What type of chromosomes are seen in meiosis 2?

DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

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The Biology Project > Cell Biology > Meiosis > Review

A review of meiosis

Comparing Meiosis and Mitosis

• Chromosome behavior
  1. Mitosis: Homologous chromosomes independent
  2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase I
• Chromosome number- reduction in meiosis
  1. Mitosis- identical daughter cells
  2. Meiosis- daughter cells haploid
• Genetic identity of progeny:
  1. Mitosis: identical daughter cells
  2. Meiosis: daughter cells have new assortment of parental chromosomes
  3. Meiosis: chromatids not identical, crossing over

Meiotic errors

• Nondisjunction- homologues don't separate in meiosis 1
  1. Results in aneuploidy
  2. Usually embryo lethal
  3. Trisomy 21, exception leading to Downs syndrome
  4. Sex chromosomes
     1. Turner syndrome: monosomy X
     2. Klinefelter syndrome: XXY
• Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently
• Meiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid).
• Trisomy 21 does not prevent meiosis

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The Biology Project > Cell Biology > Meiosis > Review

The Biology Project
Department of Biochemistry and Molecular Biophysics
University of Arizona
April 1997 Revised: August 2004

Contact the Development Team

http://www.biology.arizona.edu
All contents copyright © 1997 - 2004. All rights reserved.

• 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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Meiosis is how eukaryotic cells (plants, animals, and fungi) reproduce sexually. It is a process of chromosomal reduction, which means that a diploid cell (this means a cell with two complete and identical chromosome sets) is reduced to form haploid cells (these are cells with only one chromosome set). The haploid cells produced by meiosis are germ cells, also known as gametes, sex cells or spores in plants and fungi. These are essential for sexual reproduction: two germ cells combine to form a diploid zygote, which grows to form another functional adult of the same species.

The process of chromosomal reduction is important in the conservation of the chromosomal number of a species. If chromosome numbers were not reduced, and a diploid germ cell was produced by each parent, then the resulting offspring would have a tetraploid chromosome set: that is, it would have four identical sets of chromosomes. This number would keep increasing with each generation. This is why the chromosomal reduction is vital for the continuation of each species.

Meiosis occurs in two distinct phases: meiosis I and meiosis II. There are many similarities and differences between these phases, with each phase producing different products and each phase being as crucial to the production of viable germ cells.

What Happens Before Meiosis?

Before meiosis, the chromosomes in the nucleus of the cell replicate to produce double the amount of chromosomal material. After chromosomal replication, chromosomes separate into sister chromatids. This is known as interphase, and can be further broken down into two phases in the meiotic cycle: Growth (G), and Synthesis (S). During the G phase proteins and enzymes necessary for growth are synthesized, while during the S phase chromosomal material is doubled.

Meiosis is then split into two phases: meiosis I and meiosis II. In each of these phases, there is a prophase, a metaphase, and anaphase and a telophase. In meiosis I these are known as prophase I, metaphase I, anaphase I and telophase I, while in meiosis II they are known as prophase II, metaphase II, anaphase II and telophase II. Different products are formed by these phases, although the basic principles of each are the same. Also, meiosis I is preceded in interphase by both G phase and S phase, while meiosis II is only preceded by S phase: chromosomal replication is not necessary again.

The Phases of Meiosis I

After Interphase I meiosis I occurs after Interphase I, where proteins are grown in G phase and chromosomes are replicated in S phase. Following this, four phases occur. Meiosis I is known as reductive division, as the cells are reduced from being diploid cells to being haploid cells.

1. Prophase I

Prophase I is the longest phase of meiosis, with three main events occurring. The first is the condensation of chromatin into chromosomes that can be seen through the microscope; the second is the synapsis or physical contact between homologous chromosomes; and the crossing over of genetic material between these synapsed chromosomes. These events occur in five sub-phases:

• Leptonema – The first prophase event occurs: chromatin condenses to form visible chromosomes. Condensation and coiling of chromosomes occur.
• Zygonema – Chromosomes line up to form homologous pairs, in a process known as the homology search. These pairs are also known as bivalents. Synapsis happens when the homologous pairs join. The synaptonemal complex forms.
• Pachynema – The third main event of prophase I occurs: crossing over. Nonsister chromatids of homologous chromosome pairs exchange parts or segments. Chiasmata form where these exchanges have occurred. Each chromosome is now different to its parent chromosome but contains the same amount of genetic material.
• Diplonema – The synaptonemal complex dissolves and chromosome pairs begin to separate. The chromosomes uncoil slightly to allow DNA transcription.
• Diakinesis – Chromosome condensation is furthered. Homologous chromosomes separate further but are still joined by a chiasmata, which moves towards the ends of the chromatids in a process referred to as terminalization. The nuclear envelope and nucleoli disintegrate, and the meiotic spindle begins to form. Microtubules attach to the chromosomes at the kinetochore of each sister chromatid.

2. Metaphase I

Homologous pairs of chromosomes align on the equatorial plane at the center of the cell. Independent assortment determines the orientation of each bivalent but ensures that half of each chromosome pair is oriented to each pole. This is to ensure that homologous chromosomes do not end up in the same cell. The arms of the sister chromatids are convergent.

3. Anaphase I

Microtubules begin to shorten, pulling one chromosome of each homologous pair to opposite poles in a process known as disjunction. The sister chromatids of each chromosome stay connected. The cell begins to elongate in preparation for cytokinesis.

4. Telophase I

Meiosis I ends when the chromosomes of each homologous pair arrive at opposing poles of the cell. The microtubules disintegrate, and a new nuclear membrane forms around each haploid set of chromosomes. The chromosomes uncoil, forming chromatin again, and cytokinesis occurs, forming two non-identical daughter cells. A resting phase known as interkinesis or interphase II happens in some organisms.

The Phases of Meiosis II

Meiosis II may begin with interkinesis or interphase II. This differs from interphase I in that no S phase occurs, as the DNA has already been replicated. Thus only a G phase occurs. Meiosis II is known as equational division, as the cells begin as haploid cells and end as haploid cells. There are again four phases in meiosis II: these differ slightly from those in meiosis I.

1. Prophase II

Chromatin condenses to form visible chromosomes again. The nuclear envelope and nucleolus disintegrate, and spindle fibers begin to appear. No crossing over occurs.

2. Metaphase II

Spindle fibers connect to the kinetochore of each sister chromatid. The chromosomes align at the equatorial plane, which is rotated 90° compared to the equatorial plane in meiosis I. One sister chromatid faces each pole, with the arms divergent.

3. Anaphase II

The spindle fibers connected to each sister chromatid shorten, pulling one sister chromatid to each pole. Sister chromatids are known as sister chromosomes from this point.

4. Telophase II

Meiosis II ends when the sister chromosomes have reached opposing poles. The spindle disintegrates, and the chromosomes recoil, forming chromatin. A nuclear envelope forms around each haploid chromosome set, before cytokinesis occurs, forming two daughter cells from each parent cell, or four haploid daughter cells in total.

Figure 1. The phases of meiosis I and meiosis II, showing the formation of four haploid cells from a single diploid cell.

How is Meiosis I Different from Meiosis II?

Meiosis is the production of four genetically diverse haploid daughter cells from one diploid parent cell. Meiosis can only occur in eukaryotic organisms. It is preceded by interphase, specifically the G phase of interphase. Both Meiosis I and II have the same number and arrangement of phases: prophase, metaphase, anaphase, and telophase. Both produce two daughter cells from each parent cell.

However, Meiosis I begins with one diploid parent cell and ends with two haploid daughter cells, halving the number of chromosomes in each cell. Meiosis II starts with two haploid parent cells and ends with four haploid daughter cells, maintaining the number of chromosomes in each cell. Homologous pairs of cells are present in meiosis I and separate into chromosomes before meiosis II. In meiosis II, these chromosomes are further separated into sister chromatids. Meiosis I includes crossing over or recombination of genetic material between chromosome pairs, while meiosis II does not. This occurs in meiosis I in a long and complicated prophase I, split into five sub-phases. The equatorial plane in meiosis II is rotated 90° from the alignment of the equatorial plane in meiosis I.

The table below summarizes the similarities and differences between meiosis I and meiosis II.

Table 1. The similarities and differences between meiosis I and meiosis II.

Why is Meiosis Important?

Meiosis is essential for the sexual reproduction of eukaryotic organisms, the enabling of genetic diversity through recombination, and the repair of genetic defects.

The crossing over or recombination of genes occurring in prophase I of meiosis I is vital to the genetic diversity of a species. This provides a buffer against genetic defects, susceptibility to disease and survival of possible extinction events, as there will always be certain individuals in a population better able to survive changes in environmental condition. Recombination further allows genetic defects to be masked or even replaced by healthy alleles in offspring of diseased parents.

Meiosis I and Meiosis II Biology Review

We now know that meiosis is the process of the production of haploid daughter cells from diploid parent cells, using chromosomal reduction. These daughter cells are genetically distinct from their parent cells due to the genetic recombination which occurs in meiosis I. This recombination is essential for genetic diversity within the population and the correction of genetic defects.

Meiosis I and II are similar in some aspects, including the number and arrangement of their phases and the production of two cells from a single cell. However, they also differ greatly, with meiosis I being reductive division and meiosis II being equational division. In this way, meiosis II is more similar to mitosis. Both stages of meiosis are important for the successful sexual reproduction of eukaryotic organisms.

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