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Meiosis
Importance in genetic diversity
Definitions
• Meiosis is a type of cell division which reduces the chromosome number of the parent cell by half to forms four haploid gametes
• Involves two successive divisions: meiosis I and meiosis 2
Meiosis I
• Prophase I: – Chromosomes become visible and condense
(become thicker and shorter) into rod- like structures
– Pairing between homologous chromosomes start– Replication of chromosomes happen; each
chromosome appears to be composed of sister chromatids
– Crossing over happens: the exchange of portions between homologous chromosomes.
Meiosis I
• Prophase I Nuclear membrane
Chromosomes
Cytoplasm
Cell membrane
Meiosis I
• Prophase I
Homologous chromosomes
Meiosis I• Homologous chromosomes: similar in size and shape and contain the
same kinds of genes; remember your alleles? Notice the presence of different alleles carried by the parental gametes.
Male gamete Female gamete
gene
chromosome
Meiosis I• Homologous chromosomes: similar in size and shape and contain the
same kinds of genes; remember your alleles? Notice the presence of different alleles carried by the parental gametes.
zygote
Homologous chromosomes
Genotype:AaRrBb
Meiosis I
• Prophase I Sister chromatids
Light colored: maternal
Dark colored: paternal
Meiosis I
• Prophase I Spindle fibers: responsible for pulling thechromosomes apart
Crossing over: exchange of parts;configuration: tetrad
Meiosis I
• Metaphase I: – Homologous chromosomes migrate to the middle
plane of the cell– spindle fibers attach to the centromeres (area
where the chromatids of a chromosome are attached)
Meiosis I• Anaphase I:
– Spindle fibers pull the homologous chromosomes towards opposite ends of the cell.
Spindle fiber pulling a chromosome
Meiosis I• Telophase I:
– First set of two daughter cells
Sister chromatids
p (petite) arm
q arm
Meiosis II
• Prophase II:– Nothing special; reorganization and preparation
for the next meiotic division
• Metaphase II– Replicated chromosomes move towards the
middle plane; spindle fibers attach to the centromeres
Meiosis II
• Metaphase II
Meiosis II• Anaphase II
– Separation of sister chromatids; note: sister chromatids are separated not homologous chromosomes.
– Migration towards the opposite sides
Used to be sister chromatids
Meiosis II • Telophase II
– Second set of 2 haploid daughter cells– Total gametes produced: 4
Light colored: maternal
Dark colored: paternal
Independent assortment• Members of the different pairs of homologous chromosomes separate
randomly from each other. Paternal chromosomes do not stay only on the right side and maternal only on the left.
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Independent assortment• Independent assortment produces variations in the combination of
chromosomes in the gametes.
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Example
If crossing over did not happen
Contains dominant allele S (smooth skin form)
Contains recessive allele s (wrinkled skin form)
Example
Recessive allele s
ZygoteSkin form ss (wrinkled skin)
If crossing over did not happen:Zygote
Skin form Ss (smooth skin)
Human chromosomes
Fruit fly (Drosophila melanogaster) chromosomes
