Course 9.2Lesson 2 of 3

Meiosis II and the four haploid cells

After prophase I, the pairs line up and the two members of each pair are pulled apart. Meiosis II then separates the sister chromatids as in mitosis. One cell ends as four cells, each with half the chromosomes. This lesson follows the chromosomes and the DNA through both divisions.

about 30 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

From one cell to four

Meiosis I ends with two cells. Meiosis II divides each of them, so that four cells result. The two divisions do different jobs.

A drawing of meiosis in a cell with four chromosomes, two long and two short, one of each pair blue and one red. In the top row, meiosis I: prophase I with a bivalent for each pair, metaphase I with the two bivalents on the equator, anaphase I with whole chromosomes of two chromatids moving to the poles, and telophase I with two cells. In the bottom row, meiosis II in each of the two cells: metaphase II with two chromosomes on the equator, anaphase II with chromatids moving apart, and at the end four cells, each with two single chromosomes.

Meiosis I, after prophase I

Metaphase I. The bivalents line up on the equator. Spindle fibres from opposite poles join the two homologues of a pair. The two members of each pair face opposite poles.
Anaphase I. The homologous chromosomes separate. The sister chromatids stay together, because the centromeres do not split. Each pole receives half the chromosomes, and each chromosome still has two chromatids.
Telophase I. The nuclear envelope and the nucleolus reappear, and the cytoplasm divides. The two cells are a dyad.

Which member of a pair goes to which pole is a matter of chance. Each pair is independent of the others. This matters for variation, as the next lesson shows.

The stage between the two divisions is interkinesis. It is short. The chromosomes may spread out a little, but they do not return to the interphase state, and the DNA is not copied again. Prophase II is much simpler than prophase I.

Meiosis II

Meiosis II is like a normal mitosis.

Prophase II. The nuclear envelope disappears, and the chromosomes compact again.
Metaphase II. The chromosomes line up on the equator. The kinetochores of the two sister chromatids of every chromosome face opposite poles.
Anaphase II. The centromeres split at the same time. The sister chromatids move to opposite poles.
Telophase II. Nuclear envelopes form round the two groups of chromosomes. Cytokinesis follows. The result is a tetrad of cells: four haploid cells.

Three cells in anaphase side by side, each with four chromosomes. Mitosis anaphase: the centromeres have split and eight single chromatids move to the poles, four to each pole. Anaphase I of meiosis: the centromeres have not split, and whole chromosomes of two chromatids move to the poles, two to each pole. Anaphase II of meiosis: the two cells of the dyad side by side. In each cell the centromeres of the two chromosomes have split, and single chromatids move to the poles, two to each pole.

The first division is the reduction division, because it halves the chromosome number. The second is the equational division, because the number stays the same, as in mitosis.

In animals, the cytoplasm does not always divide equally. In the formation of eggs, one cell takes nearly all the cytoplasm, and the others are small polar bodies. In the formation of sperm, the four cells are equal.

Counting through meiosis

The numbers below are for a human cell with 46 chromosomes and 2C of DNA in G1. They are in the form that NEET likes to ask.

G1: 46 chromosomes, one chromatid in each, 2C of DNA.
Prophase I to anaphase I (the whole cell): 46 chromosomes, 92 chromatids, 4C.
One cell after meiosis I: 23 chromosomes, 46 chromatids, 2C.
Metaphase II (one cell): 23 chromosomes, 46 chromatids, 2C.
Anaphase II (one cell): 46 chromosomes, 46 chromatids, 2C.
Each of the four cells: 23 chromosomes, 23 chromatids, 1C.

Look at two surprises. A cell in anaphase II has 46 chromosomes, the same as a body cell, because the sister chromatids are counted as chromosomes. Its DNA is only 2C. And the chromosomes of a cell are halved at the end of meiosis I, but the DNA is halved again at the end of meiosis II.

A step graph of the DNA in one cell against time through meiosis, in units of C. The line is level at 2C in G1, rises to 4C in S phase, stays at 4C through meiosis I until anaphase I and telophase I, falls to 2C when the first division is complete, stays at 2C through meiosis II until anaphase II, and falls to 1C when the cells of the second division separate. A row below gives the number of chromosomes in each cell as 2n in G1, 2n in S, 2n and then n in meiosis I, and n after it.

You should see that the chromosome number falls by half at stage 4 and rises again at stage 6, while the DNA falls by half twice: at stage 4 and at stage 7.

Try it in Python

The program prints the chromosomes, the DNA molecules and the DNA content at each stage for a cell with 2n = 8.

Practise with new numbers

Common mistakes

Saying that the centromeres split in anaphase I. They split in anaphase II. In anaphase I the homologues separate.
Saying that meiosis II halves the chromosomes again. It does not. It separates the sister chromatids and keeps the number.
Saying that a cell in anaphase II has n chromosomes. It has 2n, because the separated chromatids are chromosomes.
Saying that the DNA is copied between the two divisions. Interkinesis has no copying.
Forgetting that the DNA falls twice. It falls at the end of meiosis I and again at the end of meiosis II.

Remember these

Meiosis I separates the homologues. It is the reduction division. The sister chromatids stay together.
Meiosis II separates the sister chromatids. It is the equational division, like mitosis.
Interkinesis is short, with no DNA copying. The end result is four haploid cells.
Human cell: 4C in meiosis I, 2C after it, 1C in each gamete.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What separates in anaphase I? What separates in anaphase II?

A cell has 2n = 20 chromosomes in G1. How many chromosomes are in each cell after meiosis I? And in one cell at anaphase II?

Why is meiosis I called the reduction division and meiosis II the equational division?

What is interkinesis? Does the DNA copy again?

A human cell has 2C of DNA in G1. How much DNA does it have after S phase, after meiosis I and in each gamete?

References

NCERT (2023). Biology, Textbook for Class XI, chapter 10.
Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 11.1, The Process of Meiosis.