Meiosis and Variation

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Meiosis I:
Prophase I:
1. The chromatin condenses and supercoils.
2. The chromosomes come together in their homologous pairs to form a bivalent. Each member of the pair has the same genes at the same loci. Each pair consists of one maternal and one paternal chromosome.
3. The non-sister chromatids wrap around each other and attach at points called chiasmata.
4. They may cross over and swap sections of chromatids with each other.
5. The nucleolus dissappears and the nuclear envelope breaks down.
6. A spindle forms.

Metaphase I:
1. Bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres.
2. The bivalents are arranged randomly (random assortment) with each member of the homologous pair facing opposite poles.

Anaphase I:
1. The homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles.
2, The centromeres do not divide.
3. The chiastmata separate and the lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.

Telophase I:
1. In most animal cells two new nuclear envelopes form - one around each set of chromosomes at each pole and the cell divides by cytokinesis. There is a brief interphase and the chromosomes uncoil.
2. In most plant cells the cell goes straight from anaphase I to meiosis II.

Meiosis II:
This occurs at right angles to meiosis I.
Prophase I:
1. If a nuclear envelope has reformed, it breaks down again.
2. The nucleolus dissapears, chromosomes condense and spindles form.

Metaphase II:
1. The chromosomes arrange themselves on the equator of the spindle. They are attached to spindle fibres at the centromeres.
2. The chromatids of each chromosome are randomly assorted.

Anaphase II:
1. The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate.

Telophase II:
1. Nuclear envelopes reforms around the haploid daughter nuclei.
2. In animals, the two cells now divide to give four daughter cells.
3. In plants, a tetrad of four haploid cells is formed.

Allele: An alternative version of a gene.
Locus: Specific position on a chromosome, occupied by a specific gene.
Phenotype: Observable characteristics of an organism.
Genotype: The combination of alleles possesed by an organism.
Dominant: Characteristic in which the allele responsible is expressed in the phenotype, even in those with heterozygous genotypes.
Codominant: A characteristic in which both alleles contribute to the phenotype.
Recessive: Characteristic in which the allele responsible is only expressed in the phenotype if there is no dominant allele present. It is not expressed when heterozygous and the expression is masked by the dominant allele.

Linkage: Genes for different characteristics are present at different loci on the same chromosome are linked.
Crossing-over: Where non-sister chromatids exchange alleles during prophase I of meiosis I.

Meiosis and fertilisation can lead to variation through the independent assortment of alleles. In meiosis, crossing-over 'shuffles' the alleles, random distribution and subsequent segregation of maternal and paternal chromosomes in…


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