Cell Cycle and Cell Division

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 10.

Why does a dividing cell spend almost all its time not dividing?

Take a human cell growing in a dish. It divides roughly once every 24 hours. The dramatic part, when chromosomes line up and are pulled apart, lasts only about one hour. The other 23 hours look like nothing is happening, and that is the surprise. Dividing is the easy part. Before a cell can split, it must copy its entire genome perfectly, build enough organelles and proteins for two cells, and check that nothing went wrong. This chapter follows that whole cycle, then shows how the same machinery is rewired in meiosis to make gametes.

The cell cycle: four phases, two big blocks

The cell cycle is the ordered sequence of events by which a cell duplicates its genome, makes the other things it needs, and divides into two daughter cells. Cell growth is continuous, but DNA synthesis happens in only one window, and the sharing out of chromosomes is controlled by genes.

The cycle has two blocks. Interphase is everything between two divisions and takes up more than 95 per cent of the time. The M phase (mitotic phase) is the actual division. Yeast, as a comparison, completes the whole cycle in about 90 minutes.

The cycle in one line: G1 → S → G2 → M (prophase → metaphase → anaphase → telophase → cytokinesis) → back to G1.

Why "resting phase" is a bad name. Interphase is often called the resting phase, but the cell is working harder than at any other time: it is growing, copying DNA and stocking up for division. "Resting" only means it is not visibly dividing.

The S-phase puzzle. DNA doubles but the chromosome number does not. Each chromosome is now made of two identical sister chromatids, held together at the centromere, so a cell with 2n chromosomes before S still has 2n chromosomes after it. Count centromeres, not DNA, when you count chromosomes.

G0: stepping off the treadmill. Some cells leave G1 and enter a quiescent stage called G0. They stay metabolically alive and do their job, but they do not divide unless the body needs them to. Many cells in adult animals, such as heart cells, are in G0, and others divide only occasionally to replace cells lost to injury or death.

Pause and predict: a student says, "G0 cells are dead, since they never divide." What is wrong with this?

Check your answer: G0 cells are alive and metabolically active, they have simply stopped proliferating. They can be called back into the cycle when the organism requires it.

Beyond the textbook. For the exam: the cell cycle is G1, S, G2 and M, and division is under genetic control. In reality: that genetic control works through checkpoints. A cell checks its DNA before entering S phase, again before entering M, and checks that every chromosome is properly attached to the spindle before anaphase begins. Proteins called cyclins and cyclin dependent kinases act as the switches. Why it matters: cancer is, in essence, a failure of these brakes, which is why the cell cycle sits at the centre of cancer research.

Counting DNA and chromosomes: the part everyone gets wrong

NTA loves to ask "what is the DNA content or chromosome number at this stage?" Work it out from two ideas. DNA doubles only in S phase and halves only when the cell splits. Chromosome number changes only when centromeres separate or when a nucleus splits.

Use an onion root tip cell, which has 16 chromosomes (2n = 16), and call its G1 DNA content 2C.

Mitosis: one nucleus becomes two identical copies

Mitosis is called equational division because the chromosome number in the daughter cells equals that of the parent. Karyokinesis is described in four stages, but remember it is one smooth movie, not four photographs. A good order to remember is PMAT: Prepare, Make a line, Apart, Two nuclei.

Beyond the textbook. For the exam: the complete disintegration of the nuclear envelope marks the start of metaphase, so write that. In reality: the envelope usually breaks down at the end of prophase, in a short transition that many textbooks call prometaphase, when the spindle first captures the chromosomes. Why it matters: if a question says "nuclear envelope disappears" without more, the stage is the end of prophase or the start of metaphase, so read the options and choose the one that fits.

Beyond the textbook. For the exam: centrosomes, asters and centrioles organise the spindle. In reality: the cells of higher plants have no centrioles and no asters, yet they still build a perfectly good spindle. Why it matters: it shows that the spindle can be organised without centrioles, so do not assume that every mitotic cell has them.

Cytokinesis: splitting the cytoplasm

Mitosis is not complete until the cytoplasm is divided too, and the mechanism depends on whether the cell has a wall.

Organelles such as mitochondria and plastids get shared between the daughter cells during this step.

Syncytium. Sometimes karyokinesis is not followed by cytokinesis. The result is a cell with many nuclei in one cytoplasm, called a syncytium. The liquid endosperm of coconut is the standard example, and it is the reason the coconut water you drink is full of free nuclei.

Where does mitosis happen? In animals, mainly in diploid somatic cells. A well known exception is the male honey bee, whose cells are haploid and still divide by mitosis. Plants show mitosis in both haploid and diploid cells.

Why mitosis matters. It is the basis of growth in multicellular organisms. As a cell grows, its volume rises faster than its surface, which upsets the nucleo-cytoplasmic ratio, and division restores it. Mitosis also repairs and replaces cells: the upper layer of epidermis, the lining of the gut and blood cells are renewed constantly. In plants, mitosis in the meristems (apical meristems and the lateral cambium) keeps the plant growing throughout life.

Pause and predict: why does a plant cell not form a cleavage furrow?

Check your answer: its rigid wall resists inward pinching, so the cell builds a new wall from the inside out. Cell plate formation starts in the middle and grows toward the old walls.

Meiosis: halving the chromosome number

Sexual reproduction joins two gametes. If each gamete were diploid, the chromosome number would double in every generation. Meiosis prevents this by halving the number in specialised diploid cells, producing haploid cells, and fertilisation restores diploidy. This is why meiosis, while it reduces the number, conserves the species' chromosome number across generations.

Four features define meiosis. It has two sequential divisions, meiosis I and meiosis II, but only a single round of DNA replication (in the S phase before meiosis I). Homologous chromosomes pair and non sister chromatids exchange segments. And it ends with four haploid cells.

Meiosis I: the reductional division

Prophase I is long and complex, so it is divided into five sub-stages according to chromosome behaviour.

Mnemonic for the five: Lines up, Zips, Passes, Dissolves, Detaches. The chromosomes line up as threads (leptotene), zip together (zygotene), pass segments across (pachytene), the zipper dissolves (diplotene) and the pairs detach towards the ends (diakinesis).

Why crossing over matters. Each crossover swaps real stretches of DNA between the maternal and paternal chromosome, so the chromatids that reach the gametes are new combinations that neither parent possessed. That is one of the reasons you do not look exactly like your siblings.

Beyond the textbook. For the exam: in the oocytes of some vertebrates, diplotene can last for months or years. In reality: in human females, primary oocytes begin meiosis I before birth and remain arrested at the diplotene stage until puberty, and each month an oocyte completes meiosis I shortly before ovulation, then pauses again until fertilisation triggers the completion of meiosis II. Why it matters: the long wait of the egg cell is one reason the chromosome errors you meet in Class XII become more likely with maternal age.

After prophase I, the remaining steps are short to describe and easy to confuse with mitosis, so track what each one separates.

Interkinesis is the short gap between the two meiotic divisions. There is no DNA replication in it, which is why meiosis has one S phase, not two.

Meiosis II: the equational division

Meiosis II is simpler and resembles a normal mitosis. Prophase II begins at once after cytokinesis, often before the chromosomes have fully elongated, and the nuclear membrane disappears by its end. In metaphase II, the spindle attaches to the kinetochores of sister chromatids. Anaphase II starts when the centromere of each chromosome splits simultaneously and sister chromatids move apart. Telophase II ends in nuclear envelopes reforming and cytokinesis gives a tetrad of cells: four haploid daughter cells.

Counting through meiosis. Take a human cell (2n = 46) with 2C DNA in G1, where C is the amount in a haploid gamete.

Pause and predict: a cell in metaphase II of a species whose gametes have 10 chromosomes. How many chromosomes and how many chromatids does it have?

Check your answer: 10 chromosomes and 20 chromatids. Meiosis I has already halved the chromosome number, but each chromosome still has two sister chromatids until anaphase II.

Mitosis versus meiosis

Why it matters. Meiosis is the reason sexual reproduction produces variety, and variation is the raw material of evolution. It is the cell level engine behind what you will meet in Class XII under inheritance and evolution.

Exam traps

60-second revision

Where this chapter connects

• Previous: Cell: The Unit of Life describes the centrosome, nucleus and chromosomes that you see moving here.
• Next: Photosynthesis in Higher Plants starts the plant physiology unit, which also depends on meristem activity covered in Plant Growth and Development.
• Class XII link: Sexual Reproduction in Flowering Plants and Human Reproduction use meiosis to form pollen, ovules, sperm and eggs, while Principles of Inheritance and Variation explains crossing over, linkage and chromosomal disorders.