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Stages of Meiosis

Meiosis I is often called reduction division because the diploid, or 2n, chromosome number is reduced to the haploid, or n chromosome number

The second phase, meiosis II, is marked by the separation of the two chromatids.

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Meiosis I

During prophase I, the nuclear membrane begins to dissolve, the centriole splits and its parts move to opposite poles within the cell, and spindle fibres are formed.

The chromosomes come together in homologous pairs. Each chromosome of the pair is a homologue and is composed of a pair of sister chromatids. The whole structure is then referred to as a tetrad because each pair is composed of four chromatids.

When the chromosomes come together the contact areas are referred to as synapsis.

As the chromosomes synapse, the chromatids often intertwine. Sometimes the intertwined chromatids from different homologues break and exchange segments this is called crossing over.

* Crossing over occurs between homologous pairs of chromosomes during prophase I of meiosis.

Steps of Meiosis

Meiosis I

* During meiosis I, homologous chromosomes are segregated.

Meiosis II

During meiosis II, pairs of chromatids will separate and move to opposite poles.

Note that, unlike with mitosis and meiosis I, there is no replication of chromosomes prior to meiosis II.

During this stage,

the nuclear membrane dissolves and the spindle fibres begin to form.

Arrangement of the

chromosomes, each with two chromatids, along the equatorial plate.

Breaking of the attachment between the two chromatids and by their movement to the opposite poles.

The nuclear membrane begins

to form around the chromatids, now referred to as chromosomes.

Comparing Mitosis and Meiosis

Mitosis	Meiosis
Occurs on one phase (division)	Occurs on two phases
Results in two daughter cells that are genetically identical to each other and to the parent cell. (diploid cells 2n)	Results in four cells that are genetically different from each other and from the parent cell. (haploid cells n)
No crossing-over	Crossing-over takes place in prophase I
Occurs all the time, for growth and repair, all over the body resulting in new somatic cells.	Occurs only for reproduction, only in the testes and ovaries, resulting in gametes.
The two daughter cells live.	Only one of the four female sex cells live and three die.

Explain the possible processes [crossing over, law of segregation, law of independent assortment] that provide genetic variation.�

When homologous chromosomes form pairs during prophase I of meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. It results in new combinations of genes on each chromosome.

As a result of the law of segregation, each diploid parent passes a random allele for each trait to his/her offspring during fertilization. Thus, segregation increases variation within a species.

When cells divide during meiosis, homologous chromosomes are randomly distributed during anaphase I, separating and segregating independently of each other. This is called independent assortment. It results in gametes that have unique combinations of chromosomes.

In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring. This is known as random fertilization.