Meiosis II

NCERT grounding

NCERT Class 11 Biology, Chapter 10, places meiosis II under section 10.4.2. The text is precise about its character: "In contrast to meiosis I, meiosis II resembles a normal mitosis." It is initiated immediately after the cytokinesis of meiosis I, the chromosomes condense again, and the nuclear membrane disappears by the end of prophase II. The stage that precedes it, interkinesis, is described as short-lived, and the chapter states categorically that there is no replication of DNA during interkinesis.

"Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication... Four haploid cells are formed at the end of meiosis II."

Two grounding facts from this passage anchor every NEET question on the topic. First, the single replication occurs in the S phase before meiosis I begins, never before meiosis II. Second, the end product is a tetrad of four haploid daughter cells. Hold these two while reading the mechanism below.

Meiosis II: the four phases

Meiosis I has done the hard genetic work. Homologous chromosomes have paired, crossed over, and been pulled apart, so each of the two cells emerging from meiosis I already carries a haploid number of chromosomes. But there is a structural catch: every one of those chromosomes still consists of two sister chromatids joined at a centromere. The cells are haploid in chromosome number but still carry double the DNA per chromosome. Meiosis II exists to resolve exactly this — to separate the sister chromatids so that each final cell holds single, unreplicated chromosomes.

This is why meiosis II is called an equational division. It does not reduce the chromosome number; that reduction was completed in meiosis I. Meiosis II merely splits chromatids, exactly as mitosis does. The critical distinction is what it acts upon and what precedes it. Mitosis follows an S phase and acts on cells whose chromosomes were just replicated. Meiosis II follows interkinesis, a brief gap with no DNA synthesis, and acts on the already-haploid products of meiosis I.

Prophase II

Prophase II is short and uncomplicated, in deliberate contrast to the long, five-substage prophase I. It begins almost immediately after the cytokinesis of meiosis I, usually before the chromosomes have fully elongated from telophase I. The chromosomes condense and become compact again, and the nuclear membrane disappears by the end of prophase II. There is no synapsis, no bivalent formation, and no crossing over here — those events belong solely to prophase I. A new spindle apparatus forms in each of the two haploid cells.

Metaphase II

The chromosomes align at the equator of the spindle. Each chromosome still comprises two sister chromatids, and the microtubules from opposite poles of the spindle attach to the kinetochores of these sister chromatids. The arrangement is single-file, exactly as in mitotic metaphase: one chromosome thick along the metaphase plate. This is the structural feature that distinguishes it from metaphase I, where the units on the plate were bivalents (paired homologues), forming two parallel rows of chromosomes.

Anaphase II

Anaphase II is the decisive event of the whole second division. It begins with the simultaneous splitting of the centromere of each chromosome — the centromere that had been holding the two sister chromatids together. Once the centromere divides, the two sister chromatids are free to move toward opposite poles, pulled by the shortening of the microtubules attached to their kinetochores. This is the only point in meiosis where the centromere splits; in meiosis I the centromeres stayed intact while whole homologues separated.

Telophase II

Meiosis ends with telophase II. The two groups of chromosomes at each pole, in each of the two dividing cells, become enclosed once again by a nuclear envelope. Cytokinesis follows, and the result is a tetrad of cells — four haploid daughter cells. Each of these now contains single, unreplicated chromosomes and a haploid genome ready to function as, or differentiate into, a gamete.

Why "mitosis-like" yet acting on haploid cells

The phrase NCERT uses — meiosis II "resembles a normal mitosis" — is worth dissecting because it is the source of both clarity and confusion. The resemblance is purely mechanical. In both, the chromosome condenses, aligns single-file at the equator, has its centromere split, and sends one chromatid to each pole. Neither involves homologous pairing or crossing over. In both, the chromosome number of the dividing cell is conserved across the division — hence both are equational.

The differences are about context, not mechanism. A typical mitosis is preceded by an S phase, so the chromatids being separated are freshly replicated copies, and the parent cell is usually diploid. Meiosis II is preceded by interkinesis with no replication, and the cell it divides is already haploid — the chromatids being separated are the leftover sister copies inherited from the single replication done before meiosis I. The outcome, therefore, differs: mitosis on a diploid cell yields two diploid cells, whereas meiosis II on the two haploid cells of the dyad yields four haploid cells.

The net result of meiosis I and II read together is the single most quoted figure in this chapter: one diploid parent cell gives rise to four haploid daughter cells. Meiosis I performs the reductional step (2n cell → two n cells, the dyad), and meiosis II performs the equational step (each n cell → two n cells), so the dyad becomes the tetrad. The single round of DNA replication, done before meiosis I, is divided across two divisions, which is precisely how four cells each end up with a single haploid genome.

Worked examples

Common confusion & NEET traps

Meiosis II is a reliable NEET earner because three crisp facts can be tested by a single options list, and each has a near-identical "decoy" drawn from meiosis I or mitosis. The three callouts below cover the recurring traps.