Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction

• Chapter 9

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Asexual Reproduction

• Single parent produces offspring
• All offspring are genetically identical to one
  another and to parent

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Sexual Reproduction

• Chromosomes are duplicated in germ cells
• Germ cells undergo meiosis and cytoplasmic
  division
• Cellular descendents of germ cells become gametes
• Gametes meet at fertilization

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Sexual Reproduction

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

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Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit
  new combinations of alleles, which leads to
  variations in traits
• This variation in traits is the basis for
  evolutionary change

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Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome
• One chromosome in each pair from mother, other
  from father
• Paternal and maternal chromosomes carry different
  alleles

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Gamete Formation

• Gametes are sex cells (sperm, eggs)
• Arise from germ cells

ovaries
anther
ovary
testes
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Chromosome Number

• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number

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Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei are formed

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Meiosis I
Each homologue in the cell pairs with its
partner,
then the partners separate
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Meiosis II

• The two sister chromatids of each duplicated
  chromosome are separated from each other

two chromosomes (unduplicated)
one chromosome (duplicated)
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Stages of Meiosis

• Meiosis I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

• Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

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Meiosis I - Stages
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Prophase I

• Each duplicated, condensed chromosome pairs with
  its homologue
• Homologues swap segments
• Each chromosome becomes attached to microtubules
  of newly forming spindle

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Metaphase I

• Chromosomes are pushed and pulled into the middle
  of cell
• Sister chromatids of one homologue orient toward
  one pole, and those of other homologue toward
  opposite pole
• The spindle is now fully formed

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Anaphase I

• Homologous chromosomes segregate from each other
• The sister chromatids of each chromosome remain
  attached

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Telophase I

• The chromosomes arrive at opposite poles
• The cytoplasm divides
• There are now two haploid cells
• This completes Meiosis I

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Meiosis II - Stages
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Prophase II

• Microtubules attach to the kinetochores of the
  duplicated chromosomes
• Motor proteins drive the movement of chromosomes
  toward the spindles equator

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Metaphase II

• All of the duplicated chromosomes are lined up at
  the spindle equator, midway between the poles

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Anaphase II

• Sister chromatids separate to become independent
  chromosomes
• Motor proteins interact with microtubules to move
  the separated chromosomes to opposite poles

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Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• The cytoplasm divides
• There are now four haploid cells

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Crossing Over

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

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Effect of Crossing Over

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

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Random Alignment

• During transition between prophase I and
  metaphase I, microtubules from spindle poles
  attach to kinetochores of chromosomes
• Initial contacts between microtubules and
  chromosomes are random

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Random Alignment

• Either the maternal or paternal member of a
  homologous pair can end up at either pole
• The chromosomes in a gamete are a mix of
  chromosomes from the two parents

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Possible Chromosome Combinations

• As a result of random alignment, the number of
  possible combinations of chromosomes in a gamete
  is
• 2n
• (n is number of chromosome types)

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Possible ChromosomeCombinations
1
2
3
or
or
or
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Plant Life Cycle
multicelled sporophyte
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
spores
gametes
multicelled gametophytes
mitosis
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Animal Life Cycle
multicelled body
mitosis
zygote
Diploid
meiosis
fertilization
Haploid
gametes
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Spermatogenesis
secondary spermatocytes (haploid)
spermato- gonium (diploid male germ cell)
primary spermatocyte (diploid)
spermatids (haploid)
Mitosis I, Cytoplasmic division
Meiosis II, Cytoplasmic division
Growth
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Oogenesis
three polar bodies (haploid)
first polar body (haploid)
primary oocyte (diploid)
oogonium (diploid reproductive cell)
secondary oocyte (haploid)
ovum (haploid)
Mitosis I, Cytoplasmic division
Meiosis II, Cytoplasmic division
Growth
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Fertilization

• Male and female gametes unite and nuclei fuse
• Fusion of two haploid nuclei produces diploid
  nucleus in the zygote
• Which two gametes unite is random
• Adds to variation among offspring

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Factors Contributing 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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Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs in somatic cells
• Produces clones

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring

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Prophase vs. Prophase I

• Prophase (Mitosis)
• Homologous pairs do not interact with each other
• Prophase I (Meiosis)
• Homologous pairs become zippered together and
  crossing over occurs

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Anaphase, Anaphase I, and Anaphase II

• Anaphase I (Meiosis)
• Homologous chromosomes are separated from each
  other
• Anaphase/Anaphase II (Mitosis/Meiosis)
• Sister chromatids of a chromosome are separated
  from each other

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Results of Mitosis and Meiosis

• Mitosis
• Two diploid cells produced
• Each identical to parent
• Meiosis
• Four haploid cells produced
• Differ from parent and one another