Meiosis

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Meiosis

• Biology 1114

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Life Cycles are a Diagrammatic Representation of
the Events in the Organisms Development and
Reproduction
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The Problem

• Mitosis produces two cells with the same number
  of chromosomes as the parent cell. Mitosis of a
  diploid cell (2n) produces two diploid daughter
  cells. If two diploid cells went on to
  participate in sexual reproduction, their fusion
  would produce a tetraploid (4n) zygote.

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Ploidy

• Diploid
• - Two sets of chromosomes
• - 2n
• Haploid
• - 1 set of chromosomes
• - 1n
• Polyploid
• - More than two sets of chromosomes

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Homologous Chromosomes carry the same
genes Alleles are a type of gene that carry
traits
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Gametes are Sex Cells Sperm (male) Egg (female)
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Gametes each contain one set of genetic
information (Haploid) Gametes unite to form a
zygote (Diploid) The zygote then divides by
mitosis to form an organism
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It is essential for organisms that reproduce
sexually to half their number of chromosomes,
otherwise, with each successive generation, the
number of chromosomes would double.
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Plant Life Cycles (Alternation of Generations)

• Sporophyte Phase
• 1. Diploid Phase
• 2. Meiosis Occurs Here
• Gametophyte Phase
• 1. Haploid Phase
• 2. Gametes are Produced by Mitosis

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Meiosis

• The process in which haploid gametes (egg and
  sperm) are formed.
• Consists of Two Phases
• Meiosis I
• Meiosis II

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Interphase
Identical to Interphase in Mitosis. Start out
with a diploid cell.
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Prophase I
Prophase I is virtually identical to prophase in
mitosis. Chromosomes and spindle fibers
appear. Nuclear membrane and nucleolus
disappears. Homologous chromosomes pair up.
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Metaphase I
Chromosomes line up in homologous pairs across
equator of cell forming tetrads. Spindle fibers
attach to chromosomes. Crossing-over may occur.
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Anaphase I
Spindle fibers contract, separating the
homologous pairs of chromosomes.
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Telephase I
Two daughter cells form each with a single set of
chromosomes, half the total number in the
original cell. Nuclear membrane and nucleolus may
reappear. Cell is now haploid.
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Prophase II
Nuclear membrane and nucleolus disappear. Centriol
es migrate toward poles. A new set of spindle
fibers form and chromosomes begin to move toward
the equator of the cell.
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Metaphase II
Chromosomes line up across the equator of the
cell.
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Anaphase II
The spindle fibers shorten, drawing the
chromosomes toward each pole of the cell.
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Teleophase II
Formation of new nuclear membrane. Fading of
chromosomes. Cytokinesis occurs resulting in four
haploid daughter cells.
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Comparison of Mitosis and Meiosis

• Mitosis
• Results in two genetically identical daughter
  cells.
• Diploid cell ? 1 cell division ? 2 diploid
  daughter cells
• Involves 1 single nuclear division after DNA
  replication.

• Meiosis
• Results in four genetically different daughter
  cells
• Diploid cell ? 2 cell divisions ? 4 haploid
  daughter cells
• Involves 2 nuclear divisions with no DNA
  replication between them.

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This Part Reduces Chromosome Number
This Part is Simply Mitosis All Over Again to
Duplicate Cells
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Gender Based

• Spermatogenesis
• Meiosis and gamete formation in males.
• 1 diploid cell ? 4 mature sperm
• Oogenesis
• Meiosis and gamete formation in females.
• 1 diploid cell ? 1 mature ovum 3 polar bodies.