Meiosis II — Overview & Stages

Meiosis II: The Sequel

One cell division was not enough. Now the daughter cells of meiosis I are back for round two.

Remember, meiosis happens in sex cells (sperm or egg cells) only. At the end of meiosis II, we form special sex cells called gametes. This process ensures that these gametes only contain one set of chromosomes. They get their other set of chromosomes when they are fertilized by another gamete.

Meiosis II goes through similar phases to meiosis I, but now we are starting with two haploid cells. DNA does not get replicated before this division, but each chromosome consists of two chromatids.

Prophase II:

Once again, the nuclear membrane disintegrates and chromosomes are free to roam the cytoplasm. The mitotic spindle begins to form in preparation for the following phases.

Crossing-over does not happen again because there are no homologous pairs of chromosomes.

Metaphase II:

The chromosomes begin to line up along the center of the cell, and the spindle fibers attach at the centromeres. This looks similar to metaphase of mitosis, but remember that there are no homologous chromosomes here.

Remember, just like in meiosis I, the cell needs to pass a checkpoint before heading to the next phase. The cell makes sure that each chromatid is attached to spindle fibers.

Anaphase II:

During anaphase I, the spindle pulled apart homologous chromosomes. This time, the spindle pulls apart the sister chromatids of each chromosome. This means that after meiosis II, the daughter cells will have one copy of each chromosome, and each chromosome will have one chromatid.

Telophase II:

A new nuclear envelope can now form around each cluster of chromosomes. This forms two nuclei. Simultaneously, a process called cytokinesis splits the cytoplasm to form two daughter cells.

Remember that this is happening for each of the daughter cells that resulted from meiosis I. This means that we now have a total of four daughter cells.

Although not specifically pictured, each daughter cell is genetically different. This is due to the crossover that occurred in meiosis I.

Understanding Chromosomal Numbers

Each cell is haploid because it only has one copy of each chromosome. This is very important. Meiosis only occurs in sex cells, or the egg and sperm. An egg cell will have one set of chromosomes, and a sperm cell will have one set. When they combine during fertilization, they create a diploid cell called a zygote. In this way, you get half of your DNA from your mom and half from your dad.

What's Meiosis II?

Immediately after a cell finishes meiosis I, the two haploid daughter cells enter meiosis II. This process is basically identical to mitosis except for the number of chromosomes. It follows the same pattern of prophase II, metaphase II, anaphase II, and telophase II.

At the end, we get a haploid cell, called a gamete. This is a sperm or egg cell which contains just one set of chromosomes. It will get a second set of chromosomes if it's fertilized by another gamete.

Prophase II

Prophase II is identical to the prophase stage of mitosis. The nuclear membrane dissolves and spindles form. The centrioles begin to move apart to opposite sides of the cell.

Metaphase II

During metaphase II (a bit different than metaphase of mitosis), the sister chromatids are lined up in the middle of the cell by spindle fibers. This is different from metaphase I where the homologous pairs lined up side-by-side.

Anaphase II

Anaphase II is the next step. The sister chromatids are pulled apart by the spindles towards opposite sides of the cell.

Telophase II and Cytokinesis

During telophase II and cytokinesis, the spindles dissolve and new nuclear envelopes appear. A cleavage furrow forms and the cytoplasm separates until four haploid cells are produced. They are all unique and different from one another.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Interphase}}$ : The longest part of the cell cycle where the cell grows and DNA is replicated to prepare for nuclear division

$\color{#8925AE}{\text{Nuclear Division}}$ : The process of dividing the nucleus

$\color{#8925AE}{\text{Meiosis}}$ : The process of dividing one nucleus into four unique haploid cells. Only occurs in sex cells

$\color{#8925AE}{\text{Cytokinesis}}$ : The final step of cell division where the cytoplasm is divided to form daughter cells

$\color{#8925AE}{\text{Diploid Cell}}$ : Cell that contains two sets of chromosomes

$\color{#8925AE}{\text{Haploid Cell}}$ : Cell that contains only half the number of chromosomes. These are sperm and egg cells

Summary of Meiosis II

Meiosis II is the second round of division that sex cells go through to create haploid gametes. Haploid means that the cells only have half of the genetic material that a normal somatic cell has. After meiosis I, there are two cells that are technically called haploid because they only contain one copy of each chromosome. However, each of those chromosomes has two chromatids. These chromosomes have also experienced crossing-over. Now they will go through a second round of meiosis and end up as four cells with half of the normal genetic material.

Prophase II

Here, a new set of spindle fibers forms and attaches to the centromeres of the chromosomes.

Metaphase II

During metaphase II (a little different than metaphase of mitosis), the chromosomes are aligned in the middle of the cell, along the metaphase plate.

Anaphase II

The spindle fibers pull the sister chromatids apart to opposite sides of the cell.

Telophase II and Cytokinesis

The nucleus reforms and the spindle fibers disintegrate. The chromosomes unwind back into their chromatin form and the cell membrane pinches inward, creating four new cells with mixed-up genetic material and one chromosome each.

(Video) Meiosis: Where the Sex Starts

by CrashCourse

Meiosis I and meiosis II are part of one big process: how sex cells divide. In this video clip, Hank from Crash Course covers what happens in the second round of meiosis.