks down, preparing the transition to metaphase I.

How are synapsis, crossing over and chiasmata related?

Synapsis brings homologous chromosomes together in zygotene. Crossing over occurs in pachytene and leaves the chromosomes linked at exchange sites. In diplotene, their tendency to separate makes the chiasmata recognisable. These are related events, but they are not names for the same process.

Crossing over is enzyme-mediated, meaning assisted by a biological catalyst. The enzyme involved is called recombinase. The first two substages, leptotene and zygotene, are relatively short-lived compared with pachytene. Do not confuse the time of exchange with the later visibility of chiasmata.

Draw chromosomes becoming visible in leptotene, paired homologues in zygotene, exchange between non-sister chromatids in pachytene, chiasmata between separating homologues in diplotene, and condensed chromosomes with terminalising chiasmata in diakinesis. Label each substage and keep homologous chromosomes distinct from sister chromatids.

How does meiosis I separate homologous chromosomes?

Meiosis I is the division that separates the two members of each homologous pair. Each chromosome has already replicated, so separating homologues is different from splitting the sister chromatids of one chromosome. Following that distinction through the stages explains the reduction in chromosome number.

1. Prophase I: Chromosomes condense, homologues pair and non-sister chromatids exchange genetic material. The five substages prepare the bivalents for the first division.
2. Metaphase I: Bivalent chromosomes align at the equatorial plate. Microtubules from opposite spindle poles attach to the kinetochores of homologous chromosomes.
3. Anaphase I: Homologous chromosomes separate and move towards opposite poles. Sister chromatids remain associated at their centromeres, so each moving chromosome retains both chromatids.
4. Telophase I: The nuclear membrane and nucleolus reappear, and cytokinesis follows. The resulting pair of cells is called a dyad.

What happens between the two divisions?

In many cases, chromosomes undergo some dispersion after the first division, but they do not reach the extremely extended state of an interphase nucleus. Interkinesis is the interval between the two meiotic divisions and is generally short-lived.

There is no DNA replication during interkinesis. The chromosomes entering meiosis II therefore retain chromatids produced by the earlier S phase. Interkinesis is followed by prophase II, which is much simpler than prophase I.

Arrows connect drawings labelled Prophase I, Metaphase I, Anaphase I and Telophase I. Paired coloured chromosomes align centrally, separate into opposite groups and enter two cells. The chromosomes in the two final cells still show paired chromatids.

See Fig. 10.3 in your NCERT textbook

How does meiosis II produce four haploid cells?

Meiosis II begins immediately after cytokinesis, usually before the chromosomes have fully elongated. It resembles a normal mitosis because it separates sister chromatids. However, it follows the reduction already achieved by meiosis I, so it begins with the products of that division.

What is the order of events?

1. Prophase II: Chromosomes become compact again. The nuclear membrane disappears by the end of this phase. This prepares the chromosomes for interaction with the spindle in the second division.
2. Metaphase II: Chromosomes align at the equator. Microtubules from opposite poles attach to the kinetochores of sister chromatids, placing those chromatids in position for movement towards opposite ends of the cell.
3. Anaphase II: Each centromere splits simultaneously with the others. Sister chromatids move towards opposite poles as the microtubules attached to their kinetochores shorten. This contrasts with anaphase I, when sister chromatids remained associated.
4. Telophase II: Nuclear envelopes surround the chromosome groups. Cytokinesis follows and produces four haploid daughter cells, collectively called a tetrad of cells.

Why must the two meanings of tetrad be distinguished?

During prophase I, tetrad refers to the four chromatids in a bivalent. At the completion of meiosis II, tetrad of cells refers to four daughter cells. One term describes chromosome organisation inside a cell; the other describes the cellular products of division.

Two rows of drawings show Prophase II, Metaphase II, Anaphase II and Telophase II. Chromosomes align centrally, chromatids move apart, and arrows lead to four separate cells on the right.

See Fig. 10.4 in your NCERT textbook

How do mitosis and meiosis differ, and why is meiosis significant?

Mitosis maintains chromosome number during the production of daughter cells. Meiosis reduces it and contributes to genetic variation. A useful comparison follows what pairs, what separates and how many divisions occur, instead of relying solely on the names of the stages.

How can reduction conserve chromosome number?

Meiosis halves the chromosome number. Fertilisation combines two haploid complements and restores the diploid number. These complementary changes conserve the characteristic chromosome number of a species across generations in sexual reproduction. Reduction must therefore be understood together with subsequent fusion.

Why does recombination matter?

Meiosis increases genetic variability, differences in genetic constitution within a population, from one generation to the next. Crossing over provides recombined genetic material. Such variation is important for evolution, change in populations over generations.

Mitosis usually gives daughter cells with an identical genetic complement, supporting continuity during growth and repair. Meiosis combines chromosome-number reduction with genetic recombination. Both depend on ordered chromosome movement, but their outcomes serve different roles in the organism's life cycle.

Glossary

• Cell cycle — Ordered sequence of genome duplication, synthesis of cell constituents and division into daughter cells.
• Interphase — Interval between successive M phases during which growth and DNA replication prepare the cell for division.
• Quiescent stage — Metabolically active state entered from G₁ in which cells cease proliferation unless called on to divide.
• C-value — Amount of nuclear DNA present in one unreplicated haploid chromosome complement.
• Sister chromatids — Copies produced by chromosome replication that remain connected at the centromere before separation.
• Kinetochore — Disc-shaped structure on the centromere surface that provides an attachment site for spindle fibres.
• Karyokinesis — Division of the nucleus that distributes chromosomes into the daughter nuclei.
• Cytokinesis — Division of the cytoplasm that completes the separation of daughter cells.
• Synapsis — Pairing of homologous chromosomes during the zygotene substage of prophase I.
• Bivalent — Pair of synapsed homologous chromosomes containing four chromatids after DNA replication.
• Crossing over — Exchange of genetic material between non-sister chromatids of homologous chromosomes during pachytene.
• Chiasmata — X-shaped connections at crossover sites where homologous chromosomes remain associated during diplotene.
• Interkinesis — Generally short-lived interval between meiosis I and meiosis II without DNA replication.
• Equational division — Division in which daughter cells retain the chromosome number of the parent cell.
• Syncytium — Multinucleate condition resulting when nuclear division is not followed by cytoplasmic division.

Common errors and misconceptions

• Misconception: Interphase is an inactive resting period. Correct: The cell grows, carries out metabolism and replicates DNA during its appropriate interphase stage.
• Misconception: DNA replication doubles chromosome number. Correct: S phase doubles DNA content from 2C to 4C in a diploid cycle, while chromosome number remains 2n.
• Misconception: G₀ cells have no metabolic activity. Correct: Quiescent cells remain metabolically active but do not proliferate unless called on to do so.
• Misconception: Mitosis always occurs only in diploid cells. Correct: Haploid cells can divide mitotically, including those of male honey bees; plants also show haploid mitosis.
• Misconception: Crossing over occurs during diplotene. Correct: It occurs during pachytene; chiasmata are recognised when homologues tend to separate in diplotene.
• Misconception: Sister chromatids separate during anaphase I. Correct: Homologous chromosomes separate then, while sister chromatids remain associated. Sister chromatids separate during anaphase II.
• Misconception: DNA is replicated again between meiotic divisions. Correct: Interkinesis has no DNA replication; one earlier replication is followed by two divisions.
• Misconception: Nuclear division always produces separate cells immediately. Correct: Cytokinesis must also occur for cell separation; nuclear division without it can produce a syncytium.

Exam-style questions with model answers

Q1. Define karyokinesis and cytokinesis, distinguishing what each divides. [2 marks]

1. Karyokinesis is nuclear division, distributing chromosomes into daughter nuclei.
2. Cytokinesis is division of the cytoplasm, completing the separation of daughter cells.

Q2. Describe the main events in G₁, S and G₂ of interphase. [3 marks]

1. During G₁, between mitosis and DNA replication, the cell is metabolically active and grows continuously, but does not replicate DNA.
2. During S phase, DNA is replicated and its amount doubles. Chromosome number remains unchanged; in animal cells the centriole also duplicates.
3. During G₂, cell growth continues and proteins are synthesised in preparation for the mitosis that follows this phase.

Q3. An onion root-tip cell has 16 chromosomes and DNA content 2C in G₁, where C denotes the DNA amount in an unreplicated haploid complement. It completes normal S phase and mitosis. State its chromosome number and DNA content after S phase, then the corresponding values in each daughter cell after cytokinesis. Explain each result. [4 marks]

1. After S phase, the chromosome number remains 16, because DNA replication produces joined sister chromatids without increasing chromosome number.
2. DNA content after S phase is 4C, because the supplied G₁ amount of 2C doubles during replication.
3. Each completed daughter cell has 16 chromosomes, because mitosis conserves the chromosome number of the parent cell.
4. Each daughter cell contains 2C DNA, because the replicated material is distributed between the two daughter cells during completed division.

Q4. Describe the five substages of prophase I in order, giving a characteristic chromosome event in each. [5 marks]

1. Leptotene: Chromosomes gradually become visible under the light microscope and continue to compact. This precedes the pairing of homologous chromosomes.
2. Zygotene: Homologous chromosomes pair by synapsis, accompanied by formation of the synaptonemal complex. The paired chromosomes constitute bivalents or tetrads.
3. Pachytene: Four chromatids become distinct, and crossing over occurs between non-sister chromatids at recombination nodules. Recombination is completed by its end.
4. Diplotene: The synaptonemal complex dissolves and homologues tend to separate, remaining joined at crossover sites visible as X-shaped chiasmata.
5. Diakinesis: Chiasmata terminalise, chromosomes become fully condensed and the spindle assembles. The nucleolus and nuclear envelope disappear by its end.

Q5. Compare cytokinesis in animal and plant cells using the initiating structure, starting position, direction of progression and completion. [4 marks]

1. Initiating structure: Animal cytokinesis begins with a furrow in the plasma membrane, whereas plant cytokinesis begins with formation of a cell plate.
2. Starting position: The animal furrow starts at the cell surface; the plant cell plate begins in the centre of the cell.
3. Direction: The animal furrow deepens inwards, while the plant partition grows outwards towards the existing lateral cell walls.
4. Completion: The animal furrow joins centrally to divide the cytoplasm; the plant partition meets the lateral walls and separates the daughter cells.

Q6. Give six differences between normal mitosis and meiosis: nuclear divisions, homologous pairing, crossing over, first anaphase, daughter-cell number and daughter-cell chromosome number. Assume cytokinesis is completed. [6 marks]

1. Nuclear divisions: Mitosis involves one nuclear division. Meiosis involves two successive nuclear divisions, called meiosis I and meiosis II, after one DNA replication.
2. Homologous pairing: Normal mitosis does not include synapsis. During meiosis, homologous chromosomes pair in zygotene of prophase I to form bivalents.
3. Crossing over: It is not a feature of normal mitosis. It occurs between non-sister chromatids of homologous chromosomes during pachytene of meiosis.
4. First anaphase: Mitotic anaphase separates sister chromatids after centromeres split. Anaphase I separates homologues while sister chromatids remain associated at their centromeres.
5. Daughter-cell number: Completed mitotic division produces two daughter cells, whereas the two completed meiotic divisions produce four daughter cells.
6. Chromosome number: Mitotic daughters retain the parental chromosome number. Meiotic daughters are haploid, with half the chromosome number of the original diploid cell.

Q7. Explain three aspects of the biological significance of meiosis: the haploid phase, chromosome-number conservation and genetic variability. [3 marks]

1. Meiosis produces haploid cells from a diploid cell by reducing chromosome number by half, establishing the haploid phase in sexual life cycles.
2. Fertilisation restores the diploid number by combining haploid complements. Together, reduction and fusion conserve the characteristic chromosome number across generations.
3. Meiosis increases genetic variability from one generation to the next. Recombination reshuffles genetic material, and variation is important for evolution.

Q8. Describe G₀ and state how its cells differ from cells actively progressing towards division. [2 marks]

1. G₀ is the quiescent stage entered when cells leave G₁ and cease further division.
2. These cells remain metabolically active but no longer proliferate unless called on to do so according to the organism's requirements.

Key takeaways

• The cell cycle coordinates genome duplication, synthesis of cell constituents and division into daughter cells.
• Interphase contains G₁, S and G₂; DNA replication occurs during S phase, while growth and preparation span interphase.
• DNA content doubles during S phase, but chromosome number remains unchanged because sister chromatids remain associated.
• Mitosis separates sister chromatids and maintains chromosome number, supporting growth, replacement and repair of cells.
• Animal cytokinesis uses an inward-deepening furrow; plant cytokinesis begins centrally with a cell plate growing outwards.
• Prophase I proceeds through leptotene, zygotene, pachytene, diplotene and diakinesis, with crossing over occurring during pachytene.
• Meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids without an intervening DNA replication.
• Meiosis and fertilisation together conserve chromosome number across generations, while meiotic recombination contributes to genetic variation.

Test yourself

Why is interphase not truly a resting period?

The cell is metabolically active, grows and prepares for division, with DNA replication occurring during its S phase.

Which mitotic stage is best for studying chromosome morphology, and why?

Metaphase, because chromosome condensation is complete and the chromosomes can be seen clearly under the microscope.

What attaches a chromosome to spindle fibres?

The kinetochore, a small disc-shaped structure on the centromere surface, provides the attachment site for spindle fibres.

How does a bivalent differ from one replicated chromosome?

A bivalent contains two paired homologous chromosomes and four chromatids. One replicated chromosome has two sister chromatids.

When do synapsis, crossing over and visible chiasmata occur?

Synapsis occurs in zygotene, crossing over in pachytene, and chiasmata are recognised in diplotene as homologues tend to separate.

What remains joined during anaphase I but separates during anaphase II?

Sister chromatids remain associated at their centromeres during anaphase I and separate when centromeres split during anaphase II.

Why is interkinesis different from the interphase before meiosis I?

No DNA replication occurs during interkinesis. The DNA needed for the two meiotic divisions was copied before meiosis I.

What is the outcome when karyokinesis is not followed by cytokinesis?

A multinucleate condition called a syncytium can form, as exemplified by the liquid endosperm in coconut.