Cell Cycle and Cell Division

Every new cell arises from an existing cell by division, and before a cell divides it must do two things: copy its DNA and grow. NCERT defines the cell cycle as 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. The three processes, DNA replication, cell growth and cell division, have to happen in a coordinated way so that each progeny cell receives an intact genome. That coordination is what the word 'cycle' captures: a dividing cell goes round the same sequence again and again.

Two numbers from this section are asked directly. A human cell growing in culture divides about once every 24 hours; this is the average cell cycle span for a mammalian cell. The duration varies from organism to organism and from cell type to cell type. Yeast, a single-celled fungus, can complete its whole cycle in about 90 minutes.

Two more ideas belong here. First, a cell that has just been produced by division is small; it grows through the cycle and reaches division size again. Second, DNA replication happens only once per cycle, at one defined point, and the chromosomes are then shared out so that each daughter gets one full copy. Everything else in the chapter is a description of how a cell achieves that. Keep the definition in NCERT's words; a 2-mark 'define the cell cycle' answer is expected to mention genome duplication, synthesis of other constituents, and division into two daughter cells.

The cycle has two basic phases. The M phase (mitosis phase) is when the actual division happens; it starts with the division of the nucleus (karyokinesis) and usually ends with the division of the cytoplasm (cytokinesis). The interphase is everything between two M phases. It used to be called the resting phase, but nothing rests: the cell grows and replicates its DNA in an orderly way. In a 24-hour human cell cycle, M phase lasts only about an hour; interphase takes more than 95 percent of the cycle.

Interphase is divided into three sub-phases.

• G₁ phase (Gap 1) is the interval between mitosis and the start of DNA replication. The cell is metabolically active and grows continuously but does not replicate its DNA.
• S phase (Synthesis) is when DNA replication occurs. The amount of DNA per cell doubles: if the cell starts with 2C it becomes 4C. The number of chromosomes does not change: a 2n cell stays 2n, each chromosome now having two sister chromatids. In animal cells the centriole also duplicates in the cytoplasm during S phase while DNA replicates in the nucleus.
• G₂ phase (Gap 2) is when proteins are synthesised in preparation for mitosis while cell growth continues.

Not every cell keeps cycling. In adult animals some cells, such as heart cells, do not divide at all; many others divide only occasionally, to replace cells lost by injury or death. Cells that do not divide further exit G₁ to enter an inactive stage called the quiescent stage, G₀. G₀ cells remain metabolically active but no longer proliferate unless called on to do so by the needs of the organism.

One more sentence from NCERT is a favourite MCQ. In animals, mitotic division is seen only in diploid somatic cells, with a few exceptions where haploid cells divide by mitosis, for example male honey bees. Plants, by contrast, show mitotic divisions in both haploid and diploid cells.

M phase is the most dramatic period of the cycle, involving a major reorganisation of virtually all components of the cell. Because the number of chromosomes in the parent and in the progeny cells is the same, mitosis is called equational division. A diploid parent (2n) gives two diploid daughters (2n); the DNA copied in S phase (4C) is shared so each daughter returns to 2C.

For convenience, mitosis is divided into four stages of nuclear division: prophase, metaphase, anaphase and telophase. NCERT stresses that this is only for description; mitosis is a continuous process and one stage flows into the next with no sharp boundary. Nuclear division (karyokinesis) is then followed by cytokinesis, the division of the cytoplasm, which completes cell division.

It helps to know what the cell is trying to do at each stage before you learn the details. Prophase packages the replicated chromatin into visible chromosomes and builds a spindle. Metaphase lines every chromosome up at the equator with its two chromatids facing opposite poles. Anaphase splits the centromeres and drags one chromatid of each chromosome to each pole. Telophase rebuilds a nucleus around each set. Then cytokinesis cuts the cell in two.

Examiners ask for the four stages with a labelled diagram of each (NCERT Figure 10.2), and for the 'key features' NCERT lists under metaphase, anaphase and telophase. Learn those lists word for word; they are the marking points. Also remember that mitosis in animals is usually restricted to diploid cells, while in some lower plants and in some social insects haploid cells divide by mitosis.

Prophase is the first stage of karyokinesis and follows the S and G₂ phases of interphase. At the start the chromosomal material is a diffuse, untangled mass. Through prophase it condenses to form compact mitotic chromosomes. Each chromosome is now seen to be composed of two chromatids attached together at the centromere, because the DNA was replicated in S phase.

The other job of prophase is to start assembling the mitotic spindle. The centrosome, which had duplicated during S phase, now begins to move to the opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with the spindle fibres form the mitotic apparatus.

By the end of prophase, the internal organisation of the cell has changed so much that, when viewed under the microscope, the cell does not show Golgi complexes, endoplasmic reticulum, nucleolus or the nuclear envelope. That sentence is asked as a 1-mark item ('which structures are not seen at the end of prophase?').

For a diagram, draw a cell with condensed thread-like chromosomes, each visibly double, scattered inside a fading nuclear envelope, with two centrosomes at opposite poles and short radiating microtubules (asters). Label the chromosome (two chromatids joined at a centromere), the centrosome, the aster and the forming spindle. The marking scheme rewards showing the two chromatids per chromosome and the centrosomes at opposite poles.

The complete disintegration of the nuclear envelope marks the start of metaphase. With no envelope in the way, the chromosomes spread through the cytoplasm of the cell. By now chromosome condensation is complete, so the chromosomes can be observed clearly; this is the stage at which chromosome morphology is most easily studied, and it is why karyotypes are prepared from metaphase cells.

Each metaphase chromosome is made of two sister chromatids held together by the centromere. On the surface of the centromere are small disc-shaped structures called kinetochores; they are the sites where the spindle fibres attach to the chromosomes. Metaphase is characterised by all the chromosomes coming to lie at the equator of the cell, with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid 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.

NCERT lists two key features of metaphase, and these are the two marks in any 'describe metaphase' question:

• Spindle fibres attach to the kinetochores of chromosomes.
• Chromosomes are moved to the spindle equator and get aligned along the metaphase plate through spindle fibres to both the poles.

In the diagram, draw the spindle as a football-shaped set of lines between two poles, with all chromosomes lined up in one plane across the middle. Show each chromosome as an X with its centromere on the plate and a fibre from each pole attached at the centromere. Label the spindle fibre, the kinetochore, the metaphase plate and the poles.

At the onset of anaphase, each chromosome arranged at the metaphase plate splits simultaneously at its centromere. 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, the centromere of each chromosome is towards the pole and hence leads, while the arms of the chromosome trail behind. This is why anaphase chromosomes look V-shaped, J-shaped or L-shaped depending on the position of the centromere.

NCERT's key features of anaphase:

• Centromeres split and chromatids separate.
• Chromatids move to opposite poles.

Two counting points matter in numerical questions. First, the moment the centromeres split, each former chromatid counts as an independent chromosome, so a 2n cell momentarily contains 4n chromosomes in anaphase, although the DNA content is still 4C and will be halved only by cytokinesis. Second, the number of chromosomes reaching each pole equals the original chromosome number of the cell (2n), which is exactly what makes mitosis equational.

Anaphase of mitosis is contrasted with anaphase I of meiosis in almost every exam (NCERT exercise 10). In mitotic anaphase the centromere divides and sister chromatids go to opposite poles, so each pole receives one chromatid from every chromosome. In anaphase I the centromere does not divide; whole homologous chromosomes, each still made of two chromatids, go to opposite poles, so each pole receives one member of every homologous pair. In the diagram, draw the two groups of V-shaped chromatids being pulled apart with the centromere leading, spindle fibres shortening towards the poles, and label the daughter chromosomes, the centromere and the spindle fibres.

Telophase is the final stage of karyokinesis. The chromosomes that have reached their respective poles decondense and lose their individuality. The chromatin material tends to collect in a mass at each of the two poles. Around each cluster the nuclear envelope reassembles, and the structures that had vanished by the end of prophase, the nucleolus, the Golgi complex and the endoplasmic reticulum, reform.

NCERT's key features of telophase, which are the marking points:

• Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
• The nuclear envelope assembles around the chromosome clusters.
• The nucleolus, Golgi complex and ER reform.

It is useful to see telophase as prophase run backwards: chromosomes condense in prophase and decondense in telophase; the envelope and nucleolus disappear in prophase and reappear in telophase; the spindle is built in prophase and dismantled in telophase. If a question asks you to compare the two, that mirror-image framing gives you three clean differences.

Telophase completes nuclear division: the cell now has two daughter nuclei, each with the same chromosome number as the parent nucleus. But it is still one cell. Division of the cytoplasm, cytokinesis, is a separate process that usually begins during telophase. For the diagram, draw a cell that has begun to elongate, with two clusters of decondensing chromatin at the poles, each surrounded by a new nuclear envelope with a nucleolus inside, and label the daughter nuclei, the reforming nuclear envelope and the nucleolus.

Mitosis segregates the duplicated chromosomes into two daughter nuclei; this is karyokinesis. The cell itself is divided into two daughter cells by a separate process, cytokinesis, at the end of which cell division is complete. NCERT exercise 2 asks you to distinguish the two: karyokinesis is division of the nucleus and cytokinesis is division of the cytoplasm; karyokinesis has four stages and cytokinesis has none; karyokinesis can occur without cytokinesis but cytokinesis normally follows karyokinesis.

Animal and plant cells divide their cytoplasm differently, and this difference is asked in nearly every paper (NCERT exercise 8).

• In an animal cell, cytokinesis is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre, dividing the cell cytoplasm into two. The furrow works from the outside inward, that is, centripetally.
• In a plant cell, the rigid cell wall cannot furrow. Wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls, that is, centrifugally. The formation of the new cell wall begins with a simple precursor called the cell plate, which represents the middle lamella between the walls of the two adjacent cells. The cell plate is laid down from Golgi-derived vesicles that fuse at the equator.

At the time of cytoplasmic division, organelles such as mitochondria and plastids are distributed between the two daughter cells.

Karyokinesis is not always followed by cytokinesis. In some organisms the nucleus divides repeatedly without the cytoplasm dividing, which leads to a multinucleate condition called a syncytium. NCERT's example is the liquid endosperm of coconut, a free-nuclear tissue with many nuclei in a common cytoplasm. This is also the answer to exercise 15: DNA replication without cell division does occur.

Mitosis is usually an equational division restricted to diploid cells, but NCERT reminds you again that in some lower plants and in some social insects haploid cells also divide by mitosis. Its significance is asked as a 3-mark question, and the examiner wants four clear points, ideally with NCERT's examples.

• Genetic constancy. The two daughter cells produced have the same chromosome number and the same genetic complement as the parent, so all somatic cells of an organism carry identical genetic information.
• Growth. Mitosis is the basis of the growth of multicellular organisms. A single-celled zygote becomes a body of trillions of cells purely by repeated mitotic divisions.
• Restoring the nucleo-cytoplasmic ratio. As a cell grows, the cytoplasm enlarges but the nucleus does not, which disturbs the ratio between the nucleus and the cytoplasm. Division restores the ratio, so mitosis is a corrective as much as a multiplying process. This point is the one most students miss.
• Cell repair and replacement. Cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced by mitosis.
• Continuous growth of plants. Mitotic divisions in the meristematic tissues, the apical meristems and the lateral cambium, result in the continuous growth of plants throughout their life.

Add, if asked for more, that mitosis is the mode of asexual reproduction in many organisms and the means of regeneration, but keep NCERT's points first because the marking scheme is drawn from them.

NCERT exercise 5 asks why mitosis is called equational division. The answer: because the number of chromosomes in the parent and progeny cells is the same, a diploid cell giving two diploid cells, since sister chromatids of each chromosome separate and one goes to each daughter.

Sexual reproduction involves the fusion of two gametes, each carrying a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells, so there must be a kind of cell division that reduces the chromosome number by half. That division is meiosis. Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms, and fertilisation restores the diploid phase. Meiosis takes place in diploid cells only.

NCERT lists four key features of meiosis; reproduce them in a 'what is meiosis' answer:

• 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 the pairing of homologous chromosomes and recombination between the non-sister chromatids of homologous chromosomes.
• Four haploid cells are formed at the end of meiosis II.

The two divisions each have the four familiar stages. Meiosis I: prophase I, metaphase I, anaphase I, telophase I. Meiosis II: prophase II, metaphase II, anaphase II, telophase II. Meiosis I is the reductional division, because homologous chromosomes separate and the chromosome number falls from 2n to n. Meiosis II is equational, like mitosis, because sister chromatids separate and the number stays n. In terms of DNA, a 4C cell becomes two 2C cells after meiosis I and then four 1C cells after meiosis II.

Why does the cell need two divisions? Because the one round of replication in S phase leaves each chromosome as two chromatids. Meiosis I halves the number of chromosomes by separating homologues; meiosis II is then needed to separate the chromatids so that each gamete carries single-chromatid chromosomes with 1C DNA.

Prophase I is typically longer and more complex than the prophase of mitosis. It is subdivided into five phases based on the behaviour of the chromosomes: leptotene, zygotene, pachytene, diplotene and diakinesis. Learn one defining event for each; this list is the single most examined item in the chapter.

• Leptotene. The chromosomes become gradually visible under the light microscope as long thin threads. Compaction of chromosomes continues throughout leptotene.
• Zygotene. Chromosomes start pairing together, a process called synapsis. The paired chromosomes are the homologous chromosomes. Electron micrographs show that synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The structure formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad (four chromatids). Bivalents are more clearly seen in the next stage.
• Pachytene. The four chromatids of each bivalent become distinct and clearly appear as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. It is an enzyme-mediated process; 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.
• Diplotene. The beginning of diplotene is recognised by the dissolution of the synaptonemal complex. 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. In oocytes of some vertebrates, diplotene can last for months or years.
• Diakinesis. The final stage of prophase I, marked by terminalisation of chiasmata, the movement of chiasmata towards the ends of the chromosomes. 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 also breaks down. Diakinesis represents the transition to metaphase I.

A common mnemonic is 'Little Zebras Pace Down Dales' for the order. When you write the 5-mark answer, give each substage its own line with the defining term in bold or underlined: visible threads, synapsis and synaptonemal complex, recombination nodules and crossing over with recombinase, chiasmata and dissolution of the complex, terminalisation and spindle assembly.

After diakinesis the nuclear envelope is gone and the spindle is ready. The rest of meiosis I looks like mitosis at a glance, but the unit that moves is the bivalent, not the single chromosome, and that changes everything.

Metaphase I. The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes; that is, each homologue of a bivalent is attached to fibres from one pole only, so its two sister chromatids face the same pole. Compare mitotic metaphase, where the two chromatids of one chromosome face opposite poles.

Anaphase I. The homologous chromosomes separate, while the sister chromatids remain associated at their centromeres. The centromere does not divide. Each pole receives one member of every homologous pair, still double-stranded. The chromosome number is halved here, which is why meiosis I is the reductional division, and which member of each pair goes to which pole is random, which is the independent assortment you will meet in Class 12 genetics.

Telophase I. The nuclear membrane and nucleolus reappear, and cytokinesis follows. The result is called a dyad of cells: two haploid cells, each chromosome with two chromatids, each cell with 2C DNA. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus.

Interkinesis. The stage between the two meiotic divisions is called interkinesis and is generally short-lived. There is no DNA replication during interkinesis; the chromosomes already have two chromatids from the S phase before meiosis I. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.

The anaphase I versus mitotic anaphase comparison (NCERT exercise 10) is the classic 2 or 3-mark question: in anaphase I homologous chromosomes separate, centromeres do not split, chromatids stay together, and each pole gets n chromosomes; in mitotic anaphase sister chromatids separate, centromeres split, and each pole gets 2n chromosomes.

Meiosis II is the equational half of meiosis. NCERT says it resembles a normal mitosis, and the stage-by-stage description makes that plain; the one thing to remember is that the cells entering it are already haploid, and no S phase precedes it.

• Prophase II. Meiosis II is initiated immediately after cytokinesis of meiosis I, usually before the chromosomes have fully elongated. The nuclear membrane disappears by the end of prophase II, and the chromosomes again become compact.
• Metaphase II. The chromosomes align at the equator, and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids. This is the mitotic pattern: the two chromatids of each chromosome face opposite poles.
• Anaphase II. Begins with the simultaneous splitting of the centromere of each chromosome, which was holding the sister chromatids together, allowing them to move towards opposite poles of the cell by the shortening of the microtubules attached to the kinetochores.
• Telophase II. Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope. Cytokinesis follows, resulting in the formation of a tetrad of cells, that is, four haploid daughter cells, each with n chromosomes and 1C DNA.

Two exercise questions hinge on meiosis II. Exercise 14 asks whether there can be mitosis without DNA replication in S phase. In the normal cycle, no: S-phase replication is what provides the two chromatids that mitosis separates, and without it daughter cells would receive half the genome. But meiosis II is exactly a mitosis-like division not preceded by replication, and that is the best example to give. Exercise 9 asks where the four meiotic products are equal in size and where they are unequal: they are equal in spermatogenesis (four spermatids) and in microsporogenesis (four microspores in a pollen tetrad); they are unequal in oogenesis, where one large ovum and small polar bodies form, and in megasporogenesis, where one of the four megaspores is functional and the other three degenerate.

Note the two uses of the word tetrad in this chapter: a tetrad of four chromatids is the bivalent of prophase I, and a tetrad of cells is the four haploid cells at the end of telophase II. State which one you mean.

NCERT gives two reasons why meiosis matters, and a 2-mark question expects both; a 3-mark question expects both with explanation.

• Conservation of chromosome number across generations. Meiosis is the mechanism by which the specific chromosome number of each species is conserved across generations in sexually reproducing organisms. This sounds paradoxical because meiosis itself reduces the chromosome number by half. The logic: gametes are haploid (n) only because of meiosis; when two gametes fuse at fertilisation the zygote is diploid (2n) again. Without meiosis, the chromosome number would double in every generation, 46 to 92 to 184 in humans.
• Genetic variability. Meiosis increases genetic variability in the population of organisms from one generation to the next. Two events produce it: crossing over in pachytene, which recombines alleles on homologous chromosomes so that no chromatid is exactly like a parental one, and the random orientation of bivalents at metaphase I, which makes the assortment of maternal and paternal chromosomes into gametes random. These variations are important for the process of evolution, because natural selection acts on variation.

You can add a third point drawn from the start of section 10.4: meiosis produces the haploid phase of the life cycle, that is, the gametes in animals and the spores in plants, while fertilisation restores the diploid phase. Together meiosis and fertilisation make alternation of the two phases possible.

For revision, hold the two divisions side by side. Mitosis keeps things the same: same number, same genes, for growth and repair. Meiosis changes things: half the number, new combinations, for sexual reproduction. Both start from one S phase; mitosis needs one division to share replicated chromosomes, meiosis needs two to first halve the number and then share the chromatids. Every difference in the mitosis-versus-meiosis table comes back to that.

NCERT exercise 16 asks you to analyse every stage of the cell cycle and note how two parameters change: the number of chromosomes per cell (N) and the amount of DNA per cell (C). Case-based questions are built on this, so learn the table rather than working it out under pressure. Use a human cell, 2n = 46, as the running example.

• G₁: 2n = 46 chromosomes, each with one chromatid; 2C DNA.
• S: DNA replicates; 2C rises to 4C; still 46 chromosomes, now each with two chromatids.
• G₂: 2n = 46, 4C.
• Mitosis, prophase and metaphase: 46 chromosomes, 4C, 92 chromatids.
• Mitotic anaphase: centromeres split; 92 chromosomes in the cell, 46 moving to each pole; 4C in the cell, 2C per pole.
• After telophase and cytokinesis: each daughter has 2n = 46 and 2C. The cycle is back where it started.
• Meiosis, prophase I and metaphase I: 2n = 46, 4C, 23 bivalents.
• After meiosis I (dyad): each cell n = 23 chromosomes, each two chromatids; 2C.
• Metaphase II: n = 23, 2C.
• After meiosis II (tetrad): each cell n = 23, single chromatids; 1C.

Two rules make any variant easy. First, chromosome number changes only when centromeres split (mitotic anaphase, anaphase II, doubling the count momentarily) or when homologues separate (anaphase I, halving it). Second, DNA content changes only in S phase (doubles) and at each cytokinesis (halves per cell). So if a question says a cell has 8 pg of DNA in G₁, it has 16 pg in G₂ and metaphase, 8 pg per daughter after mitosis, 8 pg per cell after meiosis I and 4 pg per gamete. If an onion cell has 2n = 16, its metaphase shows 16 chromosomes and 32 chromatids; its gametes carry 8 chromosomes.

Also note the general case of DNA replication without division: polytene chromosomes in Drosophila salivary glands and the multinucleate coconut endosperm both replicate DNA without completing cell division, and the DNA content per nucleus or per cell rises above 4C.