Cell Cycle and Cell Division

A cell does not divide at random; it passes through an orderly, repeating sequence of events called the cell cycle, in which it grows, copies its DNA and then divides into two daughter cells. In a typical human cell the whole cycle takes about 24 hours. The cycle has two broad phases — a long preparatory interphase and a short dividing M phase (mitosis) — and a common NEET fact is that interphase occupies most of the time (about 23 of the 24 hours), with the M phase lasting only about an hour.

Interphase, the so-called resting phase, is actually very active and is divided into three sub-phases. G₁ (Gap 1) follows the previous division: the cell is metabolically active and grows, but does not replicate its DNA. S (Synthesis) phase is when DNA replication takes place — the DNA content doubles (from 2C to 4C), although the chromosome number does not change because the two copies stay joined as sister chromatids; in animal cells the centriole also duplicates in S phase.

G₂ (Gap 2) is the second growth phase, during which the cell synthesises proteins and organelles and prepares for division. Some cells leave the cycle altogether and enter an inactive G₀ (quiescent) stage; they remain metabolically active but stop dividing — mature neurons and heart muscle cells are classic examples. Recognising what does (and does not) happen in each sub-phase — especially that DNA is made only in S phase — is essential.

Different cells run the cycle at very different speeds: yeast can complete a cycle in about 90 minutes, while some human cells divide only once in a year or so. The cell cycle is tightly controlled at checkpoints so that, for instance, a cell does not enter mitosis before its DNA has been completely and correctly copied. This regulation prevents errors; its failure can lead to uncontrolled division, the basis of cancer. For NEET, fix the order G₁ → S → G₂ → M, the fact that S phase = DNA replication, and the meaning of G₀.

Mitosis is the division of the M phase that produces two daughter cells with the same chromosome number as the parent — for this reason it is called the equational division. It occurs in the somatic (body) cells and underlies growth, repair and the replacement of worn-out cells. Mitosis is divided into nuclear division (karyokinesis) in four stages, followed by division of the cytoplasm (cytokinesis).

In prophase, the threadlike chromatin condenses into compact chromosomes, each already consisting of two sister chromatids joined at the centromere (the DNA having been copied in the preceding S phase). The mitotic spindle begins to form, the centrioles move toward opposite poles, and by the end of prophase the nuclear envelope and nucleolus disappear. In metaphase, the chromosomes become most condensed and line up at the equator of the cell (the metaphase plate); spindle fibres attach to the kinetochores at the centromeres.

Anaphase is the decisive step: the centromeres split, the two sister chromatids of each chromosome separate, and these daughter chromosomes are pulled to opposite poles of the cell. ('Anaphase = centromere splits, chromatids separate' is one of the most frequently asked single facts.) In telophase, the chromosomes reach the poles, decondense back into chromatin, and the nuclear envelope and nucleolus reappear, giving two nuclei.

Finally, cytokinesis splits the cytoplasm into two cells. In animal cells this happens by a cleavage furrow that pinches the cell inward, whereas in plant cells a cell plate forms from the centre outward (because the rigid wall cannot pinch). The significance of mitosis is that it produces genetically identical cells for growth, repair and the replacement of cells, and in plants it forms new tissues during vegetative growth — all while faithfully maintaining the chromosome number.

Meiosis is the special division by which diploid reproductive (germ) cells give rise to haploid gametes. Its defining feature, and a constant NEET point, is that a single round of DNA replication is followed by two consecutive divisions — meiosis I and meiosis II — producing four haploid cells from one diploid cell. Meiosis I is the reductional division because it is here that the chromosome number is halved.

The most elaborate stage is prophase I, which is divided into five substages that you must know in order: Leptotene (chromosomes begin to condense and become visible), Zygotene (homologous chromosomes pair up precisely, a process called synapsis, forming bivalents held by the synaptonemal complex), Pachytene (the crucial substage in which crossing over — exchange of segments between non-sister chromatids — occurs at recombination nodules), Diplotene (the homologues begin to separate but stay joined at X-shaped chiasmata, the sites of crossing over) and Diakinesis (chiasmata terminalise and the nuclear envelope breaks down). A handy mnemonic is 'Leptotene Zygotene Pachytene Diplotene Diakinesis'.

The remaining stages of meiosis I follow the familiar pattern but with a key twist. In metaphase I the bivalents (pairs of homologous chromosomes) line up at the equator. In anaphase I the homologous chromosomes separate and move to opposite poles, but — unlike mitosis — the sister chromatids remain joined at their centromeres. This separation of whole homologues is exactly what halves the chromosome number.

In telophase I the two nuclei reform (often briefly) and cytokinesis usually follows, giving two haploid cells, each chromosome still consisting of two chromatids. Two events of meiosis I are the engines of genetic variation: crossing over in pachytene and the independent assortment of maternal and paternal homologues at anaphase I. For NEET, the must-knows are the five prophase-I substages (with pachytene = crossing over) and that anaphase I separates homologues (reductional), not chromatids.

After meiosis I, the two haploid cells enter meiosis II without any further DNA replication (there is no S phase between the two divisions). Meiosis II is essentially a mitosis-like equational division that separates the sister chromatids that were still joined after meiosis I, so it converts the two cells into four. Its four stages mirror mitosis: prophase II (chromosomes recondense, nuclear envelope breaks down again), metaphase II (chromosomes line up at the equator), anaphase II (the centromeres finally split and the sister chromatids separate to opposite poles) and telophase II (four haploid nuclei form, followed by cytokinesis).

The pivotal contrast — and a guaranteed NEET question — is between the two anaphases. In anaphase I, homologous chromosomes separate while sister chromatids stay together (this is what reduces the number). In anaphase II, the centromeres split and the sister chromatids separate, exactly as in mitosis. So meiosis I is reductional and meiosis II is equational, and only at anaphase II do the chromatids finally part company.

The significance of meiosis is twofold and central to biology. First, by halving the chromosome number it keeps the chromosome number constant across generations in sexually reproducing organisms — the haploid gametes restore the diploid number at fertilisation, preventing a doubling every generation. Second, meiosis is the great generator of genetic variation, through crossing over (pachytene of prophase I) and the independent assortment of paternal and maternal chromosomes at metaphase/anaphase I. This variation is the raw material for natural selection and evolution and the cellular basis of Mendel's laws.

By comparison, the significance of mitosis is to provide genetically identical cells for the growth of the body, the repair of tissues and the replacement of worn-out cells, while strictly maintaining the chromosome number. A clean way to revise the whole chapter is the contrast: mitosis = one division, two identical diploid cells, growth/repair; meiosis = two divisions, four varied haploid cells, gamete formation and variation. Keeping these two outcomes straight resolves most cell-division questions.