The Process of Meiosis

Learning Objectives

By the end of this section, you will be able to:

  • Describe the behavior of chromosomes during meiosis
  • Describe cellular events during meiosis
  • Compare the differences between meiosis and mitosis
  • Distinguish between the two instances of genetic variation

Sexual reproduction requires fertilization, the union of two cells from two individual organisms.  As mentioned earlier, haploid cells contain one set of chromosomes, while diploid cells contain two sets.. For reproduction to continue, the diploid cell must reduce its number of chromosome sets before fertilization can occur.  In addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.

 In each somatic cell of the organism, the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent . Haploid cells, containing a single copy of each homologous chromosome, are found only within structures that give rise to either gametes or spores. Gametes, sperm and egg, are the sex cells of animals and some plants.  Spores are haploid cells that can produce a haploid organism or  fuse with another spore to form a diploid cell.  Some plants and all fungi produce spores for reproduction.

Meiosis is the nuclear division forming haploid cells and is similar to mitosis. As mentioned earlier, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. The starting nucleus is always diploid with the resulting nuclei being 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 of meiosis are analogous to those of mitosis, the same  names are assigned. However, there are two rounds of division in meiosis.

Meiosis I

Meiosis I is preceded by an interphase consisting of the G1, S, and G2 phases, which are very similar to the phases preceding mitosis. The G1 phase is focused on cell growth. The S phase is when the DNA of the chromosomes is replicated. Finally, the G2 phase is the third and final phase of interphase where the cell undergoes its final preparations for meiosis.

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 1. Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment at the centromere.

Early in prophase I, the homologous chromosomes are attached to the nuclear envelope.  As the nuclear envelope breaks down, proteins, associated with homologous chromosomes, bring the pair closer together. This 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 1). In humans, even though the X and Y sex chromosomes are not homologous, they have a small region that allows them to pair up during prophase I.   An exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over, may occur during synapsis(Figure 2).

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 2. Cross over occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

 

 

 

As prophase I progresses, the chromosomes begin to condense. The homologous chromosomes remain attached to each other at the centromere. Following crossing over, the connection between homologous pairs is removed. At the end of prophase I, the pairs are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible(Figure 2).  Crossing over is the first source of genetic variation in the nuclei produced by meiosis.  Now, when that sister chromatid is moved into a gamete, some DNA from each parent moves forward.  Recombinant DNA is a molecule made from a combination of maternal and paternal genes that did not exist before the crossover.

Prometaphase I

In prometaphase I, the main event is the attachment of the spindle fiber microtubules to the centromere with the kinetochore proteins.  Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes’ kinetochores.  Now, the microtubules can pull the homologous pair apart.  At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole.

Metaphase I

During metaphase I, the homologous chromosomes are arranged in the center of the cell facing opposite poles. Random orientation of the homologous pairs occurs at the equator.  This is important in determining the genes carried by a gamete.  Each gamete will only receive one of the two homologous chromosomes.  Homologous chromosomes are not identical. They contain slight differences in their genetic information, allowing each gamete to have a unique genetic makeup.

 Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent.  In humans, one set of 23 chromosomes is present in the egg from the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis I, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. There is an equal chance the microtubule fiber encounters a chromosome from mom or dad.  The arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

This random event, or independent assortment of homologous chromosomes during metaphase I, is the second mechanism that introduces variation.  In each cell that undergoes meiosis, the arrangement of the tetrads is different.  There are two possibilities for orientation at the metaphase plate.  The number of variations is dependent on the number of chromosomes making up a set.  The possible number of alignments equals 2n, where n is the number of chromosomes per set. Humans have 23 chromosome pairs, resulting in over eight million (223) possible genetically distinct gametes. This number does not include the variations created during crossing over.  Given these two mechanisms, it is highly unlikely that any two haploid cells in meiosis will have the same genetic composition (Figure 3).

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.

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 3. 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 metaphase plate 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.

Anaphase I

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere(Figure 4).

Telophase I and Cytokinesis

In telophase I, the separated chromosomes arrive at opposite poles.  Depending on the species, the other typical telophase events may or may not occur.  The chromosomes may decondense and nuclear envelopes may form around the chromatids,   Cytokinesis, the separation of the cytoplasmic components, may occur without reformation of the nuclei. As mentioned previously, in nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow. While in plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate leads to the formation of cell walls that separate the two daughter cells.

The end of Meiosis I results in two haploid cells  There is only one full set of chromosomes present, because at each pole, there is just one of each pair of the homologous chromosomes.  But, each homologous chromosomes consists of two sister chromatids. Except for changes during crossing over, sister chromatids are merely duplicates of one of the two homologous chromosomes. In meiosis II, these two sister chromatids will separate.

Link to Learning

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

Meiosis II

In some species, cells enter a brief interphase-like state 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 together. During meiosis II, the sister chromatids of the two daughter cells separate.  Four new haploid gametes are formed.  The mechanics of meiosis II is very similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.

Prophase II

What occurs in prophase II is highly dependent on the events of telophase I. If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment. The centrosomes move away from each other toward opposite poles. New spindles are begin formation.

Prometaphase II

The nuclear envelopes are completely broken down.  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. There is a physical elongation of 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 4. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes.  The homologous chromosomes are arranged at the metaphase plate 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 metaphase plate 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.   The cells produced are genetically unique due to crossing over in prophase I and the random assortment of the tetrads during metaphase I.  The entire process of meiosis is outlined in Figure 5.

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

Comparing Meiosis and Mitosis

Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but exhibit distinct differences that lead to very different outcomes (Figure 6). Mitosis is a single nuclear division that results in two nuclei that are divided into two new cells. The nuclei are genetically identical to the original nucleus.  In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are divided into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only.

The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs associate with each other, are bound together, and undergo crossing over between nonsister chromatids.  They line up along the metaphase plate as tetrads.  With pulling apart of the tetrad during anaphase I, the number of chromosomal sets has been reduced.  Mitosis has not chromosomal reduction.

Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes line up on the metaphase plate. During anaphase II, as in mitotic anaphase, the centromeres divide and one sister chromatid is pulled to one pole while the other sister chromatid is pulled to the other pole. If not for crossing over, the two products of each individual meiosis II division would be identical (like in mitosis). But there will always be some crossing over.  Meiosis II is not a reduction division because although there are fewer copies of the genome. There is still one set of chromosomes, as at the end of meiosis I.

This illustration compares meiosis and mitosis. In meiosis, there are two rounds of cell division, whereas there is only one round of cell division in mitosis. In both mitosis and meiosis, DNA synthesis occurs during S phase. Synapsis of homologous chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Crossover of chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Homologous pairs of chromosomes line up at the metaphase plate during metaphase I of meiosis, but not during mitosis. Sister chromatids line up at the metaphase plate during metaphase II of meiosis and metaphase of mitosis. The result of meiosis is four haploid daughter cells, and the result of mitosis is two diploid daughter cells.

Figure 6. Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.

 

Link to Learning

Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.

Section Summary

Sexual reproduction requires that diploid organisms produce haploid cells.  These then fuse during fertilization to form diploid offspring.  Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologous chromosomes, and the second separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei can occur due to crossing over(prophase I) and random alignment of tetrads(metaphase I). The cells produced by meiosis are genetically unique.

Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single. nuclear divisions producing daughter nuclei that are genetically identical with the same number of chromosome sets as the original cell. Meiotic divisions include two nuclear divisions producing four daughter nuclei that are genetically different, having one chromosome set instead of the two sets like the parent cell. The main differences between the processes occur in the first division of meiosis.  The second division of meiosis is more similar to a mitotic division.

Additional Self Check Questions

1.  Define tetrad.

2.  Name two methods of variation in cell division.

3.   In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosisAnswers

1. A tetrad forms when homologous chromosomes pair up during synapsis.
2.  Crossing over and random alignment of tetrads provides variation during cell division.

3.  All of the stages of meiosis I are unique because homologous chromosomes are separated, not sister chromatids.