Sexual reproduction and meiosis

1
Sexual reproduction and meiosis

• Genetic contribution from two parents

2
Discovery of gametes and somatic cells

• Each gamete has half a set of chromosomes
• Two gametes (from the same species) fuse together
  to form a diploid cell
• Sexually reproducing organisms have haploid and
  diploid stages in their life cycles

3
Sexual life cycle in animals

• germ line cells give rise to gametes
• Zygote the cell formed from the fusion of two
  gametes
• We all started out as one cell!

4
Meiosis how gametes are formed

• Two rounds of division meiosis I and meiosis II
• In humans, the cell starting meiosis I has 92
  chromosomes
• After meiosis I two cells with 46 chromosomes
  each
• After meiosis II 4 cells with 23 chromosomes each

5
Homologous chromosomes

• Synaptonemal complex holds homologues closely
  together
• Tetrad remember, at this stage the
  chromosome is actually two sister chromatids

6
Crossing over- genetic exchange

• Alleles versions of genes found in the same
  region on pairs of homologous chromosomes
• Alleles can be exchanged in crossing over
• Sites of crossing over chiasmata
• Chiasmata pulled apart in anaphase I (sister
  chromatids still attached)

7
Meiosis I still 2n
8
Meiosis II mitosis without another round of DNA
replication
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Nondisjunction results in aneuploidy

• Chromosomes dont separate during anaphase
• Euploidy (true ploidy)- the correct number of
  chromosomes
• Aneuploidy- too many or too few
• Result is usually fatal

10
Meiosis vs. mitosis structural comparisons

• Meiosis

• Mitosis

• Homologous pairing and crossing over during
  prophase I
• Paired homologues line up in metaphase I
• Sister chromatids stay together after anaphase I
• After meiosis II, sister chromatids separate and
  each (of four) cells has half the original number
  of chromosomes

• No homologous pairing
• Homologues line up in single file in metaphase
• Sister chromatids are pulled apart in anaphase
• After mitosis, each (of two) cells has the same
  number of chromosomes as the original cell

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Meiosis vs mitosis genetic comparisons

• Meiosis

• Mitosis

• Daughter cells are not identical
• Random pairing of homologues in metaphase I
• 2n possible combinations
• Crossing over

• Daughter cells are usually identical to parent
  cells

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Meiosis vs. mitosis purpose

• Meiosis

• Mitosis

• Formation of gametes (sexual reproduction)
• Genetic diversity

• Asexual reproduction in eukaryotes
• Increase in size (growth of tissues)
• Repair

13
Some tutorials to help you with mitosis and
meiosis

• http//www.hartnell.edu/tutorials/biology/mitosis.
  html
• http//www.hartnell.edu/tutorials/biology/meiosis.
  html