Meiosis and Variation

Summary notes of the stages of meiosis and how meiosis produces variation within populations. Also includes genetic drift, speciation and artificial selection

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  • Created on: 12-05-11 08:30
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Meiosis and Variation
Assexual reproduction ­ Divides by mitosis to form offspring genetically identical to the parent.
Sexual reproduction ­ Production of gametes using meiosis.
Diploid cell with 2 sets of chromosomes divides to form 4 haploid daughter cells (gametes).
When 2 gametes fuse together, they produce a diploid zygote.
Helps produce genetic variation in a population
Prophase 1 Metaphase 1 Anaphase 1 Telophase 1
Chromosomes Nuclear envelope Homologous Chromosomes reach
condense and disappears. Centrioles chromosomes of each opposite poles and
become visible. have reached poles. bivalent separate and decondense. Two
Chiasmas can form Spindle fibres form are pulled to opposite nuclear envelopes
(crossing over) and attach to poles. Halves the no. of form.
centromeres chromosomes.
Prophase 2 Metaphase 2 Anaphase 2 Telophase 2
Cytokinesis 1 forms 2 Nuclear envelope Centromeres divide, Chromosomes reach
diploid cells. disappears and so sister chromatids poles and decondense.
Chromosomes chromosomes aligned pulled to opposite Cytokinesis 2 to form 4
condense and spindle along the equator poles new daughter haploid
fibres formed. cells
Each of the 4 daughter cells has one chromatid from each homologous pair in the original cell.

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How Meiosis cause Variation
1. Independent assortment ­ Alleles originally from mother mixed with alleles originally from
No. different combinations = 2n, n= haploid no. of chromosomes (23 in humans).
Only if alleles reside on different chromosomes.
2. Crossing over ­ During Prophase 1, chromatids within a bivalent can get so close together
that they get tangled up. Chiasmata form, where the chromatids cross over.…read more

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Alleles which give an advantage become more common in a population over time. Then, if the
environment remains much the same, the alleles in the population should not change much over
successive generations ­ Stabilising selection.
Directional (evolutionary) change = Change in selection pressures means that different alleles are
selected for, so allele frequencies within a population change.
Evolution ­ changes in the environment or the appearance of a new allele which change the allele
frequencies within a population. Eg. Resistant bacteria populations.…read more

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Phylogenetic Species Concept ­ Species is a group of organisms that is geographically separated,
morphologically or behaviourally distinct from other species, even if they can still interbreed to
produce fertile offspring. (Based on evolutionary relationships eg. Common ancestors)
Artifical Selection
The choice of humans of individual organisms from which to breed over successive generations to
improve the characteristics of a population to suit the requirements of humans.…read more


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