Behold chromosome is aligned at the metaphase

Behold round two of meiotic cell division! You’ll see how meiosis II keeps the genome intact and gives the final push to produce haploid gametes, which make the world go round.

Types of Cell Division

In meiosis II, the number of chromosomes in each daughter cell is the same as in the mother cell
Cell division during meiosis II

To successfully complete meiosis and produce a gamete, a cell must undergo two rounds of division.

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The first round, conveniently known as meiosis I, reduces the number of chromosomes in the cell by half. The mechanisms the cell uses to accomplish this reductional division differ greatly from mitosis.In contrast, meiosis II is very similar to mitosis, since the goal is to segregate a chromatid of each chromosome into separate daughter cells. For this reason, meiosis II is sometimes called an equational division.

Like mitosis, the number of chromosomes in each daughter cell is unchanged compared to the number of chromosomes in the mother cell. Let’s examine the similarities and differences between meiosis II and mitosis a little more closely.Recall that meiosis I produced two cells, which will behave exactly the same during meiosis II.

Since both cells are going to undergo the same steps, let’s just consider the meosis II division of one of these cells, and then we can consider all of the products together at the end. So, this cell is going to undergo interphase, prophase II, metaphase II, anaphase II and telophase II.Meiosis II is preceded by a brief interphase. Now, it’s a special interphase, because no DNA replication occurs in this case. Recall that meiosis I left each cell with only one set of homologous chromosomes, but each homologous chromosome consists of two chromatids. Like in mitosis, the centrosome duplicates during interphase.

Meiosis II Stages

Chromosomes are positioned in the middle of each cell during the first step of meiosis II
microtubules on each side, the kinetochore can position each chromosome in the middle of the cell. By the end of prophase II, the nuclear membrane has broken down and the meiotic spindle apparatus has assembled.Metaphase II is the second step in meiosis II. At metaphase II, each chromosome is aligned at the metaphase plate. As in mitosis, each chromosome is held in place at the metaphase plate by equivalent spindle tension on each side.Anaphase II is the third step in meiosis II. Like in mitosis, anaphase II is initiated when the protein separase cleaves the protein holding the chromatids together.

Once this physical connection is broken, each chromatid is free to move to its respective pole.Telophase II is the fourth and final step in meiosis II. During telophase II, the nuclear membrane reforms and the chromosomes de-condense. Cytokinesis completes the formation of four haploid gamete cells.

Note that chromosomal content of each haploid gamete is slightly different due to homologous recombination and the manner in which the chromosomes sort into the gamete cells. We will see later how cytokinesis can be modified to produce even more specialized gametes.

The connection between sister chromatids is broken during anaphase II
Third step of meiosis II

Summary

Meiosis I is the first division in meiosis in which the number of chromosomes in the cell is reduced by half. Meiosis II is the second division in meiosis in which chromatids of each chromosome are segregated equally into daughter cells.No DNA replication occurs during the interphase which precedes meiosis II. Prophase II is the first step in meiosis II.

Like in mitosis and meiosis I, chromatin condenses, centrosomes move to opposite poles and a spindle apparatus forms. Kinetochores assembled at the centromeres work with spindle microtubules to guide the chromosomes to the metaphase plate.Metaphase II is the second step in meiosis II. By metaphase II, each chromosome is aligned at the metaphase plate.Anaphase II is the third step in meiosis II. During anaphase II, the physical connection between sister chromatids is broken, allowing them to move toward the spindle poles.

Telophase II is the fourth and final step of meiosis II. During telophase II, the nuclear membrane reforms and the chromosomes de-condense. Cytokinesis, which follows meiosis II, establishes four gametes with different genetic content.As we draw to a close on meiosis and cell division, I’ll leave you with one last poetic study-aid:Meiosis is but halfway startedBut chromatids must now be partedNo replication precedes this actYet genome size remains intactAs division number two completesMeiosis leaves haploid gametes

Lesson Objective

After watching this lesson, you should be able to define meiosis II and describe what happens in each of its stages: interphase, prophase II, metaphase II, anaphase II and telophase II.

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