In meiosis how many chromosomes did each cell end up with

• During meiosis one cell divides twice to form four daughter cells.
• These four daughter cells only have half the number of chromosomes of the parent cell – they are haploid.
• Meiosis produces our sex cells or gametes (eggs in females and sperm in males).

Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II):

Meiosis I

1. Interphase:

• The DNA in the cell is copied resulting in two identical full sets of chromosomes.
• Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division.
• During interphase, microtubules extend from these centrosomes.

2. Prophase I:

• The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope.
• Each chromosome is composed of two sister chromatids containing identical genetic information.
• The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.
• The pairs of chromosomes may then exchange bits of DNA in a process called recombination or crossing over.
• At the end of Prophase I the membrane around the nucleus in the cell dissolves away, releasing the chromosomes.
• The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.

3. Metaphase I:

• The chromosome pairs line up next to each other along the centre (equator) of the cell.
• The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them.
• The meiotic spindle fibres attach to one chromosome of each pair.

4. Anaphase I:

• The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.
• In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II.

5. Telophase I and cytokinesis:

• The chromosomes complete their move to the opposite poles of the cell.
• At each pole of the cell a full set of chromosomes gather together.
• A membrane forms around each set of chromosomes to create two new nuclei.
• The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.

Meiosis II

6. Prophase II:

• Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids).
• In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.
• The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes.
• The centrioles duplicate.
• The meiotic spindle forms again.

7. Metaphase II:

• In each of the two daughter cells the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell.
• The centrioles are now at opposites poles in each of the daughter cells.
• Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.

8. Anaphase II:

• The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle.
• The separated chromatids are now individual chromosomes.

9. Telophase II and cytokinesis:

• The chromosomes complete their move to the opposite poles of the cell.
• At each pole of the cell a full set of chromosomes gather together.
• A membrane forms around each set of chromosomes to create two new cell nuclei.
• This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis.
• Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid):
  • in males, these four cells are all sperm cells
  • in females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs).

Illustration showing the nine stages of meiosis.
Image credit: Genome Research Limited

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Like mitosis, meiosis is a form of eukaryotic cell division. However, these two processes distribute genetic material among the resulting daughter cells in very different ways. Mitosis creates two identical daughter cells that each contain the same number of chromosomes as their parent cell. In contrast, meiosis gives rise to four unique daughter cells, each of which has half the number of chromosomes as the parent cell. Because meiosis creates cells that are destined to become gametes (or reproductive cells), this reduction in chromosome number is critical — without it, the union of two gametes during fertilization would result in offspring with twice the normal number of chromosomes!

Apart from this reduction in chromosome number, meiosis differs from mitosis in yet another way. Specifically, meiosis creates new combinations of genetic material in each of the four daughter cells. These new combinations result from the exchange of DNA between paired chromosomes. Such exchange means that the gametes produced through meiosis exhibit an amazing range of genetic variation.

Finally, unlike mitosis, meiosis involves two rounds of nuclear division, not just one. Despite this fact, many of the other events of meiosis are similar to those that occur in mitosis. For example, prior to undergoing meiosis, a cell goes through an interphase period in which it grows, replicates its chromosomes, and checks all of its systems to ensure that it is ready to divide. Like mitosis, meiosis also has distinct stages called prophase, metaphase, anaphase, and telophase. A key difference, however, is that during meiosis, each of these phases occurs twice — once during the first round of division, called meiosis I, and again during the second round of division, called meiosis II.

What happens during meiosis I?

As previously mentioned, the first round of nuclear division that occurs during the formation of gametes is called meiosis I. It is also known as the reduction division because it results in cells that have half the number of chromosomes as the parent cell. Meiosis I consists of four phases: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

During prophase I, the chromosomes condense and become visible inside the nucleus. Because each chromosome was duplicated during the S phase that occurred just before prophase I, each now consists of two sister chromatids joined at the centromere. This arrangement means that each chromosome has the shape of an X.

Once this chromosomal condensation has occurred, the members of each chromosome pair (called homologous chromosomes, because they are similar in size and contain similar genes), align next to each other. At this point, the two chromosomes in each pair become tightly associated with each other along their lengths in a process called synapsis. Then, while the homologous chromosomes are tightly paired, the members of each pair trade adjacent bits of DNA in a process called crossing over, also known as recombination (Figure 1). This trading of genetic material creates unique chromosomes that contain new combinations of alleles.

At the end of prophase I, the nuclear membrane finally begins to break down. Outside the nucleus, the spindle grows out from centrosomes on each side of the cell. As in mitosis, the microtubules of the spindle are responsible for moving and arranging the chromosomes during division.

Metaphase I

Anaphase I

Telophase I

Interkinesis

At this point, the first division of meiosis is complete. The cell now rests for a bit before beginning the second meiotic division. During this period, called interkinesis, the nuclear membrane in each of the two cells reforms around the chromosomes. In some cells, the spindle also disintegrates and the chromosomes relax (although most often, the spindle remains intact). It is important to note, however, that no chromosomal duplication occurs during this stage.

What happens during meiosis II?

During meiosis II, the two cells once again cycle through four phases of division. Meiosis II is sometimes referred to as an equational division because it does not reduce chromosome number in the daughter cells — rather, the daughter cells that result from meiosis II have the same number of chromosomes as the "parent" cells that enter meiosis II. (Remember, these "parent" cells already have half the number of chromosomes of the original parent cell thanks to meiosis I.)

Prophase II

As prophase II begins, the chromosomes once again condense into tight structures, and the nuclear membrane disintegrates. In addition, if the spindle was disassembled during interkinesis, it reforms at this point in time.

Metaphase II

Anaphase II

Telophase II

Why is meiosis important?