11-4 Meiosis

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11-4 Meiosis

• Describe the process of meiosis.
• Compare meiosis and mitosis.

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Introduction

• Meiosis
• A Process by which the number of chromosomes per
  cell is cut in HALF through the separation of
  homologous chromosomes in a diploid cell
• Meiosis has TWO Phases Meiosis I and Meiosis II
• Starts with ONE DIPLOID (2n) CELL
• Ends with FOUR HAPLOID (n) CELLS that are NOT
  identical to each other or to the parent cell

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Why do Cells Undergo Meiosis?

• Gametes (aka sex cells egg and sperm) need to be
  HAPLOID (n) so that when they combine, the new
  organism is DIPLOID (2n) instead of
  TETRAPLOID(4n)

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Chromosome Number

• Each parent has a full number of chromosomes, 46
  in humans
• During meiosis this number has to be cut in half
  or the human offspring would have 92 chromosomes
• When the sperm (male sex cell) fertilizes the egg
  (female sex cell) the result is a zygote that
  contains the diploid number of chromosomes

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• Haploid (1n) a cell with half the number of
  chromosomes
• 23 chromosomes in human sex cells aka gametes
• Diploid (2n) a cell with the full number of
  chromosomes
• 46 in humans in somatic human cells
• Homologous chromosomes two chromosomes, one
  from mom and one from dad. Each one has genes
  for the same trait.
• Ex. B and b could be the dominant and recessive
  forms of hair color. One allele on each
  chromosomes located in the gene. One chromosome
  from each parent. This offspring is heterozygous
  for hair color Bb and heterozygous Aa.

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The Phases of Meiosis

• Consists of two separate cell divisions named
  Meiosis I and Meiosis II
• Starts with one diploid cell and ends with four
  haploid cells called gametes (sex cells)
• Meiosis I is known as REDUCTION

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Meiosis I REDUCTION homologous chromosomes are
separated

• Interphase I DNA is duplicated, chromatin
  centrioles appear
• Prep for cell division
• DNA duplicates S phase
• Cell grows
• Organelle duplication
• Very similar to interphase of mitosis

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Meiosis I - Prophase I

• Prophase I Nuclear envelope and nucleolus
  disappears, chromatin condenses into chromosomes
  (sister chromatids), homologous chromosomes pair
  up (TETRADS), crossing over may occur
• Synapsis - formation of tetrads - when homologous
  pairs find one another (this is the main
  difference between meiosis and mitosis)
• Tetrad two homologous chromosomes grouped
  together (total of FOUR chromatids)
• Crossing over process by which homologous
  chromosomes exchange pieces of chromatids forming
  new combinations

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Meiosis I - Prophase I

• Crossing Over - Chromatids of homologues exchange
  equal portions (usually)
• Chiasma cross-over exchange point
• New chromatids are called recombinants

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Meiosis I Metaphase Iand Anaphase I

• Metaphase I Tetrads - Homologous chromosomes
  line up in the middle (the metaphase plate)
• Anaphase I Homologous chromosomes separate, are
  pulled apart towards the poles
• Tetrads split sister chromatids are still
  together, attached at centromere

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Meiosis I Telophase I and Cytokinesis

• Telophase I Homologous chromosomes gather at
  opposite poles
• Nuclear membrane and nucleolus begin to reform
• Chromosomes begin to unwind ? chromatin
• Followed by cytokinesis I
• Cytokinesis Cytoplasm divides
• TWO new cells NON IDENTICALS (crossing over)
• Sister chromatids are still together

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Meiosis II Sister chromatids are separated
(just like mitosis)

• Just like mitosis may be preceded by brief
  period of rest (Interkinesis or Interphase II)
• NO DNA REPLICATION OCCURS

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• Meiosis II SAME AS MITOSIS
• Prophase II same as mitosis (nuclear membrane
  and nculeolus disappear, centrioles move to
  opposite poles, spindle fibers stretch across
  cell , chromatin condenses ? chromosomes
• Metaphase II sister chromatids line up in the
  middle/equator
• Anaphase II Centromeres divide, sister
  chromatids separate and move apart to the poles
• Telophase II reverse of prophase - nuclear
  envelope and nucleolus reform, chromosomes uncoil
  ? chromatin, cytokinesis begins
• Result 4 haploid cells, genetically different

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Meiosis End Result

• Start ? 1 diploid (2n) parent cell
• End ? 4 haploid (n) daughter cells
• Each cell contains HALF as much DNA as the
  original parent cell
• Each cell is genetically different (they are NOT
  all identical)

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Meiosis and Genetics

• Genetic Reassortment leads to greater genetic
  variation (why you have characteristics from both
  your parents)
• Sources
• Crossing Over (synapsis) new combinations of
  genes on chromosomes created by crossing over
  during prophase I
• Results in creation of chromosomes with genes
  from both parents on them hybrid mom-dad
  chromosome
• Humans average crossing over 2-3X per homologous
  pair
• Independent Assortment new assortment of
  chromosomes, when chromosomes line up at the
  equator during metaphase I it is completely
  random and lead to LOTS of different combinations
• Fertilization random meeting of sperm and egg

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Gamete Formation

• Males
• Spermatogenesis birth of sperm
• 4 equal size sperm produces for every meiotic
  division
• Females
• Oogenesis birth of egg (largest cell in
  female body)
• 1 egg and 3 polar bodies produced for every
  meiotic division

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Comparing Mitosis and Meiosis
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1
1
1
2
2
4
2n
n
Sister chromatids
Homologous then sister chromatids
identical
different
Production of somatic cells
Production of gametes
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Check Mitosis, Meiosis or Both Mitosis Meiosis Both
No pairing of homologous chromosomes
Two divisions
Four daughter cells produced
Associated with growth and asexual reproduction
Associated with sexual reproduction
One division
Two daughter cells produced
Involves duplication of chromosomes
Chromosome maintained
Chromosome is halved
Crossing over between homologous may occur
Daughter cells are identical to parent cell
Daughter cells are not identical to parent cell
Produces gametes
Synapsis occurs in prophase
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