In what phase of meiosis are the sister chromatids pulled towards the opposite ends of the cell pulled towards the poles )?

• 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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Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides.

• G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase.
• S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.
• G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
• M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).
• The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.

Mitosis

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.

The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.

In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase.

The phases of mitosis

Prophase

Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.

Prometaphase

The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.

Metaphase

The chromosomes align themselves along the metaphase plate of the spindle apparatus.

Anaphase

The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)

Telophase

The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.

Cytokinesis

The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions.

Meiosis

Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome). The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids.

Meiosis I

Meiosis I separates the pairs of homologous chromosomes. 

In Meiosis I a special cell division reduces the cell from diploid to haploid.

Prophase I

The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

• Leptotene: chromosomes start to condense.
• Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).
• Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma).
• Diplotene: homologous chromosomes start to separate but remain attached by chiasmata.
• Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Prometaphase I

Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.

Metaphase I

Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I

The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell

Telophase I

The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis

The final cellular division to form two new cells, followed by Meiosis II. Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome.

Meiosis II

Meiosis II separates each chromosome into two chromatids.

The events of Meiosis II are analogous to those of a mitotic division, although the number of chromosomes involved has been halved.

Meiosis generates genetic diversity through:

• the exchange of genetic material between homologous chromosomes during Meiosis I
• the random alignment of maternal and paternal chromosomes in Meiosis I
• the random alignment of the sister chromatids at Meiosis II