Meiosis

What you’ll learn to do: Identify the stages of meiosis by picture and by description of major milestones; explain why meiosis involves two rounds of nuclear division

What is the difference between mitosis and meiosis? We know that mitosis produces the somatic cells of the body like skin and muscle cells, and meiosis produces sex cells like eggs and sperm — let’s learn just how similar and different these processes are.

Learning Outcomes

  • Identify the stages of meiosis
  • Describe the steps of meiosis I
  • Describe the steps of meiosis II

Stages of Meiosis

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. Hippopotamuses give birth to hippopotamus calves, Joshua trees produce seeds from which Joshua tree seedlings emerge, and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same.

Three images are shown. Part a shows a mother and baby hippopotamus. In part b, mature trees are pictured next to saplings. In part c, a mother and baby flamingo are shown.

Figure 1. Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters; credit b: modification of work by Ken Cole, USGS; credit c: modification of work by Martin Pettitt)

As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. While many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitosis, many single-celled organisms and most multicellular organisms reproduce regularly using another method: meiosis. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce.

Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.

Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.

Meiosis I

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. Finally, the G2 phase, also called the second gap phase, is the third and final phase of interphase; in this phase, the cell undergoes the final preparations for meiosis.

During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. This prepares the cell to enter prophase I, the first meiotic phase.

Prophase I

This illustration depicts two pairs of sister chromatids joined together to form homologous chromosomes. The chromatids are pinched together at the centromere and held together by the kinetochore. A protein lattice called a synaptonemal complex fuses the homologous chromosomes together along their entire length.

Figure 2. Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.

As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. (Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair.) The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other (Figure 2). The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over occurs at chaiasmata (singular = chiasma), the point of contact between non-sister chromosomes of a homologous pair (Figure 3).

At the end of prophase I, the pairs are held together only at the chiasmata (Figure 3) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

This illustration shows a pair of homologous chromosomes that are aligned. The ends of two non-sister chromatids of the homologous chromosomes cross over, and genetic material is exchanged. The non-sister chromatids between which genetic material was exchanged are called recombinant chromosomes. The other pair of non-sister chromatids that did not exchange genetic material are called non-recombinant chromosomes.

Figure 3. Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.

A second event in Prophase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata.

In addition, the nuclear membrane has broken down entirely.

Metaphase I

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator.  Recall that homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup. This randomness is the physical basis for the creation of the second form of genetic variation in offspring. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate; the possible number of alignments therefore equals 2n, where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (223) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 4).

This illustration shows that, in a cell with a set of two chromosomes, four possible arrangements of chromosomes can give rise to eight different kinds of gamete. These are the eight possible arrangements of chromosomes that can occur during meiosis of two chromosomes.

Figure 4. Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes.

To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

Anaphase I

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together a the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart (Figure 5).

This illustration compares chromosome alignment in meiosis I and meiosis II. In prometaphase I, homologous pairs of chromosomes are held together by chiasmata. In anaphase I, the homologous pair separates and the connections at the chiasmata are broken, but the sister chromatids remain attached at the centromere. In prometaphase II, the sister chromatids are held together at the centromere. In anaphase II, the centromere connections are broken and the sister chromatids separate.

Figure 5. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated.

Telophase I and Cytokinesis

In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.

Meiosis II

In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.

Prophase II

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

Metaphase II

The sister chromatids are maximally condensed and aligned at the equator of the cell.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

This illustration compares chromosome alignment in meiosis I and meiosis II. In prometaphase I, homologous pairs of chromosomes are held together by chiasmata. In anaphase I, the homologous pair separates and the connections at the chiasmata are broken, but the sister chromatids remain attached at the centromere. In prometaphase II, the sister chromatids are held together at the centromere. In anaphase II, the centromere connections are broken and the sister chromatids separate.

Figure 6. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated.

Telophase II and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in Figure 7.

This illustration outlines the stages of meiosis. In interphase, before meiosis begins, the chromosomes are duplicated. Meiosis I then proceeds through several stages. In prophase I, the chromosomes begin to condense and the nuclear envelope fragments. Homologous pairs of chromosomes line up, and chiasmata form between them. Crossing over occurs at the chiasmata. Spindle fibers emerge from the centrosomes. In prometaphase I, homologous chromosomes attach to the spindle microtubules. In metaphase I, homologous chromosomes line up at the metaphase plate. In anaphase I, the spindle microtubules pull the homologous pairs of chromosomes apart. In telophase I and cytokinesis, the sister chromatids arrive at the poles of the cell and begin to decondense. The nuclear envelope begins to form again, and cell division occurs. Meiosis II then proceeds through several stages. In prophase II, the sister chromatids condense and the nuclear envelope fragments. A new spindle begins to form. In prometaphase II, the sister chromatids become attached to the kinetochore. In metaphase II, the sister chromatids line up at the metaphase plate. In anaphase II, the sister chromatids are pulled apart by the shortening spindles. In telophase II and cytokinesis, the nuclear envelope forms again and cell division occurs, resulting in four haploid daughter cells.

Figure 7. An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells.

Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation.

The Complete Cycle

This video walks you through meiosis I and meiosis II:

Check Your Understanding

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