Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction

• Chapter 7

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Homologous Chromosomes

• Read p . 120-124 in chapter 6 (slides 2-7)
• Each somatic cell in the human body has 46
  chromosomes
• Separated into 23 pairs of homologous chromosomes
• Homologous chromosomes
• similar in size, shape, and
• genetic information
• Like matching socks

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Chromosomes come in pairs, like socks
Chromosomes come in pairs, like socks
Onehomolog
Homologouschromosomes(unreplicated)
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Homologous Chromosomes

• Karyotype photo of chromosomes arranged in
  homologous pairs
• Diploid cells have two chromosomes from each
• pair
• Haploid cells have one chromosome from each
• pair

Karyotype of male somatic cell
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Homologous Chromosomes

• The fusion of haploid sperm and egg cells during
  fertilization results in the formation of a
  diploid zygote
• Zygote first cell of a new Individual
• The zygote receives
• one chromosome of a
• pair of homologues
• from each parent
• One from mother
• One from father

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Types of Chromosomes

• Sex Chromosomes the two chromosomes which carry
  the genetic information which determines gender
• X and Y
• Females have XX
• Males have XY
• Autosomes all other
• chromosomes
• Carry genetic info which is
• common to both genders

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Chromosomal Disorders

• A karyotype allows doctors to identify potential
  disorders, like
• Trisomy 21 Downs Syndrome
• Individuals have an extra
• copy of chromosome pair 21
• Incidence ? as mothers age ?

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Types of Reproduction

• Chapter 7
• Asexual Reproduction
• Parent copies all of its DNA and passes it onto
  offspring
• Produces clones genetically identical to
  parents
• Occurs in both eukaryotes (hydra) and prokaryotes
  (bacteria)
• Advantages
• Quick can produce many offspring
• Energy-efficient
• Disadvantage NO genetic
• variability

New starfish growing from an arm
Hydra reproducing asexually by budding
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Types of Reproduction

• Sexual Reproduction
• Two parents
• Parents form haploid gametes by meiosis
• Union of sperm (n) and egg (n) results in a
  diploid (2n) zygote
• Zygote develops into offspring
• Advantages
• Allows for genetic variability
• Allows chance for repair of damaged DNA
  (protists)

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Function of Meiosis

• Function
• Sexual Reproduction
• Results in the formation of 4 haploid gametes
• 4 viable sperm, 1 viable egg

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Stages of Meiosis

• 2 divisions
• 1st division Meiosis I reductional
• results in formation of 2 haploid cells
• 2nd division Meiosis II equational
• results in formation of 4 haploid cells

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Meiosis I

• Reductional division halves the chromosome
• Four stages
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Stages are similar to Mitosis EXCEPT
• Homologous chromosomes not sister chromatids are
  separated
• Results in two haploid cells

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Stages of Meiosis I

• Prophase I homologous chromosomes pair up and
  crossing over occurs
• Metaphase I Homologues align along equator
• Anaphase I Homologous chromosomes separate
• Telophase I Cytokinesis same as mitosis
• Resulting cells are now haploid

Prophase I Metaphase I Anaphase I
Telophase I
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Crossing-over

• Involves exchange of genetic material between
  homologoues during Prophase I
• Results in genetic recombination a reshuffling
  of possible genetic traits
• Results in 2 chromosomes which are different from
  the original two.

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Meiosis II

• Follows immediately after Meiosis I
• Four stages
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
• Stages are identical to Mitosis
• Equational division
• Results in four haploid cells

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Stages of Meiosis II

• Prophase II new spindle forms and nuc. Envelope
  disintegrates
• Metaphase II chromosomes align along equator
• Anaphase II sister chromatids separate
• Telophase II Cytokinesis same as mitosis

Prophase II Metaphase II Anaphase II
Telophase II
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Genetic Variation

• Why is it that you and your siblings have the
  same parents, but look different?
• Three Sources of Genetic Variation
• Independent Assortment
• Crossing-over
• Random fertilization
• Importance
• Results in new combinations of genes
• Allows for evolution to occur

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Independent Assortment

• Each gamete receives only 1 chromosome of each
  homologous pair
• There are 23 human chromosomes, thus
  2238,388,608 possiblities!

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Random Fertilization

• Usually 1 egg released each month which one?
• Only 1 sperm fertilizes each egg (300 M start the
  journey to the egg!)
• Random pairing of sperm and egg
• MANY possible combinations of sperm and egg

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Gamete Formation

• Meiosis results in 4 haploid cells which are
  genetically different than the parent cell.
• Occurs in sex organs (testes and ovaries)
• Formation of sperm is different than formation of
  eggs

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Spermatogenesis

• Division of cytoplasm is EQUAL in both Meiosis I
  and Meiosis II
• Results in 4 haploid spermatids
• All 4 spermatids mature into sperm and develop a
  tail

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Oogenesis

• Division of cytoplasm is UNEQUAL in both Meiosis
  I and Meiosis II
• Function creates egg will lots of cytoplasm
• Results in
• 1 large oocyte
• 3 small polar bodies (all die)
• Oocyte will develop into ovum (egg)

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Sexual Life Cycles

• Life Cycle entire span of life from one
  generation to the next generation
• Haploid Life Cycle
• spend the majority of their life as haploid
  organisms
• produce diploid zygotes only during reproduction
• Many protists

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Sexual Life Cycles

• Diploid Life Cycle
• spend the majority of their life as diploid
  organisms and produce haploid cells only for
  reproduction
• humans
• most animals

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Sexual Life Cycles

• Alternation of Generations
• Generations (individuals) alternate between
  haploid states and diploid states
• Meiosis produces spores not gametes
• Plants