Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction

• Honors Biology
• Beth Walker

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Sexual Reproduction

• Involves
• Meiosis
• Gamete production
• Fertilization
• Produces genetic variation among offspring

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Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome
• 23 pairs of chromosomes in humans
• Total of 46 chromosomes
• One chromosome in each pair from mother the
  other from father
• Paternal and maternal chromosomes carry different
  alleles (forms of a gene)

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Sexual Reproduction Shuffles Alleles

• Offspring inherit new combinations of alleles
• Leads to variations in traits
• Variation in traits is the basis for evolutionary
  change

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Gamete Formation

• Gametes are sex cells (sperm eggs)
• Arise from germ cells in reproductive organs

ovaries
testes
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Chromosome Number

• Sum total of chromosomes in a cell
• Somatic cells are diploid (2n) ? have two of each
  chromosome
• Gametes are haploid (n) ? have only one of each
  chromosome
• Meiosis halves chromosome number

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Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I (homologous chromosomes)
• Meiosis II (chromosomes)
• DNA is not duplicated between divisions
• Four haploid cells form

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Prophase I

• Each duplicated chromosome pairs with homologue
• Crossing Over ? Homologues swap segments
• Each chromosome becomes attached to spindle

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Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments

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Metaphase I

• Homologous chromosomes are pushed and pulled into
  the middle of cell
• The spindle is fully formed

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Anaphase I

• Homologous chromosomes separate
• The sister chromatids remain attached

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Telophase I

• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division
• Chromosome has been halved
• 2n ? n

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Prophase II

• Spindle fibers attach to the duplicated
  chromosomes

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Metaphase II

• Chromosomes line up at the spindle equator,
  midway between the poles

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Anaphase II

• Sister chromatids separate to become independent
  chromosomes

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Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• Four haploid (n) cells are formed

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REVIEW of MEIOSIS I
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REVIEW of MEIOSIS II
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How Does Meiosis Puts Variation in Traits?
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Crossing Over in Prophase I

• After crossing over, each chromosome contains
  both maternal and paternal segments
• Akamixes maternal and paternal genes
• Creates new allele combinations in offspring

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Random Alignment

• During the transition between prophase I and
  metaphase I, the homologous chromosomes randomly
  line up at the metaphase plate
• There are 223 possible combinations

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Possible ChromosomeCombinations
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2
3
or
or
or
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Random Fertilization

• Each female makes hundreds of eggs in her
  lifetime
• Each male makes billions of sperm in his lifetime
• Fertilization is a random combination of gametes!

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Three Factors that Contribute to Variation among
Offspring

• Crossing over during prophase I
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization

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From Gametes to Offspring
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Oogenesis ? Makes Eggs in Females
three polar bodies (haploid)
first polar body (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte (haploid)
ovum (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
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Spermatogenesis
Spermatogenesis ? Makes Sperm in Males
primary spermatocyte (diploid)
spermato-gonium (diploid )
sperm (mature, haploid male gametes)
secondary spermatocytes (haploid)
spermatids (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
cell differentiation, sperm formation
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Fertilization

• Male gamete (sperm) and female gamete (egg)
  unites
• A fertilized egg, called a zygote forms
• Mitosis creates a hollow ball of cells called a
  blastula
• The blastula folds inward to create a gastrula
• About 7 days after conception, the embryo
  attaches to the uterine lining

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Fertilization Development