Cell Cycle and Cell Division: NEET notes.

01 · NCERT §10.1

The cell cycle and its phases

• Cells reproduce by splitting into two, and the daughters can grow and split again. Repeated rounds of growth and division let one cell build a body of millions of cells.
• During a division the cell must also copy its DNA and grow, and these three processes are coordinated so that every daughter receives an intact genome.
• The cell cycle is the ordered series of events in which a cell copies its genome, makes its other components and finally divides into two daughter cells.
• Growth of the cytoplasm goes on continuously, but DNA is synthesised in only one particular stage of the cycle. The copied chromosomes are then shared out to the daughter nuclei, and all of these events are under genetic control.
• Human cells grown in culture, a typical eukaryotic example, complete one cycle in about 24 hours. The length varies between organisms and between cell types: yeast can finish a cycle in only about 90 minutes.
• The cycle has two basic phases: interphase, the stretch between two successive divisions, and the M phase (mitosis phase), when the cell actually divides.
• Of the roughly 24-hour human cycle, division proper takes only about an hour. Interphase takes up more than 95% of the cycle.
• The M phase begins with karyokinesis, the division of the nucleus in which the daughter chromosomes separate, and usually ends with cytokinesis, the division of the cytoplasm.

02 · NCERT §10.1.1

Interphase: G1, S and G2

• Interphase is often called the resting phase, but the cell is far from idle: it is getting ready to divide by growing and by replicating its DNA in an orderly way.
• Interphase has three parts in order: G1 (Gap 1), S (Synthesis) and G2 (Gap 2).
• G1 is the interval between the end of mitosis and the start of DNA replication. The cell is metabolically active and keeps growing, but it does not copy its DNA. Most organelle duplication also happens in this phase.
• S phase is when DNA is synthesised (replicated). The DNA per cell doubles: if the starting amount is called 2C, it becomes 4C.
• The chromosome number does not rise in S phase. A cell that is diploid (2n) in G1 is still 2n after S; each chromosome now simply has two copies of its DNA.
• In animal cells, DNA replication starts in the nucleus during S phase while the centriole duplicates in the cytoplasm.
• In G2 the cell makes proteins in preparation for mitosis, and growth continues.
• Onion root tip cells have 16 chromosomes. They have 16 in G1, still 16 after S, and 16 in each daughter after M. If the DNA content after M phase is 2C, it is 2C in G1 and 4C after S and in G2.

03 · NCERT §10.1.1

Quiescent stage and which cells divide

• Some cells of adult animals do not seem to divide at all, heart cells for example, and many others divide only now and then, to replace cells lost to injury or cell death.
• Cells that stop dividing leave G1 and enter an inactive stage of the cycle called the quiescent stage, G0.
• Cells in G0 stay metabolically active but no longer multiply, unless the organism needs them to and calls them back into division.
• In animals, mitosis is seen only in diploid somatic cells. The exception NCERT names is the male honey bee, whose haploid cells divide by mitosis.
• Plants, by contrast, can show mitosis in both haploid and diploid cells, as the alternation of generations in plants requires.
• Plants keep growing all their lives because their meristematic tissues keep dividing, while most cells elsewhere in the plant do not.

04 · NCERT §10.2.1–10.2.2

Prophase and metaphase

• Mitosis is called equational division because parent and daughter cells have the same number of chromosomes. Its nuclear division (karyokinesis) is split into four stages, prophase, metaphase, anaphase and telophase, but the process is continuous and the boundaries between stages are not sharp.
• Prophase follows the S and G2 phases. Straight after S and G2 the new DNA molecules are intertwined and not distinct; prophase begins as the chromosomal material starts to condense and gets untangled.
• By the end of prophase the chromosomes are compact, and each is seen to consist of two chromatids joined at the centromere.
• The centrosome, duplicated during S phase, starts moving towards opposite poles. Each centrosome sends out microtubules called asters, and the two asters with the spindle fibres form the mitotic apparatus.
• Under the microscope a cell at the end of prophase shows no Golgi complex, no endoplasmic reticulum, no nucleolus and no nuclear envelope.
• Metaphase begins with the complete disintegration of the nuclear envelope, so the chromosomes lie spread in the cytoplasm. Condensation is now complete.
• Metaphase is the stage at which chromosome morphology is easiest to study. Each metaphase chromosome is two sister chromatids held together by the centromere.
• Kinetochores are small disc-shaped structures on the surface of the centromere, where spindle fibres attach.
• All the chromosomes come to lie at the equator: one chromatid of each chromosome is attached by its kinetochore to fibres from one pole, and its sister to fibres from the other pole. The plane of alignment is the metaphase plate.

05 · NCERT §10.2.3–10.2.4

Anaphase and telophase

• At the start of anaphase every chromosome on the metaphase plate splits at the same moment: the centromeres split and the two chromatids separate.
• The separated chromatids are now called daughter chromosomes, one for each future daughter nucleus, and they move to opposite poles.
• As a chromosome moves, its centromere points towards the pole and leads, while the arms trail behind.
• For a 2n cell, each pole receives a complete set of 2n single-chromatid chromosomes.
• In telophase the chromosomes that have reached the poles decondense and lose their individuality. They can no longer be seen one by one; the chromatin collects as a mass at each pole.
• A nuclear envelope forms around each cluster, giving two daughter nuclei, and the nucleolus, Golgi complex and ER reappear.

06 · NCERT §10.2.5

Cytokinesis

• Karyokinesis separates the duplicated chromosomes into two nuclei; cytokinesis then divides the cytoplasm, and with it the cell division is complete.
• In an animal cell a furrow appears in the plasma membrane. It deepens steadily until its edges meet in the centre, cutting the cytoplasm in two.
• A plant cell sits inside a cell wall that hardly stretches, so it divides another way: the new wall is laid down first at the middle of the cell and spreads outwards until it reaches the old side walls.
• The new plant wall starts as a simple precursor, the cell plate, which becomes the middle lamella between the walls of the two neighbouring cells.
• While the cytoplasm divides, organelles such as mitochondria and plastids are shared between the two daughter cells.
• Sometimes the nucleus divides but the cytoplasm does not. The result is a cell with many nuclei, a syncytium; the liquid endosperm of coconut is NCERT's example.

07 · NCERT §10.3

Significance of mitosis

• Mitosis usually happens only in diploid cells, but some lower plants and some social insects have haploid cells that divide by mitosis too.
• Mitosis usually produces diploid daughter cells with identical genetic complements.
• Mitosis is what makes a multicellular organism grow.
• As a cell grows, the ratio between its nucleus and its cytoplasm is disturbed; dividing restores the nucleo-cytoplasmic ratio.
• Mitosis repairs and replaces cells: the outermost epidermal cells, the cells lining the gut and blood cells are replaced all the time.
• In plants, mitosis in the meristems, the apical meristem and the lateral cambium, lets the plant keep growing throughout its life.

08 · NCERT §10.4

Meiosis and its key features

• Sexual reproduction joins two gametes, each carrying a full haploid set of chromosomes. The gametes come from special diploid cells.
• Meiosis is the special division that halves the chromosome number and so produces haploid daughter cells. It creates the haploid phase of the life cycle, and fertilisation restores the diploid phase.
• Meiosis happens during gametogenesis in plants and animals and gives rise to haploid gametes.
• It has two successive rounds of nuclear and cell division, meiosis I and meiosis II, but only one round of DNA replication.
• Meiosis I begins after the parent chromosomes have replicated in S phase to give identical sister chromatids.
• Homologous chromosomes pair, and recombination takes place between non-sister chromatids of the homologues.
• At the end of meiosis II there are four haploid cells.
• Each division has four stages: prophase, metaphase, anaphase and telophase I in meiosis I, and prophase, metaphase, anaphase and telophase II in meiosis II.

09 · NCERT §10.4.1

Prophase I

• Prophase I is longer and more complex than the prophase of mitosis. By how the chromosomes behave it is split into five stages, in order: leptotene, zygotene, pachytene, diplotene and diakinesis.
• Leptotene: the chromosomes gradually become visible under the light microscope, and they keep compacting all through this stage.
• Zygotene: homologous chromosomes begin to pair up, a process called synapsis. Electron micrographs show that synapsis comes with a complex structure, the synaptonemal complex.
• Two homologues joined by synapsis form a bivalent, also called a tetrad; bivalents become clearer in the next stage. Leptotene and zygotene are short compared with pachytene.
• Pachytene: the four chromatids of each bivalent become distinct and clearly appear as tetrads. Recombination nodules appear; these are the sites where crossing over happens between non-sister chromatids of the homologues.
• Crossing over means homologous chromosomes swap pieces of genetic material. It is enzyme-mediated, the enzyme being recombinase, and it recombines the genetic material on the two chromosomes.
• Recombination is finished by the end of pachytene, and the homologues stay linked at the crossover sites.
• Diplotene begins when the synaptonemal complex dissolves. The recombined homologues of each bivalent tend to move apart, except where they crossed over; these X-shaped links are the chiasmata. In some vertebrates, an oocyte can stay in diplotene for months or even years.
• Diakinesis, the last stage, is marked by terminalisation of the chiasmata. Condensation of the chromosomes is now complete, and the meiotic spindle is assembled to get the homologues ready to separate.
• By the end of diakinesis the nucleolus has disappeared and the nuclear envelope has broken down; diakinesis is the transition into metaphase I.

10 · NCERT §10.4.1

Metaphase I to interkinesis

• Metaphase I: the bivalents line up on the equatorial plate. Microtubules from opposite spindle poles attach to the kinetochores of the homologous chromosomes.
• Anaphase I: the homologous chromosomes separate and move to opposite poles, while the sister chromatids stay joined at their centromeres.
• So each pole gets half as many chromosomes as the parent cell had, and every one of those chromosomes still has two chromatids.
• Telophase I: a nuclear membrane and a nucleolus form again, and cytokinesis follows to give a dyad of cells.
• In many cases the chromosomes spread out a little in telophase I, but they never reach the fully extended state of an interphase nucleus.
• Interkinesis is the stage between the two meiotic divisions. It is generally short, and there is no DNA replication in it.
• Interkinesis is followed by prophase II, which is much simpler than prophase I.

11 · NCERT §10.4.2

Meiosis II

• Meiosis II begins straight after cytokinesis, generally while the chromosomes are still not fully stretched out. Unlike meiosis I, it resembles an ordinary mitosis.
• Prophase II: the chromosomes become compact again, and the nuclear membrane has disappeared by the end of this stage.
• Metaphase II: the chromosomes line up at the equator, and microtubules from opposite poles attach to the kinetochores of the sister chromatids.
• Anaphase II: the centromere of each chromosome, which had been holding the sister chromatids together, splits simultaneously in all chromosomes, and the chromatids move to opposite poles as the microtubules attached to the kinetochores shorten.
• Telophase II: the two groups of chromosomes are enclosed by nuclear envelopes again, and cytokinesis follows, giving a tetrad of cells, four haploid daughter cells.
• Counting through it: a 2n cell with 4C DNA gives two cells of n chromosomes (2C each, chromatids still paired) after meiosis I, and four cells of n chromosomes (C each) after meiosis II.

12 · NCERT §10.5

Significance of meiosis

• Meiosis keeps the chromosome number of each species constant from one generation to the next in sexually reproducing organisms, even though the division itself halves that number.
• Halving in meiosis followed by restoring at fertilisation is what keeps the count steady.
• Meiosis also increases genetic variability in the population from one generation to the next; crossing over in prophase I, for instance, produces new combinations of genetic material on the chromosomes.
• Variation is very important for the process of evolution.
• Mitosis versus meiosis in short: mitosis is one division giving two identical diploid cells; meiosis is two divisions after one DNA replication, giving four haploid cells, with pairing and crossing over of homologues in prophase I.

Revise

Must-know facts

1. 01Human cells in culture divide about once every 24 hours; yeast can complete a cycle in about 90 minutes.
2. 02In the 24-hour human cycle, cell division proper takes only about an hour; interphase is more than 95% of the cycle.
3. 03Interphase order: G1 → S → G2. Then the M phase: karyokinesis, then cytokinesis.
4. 04S phase: DNA 2C → 4C, but the chromosome number stays 2n.
5. 05The centriole duplicates in the cytoplasm during S phase in animal cells.
6. 06G0 (quiescent stage) is entered from G1; cells stay metabolically active but stop multiplying. Example: heart cells.
7. 07In animals mitosis occurs only in diploid somatic cells; exception: male honey bees (haploid).
8. 08Plants show mitosis in both haploid and diploid cells.
9. 09Mitosis is equational division: parent and daughters have the same chromosome number.
10. 10End of prophase: no Golgi complex, ER, nucleolus or nuclear envelope visible.
11. 11Metaphase: the easiest stage for studying chromosome morphology; kinetochores are disc-shaped structures on the centromere.
12. 12Anaphase of mitosis: centromeres split, chromatids separate and move to opposite poles, centromere leading.
13. 13Telophase: chromosomes decondense; nuclear envelope, nucleolus, Golgi complex and ER reform.
14. 14Plant cytokinesis: cell plate forms in the centre and grows outward; it represents the middle lamella.
15. 15Karyokinesis without cytokinesis gives a syncytium, e.g. liquid endosperm of coconut.
16. 16Onion root tip cells have 16 chromosomes.
17. 17Meiosis: two divisions, one DNA replication, four haploid cells.
18. 18Prophase I order: leptotene, zygotene, pachytene, diplotene, diakinesis.
19. 19Synapsis and the synaptonemal complex appear in zygotene; a synapsed pair is a bivalent or tetrad.
20. 20Crossing over happens in pachytene, between non-sister chromatids, at recombination nodules, by the enzyme recombinase.
21. 21Chiasmata (X-shaped) become visible in diplotene after the synaptonemal complex dissolves; diplotene can last months or years in some vertebrate oocytes.
22. 22Diakinesis: terminalisation of chiasmata.
23. 23Anaphase I separates homologues; anaphase II separates sister chromatids.
24. 24Interkinesis is short and has no DNA replication.
25. 25Meiosis I ends in a dyad of cells; meiosis II ends in a tetrad of four haploid cells.

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Common traps

Glossary

Key terms

Cell cycle

The ordered series of events in which a cell copies its genome, makes its other parts and divides into two.

Interphase

The long stretch between two divisions, made up of G1, S and G2, when the cell grows and copies its DNA.

Karyokinesis

Division of the nucleus, in which the separated chromosomes are shared between two daughter nuclei.

Cytokinesis

Division of the cytoplasm that completes cell division.

G0 (quiescent stage)

A stage entered from G1 by cells that stop multiplying while staying metabolically active.

Chromatid

One of the two identical copies of a replicated chromosome, joined to its sister at the centromere.

Centromere

The point where the two sister chromatids of a chromosome are held together.

Kinetochore

Small disc-shaped structure on the centromere where spindle fibres attach.

Metaphase plate

The plane at the cell's equator in which chromosomes line up at metaphase.

Mitotic apparatus

The two asters together with the spindle fibres.

Cell plate

The precursor of the new wall in a dividing plant cell; it becomes the middle lamella.

Syncytium

A multinucleate cell formed when nuclear division is not followed by cytokinesis.

Synapsis

The pairing of homologous chromosomes during zygotene.

Synaptonemal complex

The structure formed along a pair of homologues during synapsis; it dissolves at the start of diplotene.

Bivalent (tetrad)

A pair of synapsed homologous chromosomes, four chromatids in all.

Recombination nodule

A site in pachytene where crossing over occurs between non-sister chromatids.

Crossing over

Enzyme-driven exchange of genetic material between homologous chromosomes; the enzyme is recombinase.

Chiasmata

X-shaped points where recombined homologues stay linked after the synaptonemal complex dissolves.

Terminalisation

The movement of chiasmata towards the chromosome ends during diakinesis.

Interkinesis

The short gap between meiosis I and meiosis II, with no DNA replication.

Dyad and tetrad of cells

Two cells after meiosis I; four haploid cells after meiosis II.