Meiosis I is the first division of meiosis and is responsible for separating homologous chromosomes. From metaphase I through telophase I and cytokinesis, cells establish a haploid set in each daughter cell.

Where this fits in meiosis I

By this point, homologous chromosomes have paired and condensed into tetrads (also called bivalents). Each chromosome still consists of two sister chromatids joined at a centromere, and the spindle apparatus is poised to separate homologs (not chromatids).

Metaphase I: tetrads align at the metaphase plate

In metaphase I, homologous chromosome pairs line up at the cell’s equator.

Metaphase I is characterized by tetrads (bivalents) positioned on the metaphase plate and oriented toward opposite spindle poles. This diagram emphasizes that homologs—not sister chromatids—are the units being arranged for separation in anaphase I. Source

• Tetrads align along the metaphase plate.

• Spindle microtubules from opposite poles attach to kinetochores.

• The two homologs in each pair attach to opposite poles, creating tension across the tetrad.

• Sister kinetochores on each chromosome typically orient together toward the same pole (a key feature distinguishing meiosis I from mitosis).

This alignment ensures that each homolog in a pair is connected to opposite spindle poles, preparing for separation in anaphase I.

Anaphase I: homologous chromosomes separate

In anaphase I, homologous chromosomes separate and move toward opposite poles.

• Homologs disjoin: one homolog from each pair moves to each pole.

• Sister chromatids remain attached at their centromeres; they do not separate in meiosis I.

• Chromosome movement is driven by:

  • shortening of kinetochore microtubules (pulling chromosomes poleward)

  • lengthening of non-kinetochore microtubules (pushing poles apart)

A central outcome of anaphase I is reduction of chromosome number: each pole receives one chromosome from each homologous pair, establishing a haploid set in terms of homolog identity (even though each chromosome is still duplicated).

This staged meiosis diagram summarizes how homologous chromosomes separate in anaphase I while sister chromatids remain joined, followed by telophase I and cytokinesis producing two haploid cells. The sequence helps distinguish the reduction in homolog number during meiosis I from the chromatid separation that occurs in meiosis II. Source

Telophase I: two haploid sets reach opposite poles

During telophase I, separated homologs arrive at opposite poles and the cell begins to reorganise.

• Chromosomes may partially decondense, depending on species.

• A nuclear envelope may reform around each chromosome set in some organisms.

• Spindle fibres disassemble or reorganise.

What matters for AP Biology is the chromosome content: each pole now contains a haploid set of chromosomes (one member of each homologous pair), consistent with the specification that homologs “separate.”

Cytokinesis: two haploid daughter cells form

Cytokinesis divides the cytoplasm, producing two haploid daughter cells.

• In animal cells, a cleavage furrow forms via an actin–myosin contractile ring.

• In many plant cells, division occurs by formation of a cell plate.

Each daughter cell contains:

• a haploid set of chromosomes (no homologous pairs present)

• chromosomes that are still duplicated (each with two sister chromatids)

These haploid cells are not yet gametes in most organisms; they must proceed through meiosis II to separate sister chromatids.

Key AP Biology clarifications (common errors)

• Metaphase I aligns homologous pairs, not individual chromosomes.

• Anaphase I separates homologous chromosomes, not sister chromatids.

• The reduction to haploid occurs because each daughter cell receives only one homolog from each pair, matching: “align at the metaphase plate, separate, and form two haploid daughter cells.”