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Replication and Distribution of DNA during Meiosis

A schematic illustration shows 20 sperm cells swimming towards a much larger egg cell. The egg cell is depicted as an orange sphere; only a portion of the cell is visible in the frame. The sperm are depicted as oval-shaped cells with a single anterior whip-like flagellum. One sperm is shown making contact with and implanting itself into the egg's surface in an event known as fertilization. The oviform body of this sperm is a lighter brown color than the surrounding sperm. A simplified, double-helical DNA molecule is visible inside the cell body of the fertilizing sperm.

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

A schematic shows two homologous chromosomes side-by-side. The chromosomes each have two elongated, vertical, parallel arms, held together at a pinched point above their center. The chromosome at left is mostly green, but the lower region of the right arm is orange. The chromosome at right is mostly orange, but the lower region of the left arm is green.

Figure 1: Recombination is the exchange of genetic material between homologous chromosomes.

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

A schematic shows two pairs of homologous chromosomes arranged in a vertical formation in the middle of a cell. Two developing mitotic spindles flank the four chromosomes. Long fibers radiating from the mitotic spindles are attached to the centromere of each chromosome.

Figure 2: Near the end of metaphase I, the homologous chromosomes align on the metaphase plate.

Figure Detail

At the start of metaphase I, microtubules emerge from the spindle and attach to the kinetochore near the centromere of each chromosome. In particular, microtubules from one side of the spindle attach to one of the chromosomes in each homologous pair, while microtubules from the other side of the spindle attach to the other member of each pair. With the aid of these microtubules, the chromosome pairs then line up along the equator of the cell, termed the metaphase plate (Figure 2).

Anaphase I

A schematic shows each member in a pair of homologous chromosomes separating from its partner and migrating in opposite directions. White spindle fibers attached to the centromeres of each chromosome are responsible for the movement of the chromosomes to opposite poles of the cell.

Figure 3: During anaphase I, the homologous chromosomes are pulled toward opposite poles of the cell.

Figure Detail

During anaphase I, the microtubules disassemble and contract; this, in turn, separates the homologous chromosomes such that the two chromosomes in each pair are pulled toward opposite ends of the cell (Figure 3). This separation means that each of the daughter cells that results from meiosis I will have half the number of chromosomes of the original parent cell after interphase. Also, the sister chromatids in each chromosome still remain connected. As a result, each chromosome maintains its X-shaped structure.

Telophase I

A schematic shows two overlapping, circular cells, each with a nucleus and two chromosomes. The cytoplasmic portions of the cells are light grey, and the nuclei are dark grey. The chromosomes in the left-hand cell are mostly green, but the lower regions of the right chromatids are orange. The chromosomes in the right-hand cell are mostly orange, but the lower regions of the left chromatids are green. Both cells have remnants of the mitotic spindle in the cytoplasm.

Figure 4: Telophase I results in the production of two nonidentical daughter cells, each of which has half the number of chromosomes of the original parent cell.

As the new chromosomes reach the spindle during telophase I, the cytoplasm organizes itself and divides in two. There are now two cells, and each cell contains half the number of chromosomes as the parent cell. In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I (Figure 4).

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

A schematic shows two grey, circular cells: one cell is in the background, eclipsed by a cell in the immediate foreground that appears larger. Inside the larger cell, two chromosomes are aligned along the cell's Z-axis. An orange chromosome with a small portion of green on the lower left arm is shown in the topmost position; a second orange chromosome with a small portion of green on the upper right arm is directly below it. Long fibers radiate from mitotic spindles on the cell's left and right sides and are attached to the centromere of each chromosome. Shorter fibers also emanate from the mitotic spindle, but they are not attached to chromosomes.

Figure 5: During metaphase II, the chromosomes align along the cell's equatorial plate.

The events of metaphase II are similar to those of mitotic metaphase — in both processes, the chromosomes line up along the cell's equatorial plate, also called the metaphase plate, in preparation for their eventual separation (Figure 5).

Anaphase II

A schematic shows two circular cells: one cell is in the background, eclipsed by a cell in the immediate foreground that appears larger and contains two chromosomes that have been split in half. Each chromatid is attached to spindle fibers at their centromeres. The sister chromatids of each of the chromosomes have become separated due to the action of the spindle fibers and are shown migrating to opposite poles of the cell. For example, the topmost chromosome has been split in half; one half is moving toward the left-hand pole of the cell, and the other half is moving toward the right-hand pole of the cell.

Figure 6: Anaphase II involves separation of the sister chromatids.

During anaphase II, microtubules from each spindle attach to each sister chromatid at the kinetochore. The sister chromatids then separate, and the microtubules pull them to opposite poles of the cell. As in mitosis, each chromatid is now considered a separate chromosome (Figure 6). This means that the cells that result from meiosis II will have the same number of chromosomes as the "parent" cells that entered meiosis II.

Telophase II

A schematic shows four circular cells: two cells are in the background, eclipsed by two cells in the immediate foreground that appear larger. The cytoplasm of the cells appears light grey, and the nuclei appear dark grey. The two visible cells at front each contain one orange chromatid and one orange chromatid with a green tip.

Figure 7: Telophase II results in the production of four daughter cells.

Finally, in telophase II, nuclear membranes reform around the newly separated chromosomes, which relax and fade from view. As soon as the cytoplasm divides, meiosis is complete. There are now four daughter cells — two from each of the two cells that entered meiosis II — and each daughter cell has half the normal number of chromosomes (Figure 7). Each also contains new mixtures of genes within its chromosomes, thanks to recombination during meiosis I.

Why is meiosis important?

More about meiosis

Meiosis is important because it ensures that all organisms produced via sexual reproduction contain the correct number of chromosomes. Meiosis also produces genetic variation by way of the process of recombination. Later, this variation is increased even further when two gametes unite during fertilization, thereby creating offspring with unique combinations of DNA. This constant mixing of parental DNA in sexual reproduction helps fuel the incredible diversity of life on Earth.