Meiosis I — Metaphase to Telophase I

NCERT grounding

NCERT Class 11 Biology (Chapter 10, Section 10.4.1) treats the later phases of meiosis I in a compact three-line block. Metaphase I: "The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes." Anaphase I: "The homologous chromosomes separate, while sister chromatids remain associated at their centromeres." Telophase I produces the dyad of cells, after which a short interkinesis with no DNA replication leads into prophase II. The chapter summary calls meiosis the reduction division precisely because each pole receives half the parental chromosome number here.

Metaphase I to Telophase I

By the end of diakinesis the bivalents are fully condensed, the nuclear envelope has broken down and the meiotic spindle is assembled. What follows is the business end of meiosis I — the three stages that physically convert one diploid cell into a haploid dyad. Although they echo the names of the mitotic stages, every one of them behaves differently because the working unit here is the bivalent (a pair of synapsed homologues, also called a tetrad), not a lone chromosome.

Metaphase I — bivalents on the equator

In metaphase I the bivalents migrate to the centre of the cell and arrange themselves on the equatorial plate. The crucial structural fact is that an intact bivalent — two homologous chromosomes, each still made of two sister chromatids — occupies the plate as a unit. Because two homologues sit side by side, the chromosomes form two parallel rows at the equator rather than the single file seen in mitotic metaphase and in metaphase II.

Spindle microtubules from the opposite poles attach to the kinetochores of the two homologous chromosomes. A subtle but examinable point: the two sister kinetochores of a single homologue both face the same pole, so the entire homologue is pulled as one piece. One homologue of each pair is therefore committed to one pole and its partner to the other. The random way each bivalent orients on the plate is the cytological basis of independent assortment, though NCERT keeps the spotlight on the mechanics of attachment.

Anaphase I — the reduction step

Anaphase I is the single most heavily examined moment in the whole chapter, and the reason is its quiet asymmetry with mitosis. Here the homologous chromosomes separate and travel to opposite poles, but the sister chromatids stay together, still held at their common centromere. The centromere does not split in anaphase I. What moves to each pole is therefore a whole, two-chromatid chromosome — not a single chromatid.

This is what halves the chromosome number. A diploid cell that began meiosis with 2n homologous pairs sends one member of every pair to each pole, so each pole inherits exactly n chromosomes. The reduction — 2n to n — is accomplished at the end of meiosis I, not later. Meiosis II will simply split the chromatids of those n chromosomes, keeping the count at n.

Telophase I — the dyad and interkinesis

At telophase I the chromosomes reach the poles, the nuclear membrane and nucleolus reappear, and cytokinesis follows to give what NCERT names the dyad of cells — two cells. NCERT notes that the chromosomes typically undergo some dispersion but do not return to the fully extended interphase state; in many organisms they stay relatively condensed. Each cell of the dyad is now haploid in chromosome number, yet every chromosome it carries is still made of two sister chromatids awaiting separation in meiosis II.

The gap that follows telophase I is interkinesis — a short interphase-like stage between meiosis I and meiosis II. Its defining feature, and a perennial NEET favourite, is that there is no replication of DNA during interkinesis. Meiosis runs on a single round of replication carried out in the S phase before prophase I; the cell never copies its DNA again between the two divisions. Interkinesis is generally short-lived and leads directly into prophase II, a far simpler prophase than prophase I.

Worked examples

Common confusion & NEET traps

Three closely related ideas — what separates in anaphase I, when the centromere splits, and whether interkinesis replicates DNA — generate a disproportionate share of NEET errors. Map them cleanly and the marks follow.