Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction

• Chapter 12

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Meiosis

• A specialized type of cell reproduction
  specifically designed to create gametes (ova and
  sperm or spores) for sexual reproduction
• Meiosis halves the parental cells chromosome
  number from diploid to haploid
• Occurs only in sexually reproducing eukaryotic
  species

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Terms and Concepts

• Asexual reproduction
• A single parent
• No genetic variation
• All offspring are genetically identical to each
  other and the parent
• Mechanisms
• Mitosis
• Prokaryotic fission
• Vegetative propagation
• Pros and Cons
• No energy expended to find a mate (pro)
• The phrase There goes the last female Dodo bird
  doesnt mean extinction (pro)
• No genetic variation (con)

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Fig. 9-1a, p.138
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Terms and Concepts

• Sexual Reproduction
• Two parents required
• most of the timeself fertilizing flowers
• Genetic variation
• Variation is important for survival and
  adaptation (and ultimately evolution)
• Mechanisms
• Meiosis, gamete formation, and fertilization
• Pros and Cons
• Genetic variation (pro)
• Must expend energy to attract and find a mate
  (con)
• The phrase There goes the last female Dodo bird
  does mean extinction (con)

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Fig. 9-1b, p.138
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Fig. 9-1c, p.138
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Terms and Concepts

• Diploid
• Two copies of each type of chromosome
• Haploid
• One copy of each type of chromosome
• Homologs
• Pair of chromosomes
• Somatic cells
• Typical diploid cells of an animal
• Germ cells
• Cells committed to go through meiosis to produce
  gametes
• Gamete
• Reproductive cells (Egg/ova, sperm, spores)
• Zygote
• The first cell of a new individual (result of
  fertilization)

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Meiosis

• For sexual reproduction the gametes must have 1/2
  the number of chromosomes as the normal cells of
  an organism
• For example human cells have 46 chromosomes
• (23 pairs)
• If the sperm had 46 and the ovum had 46, then the
  resulting zygote would have 92 chromosomes!
• Over time cells would have more and more
  chromosomes
• The process of meiosis produces gametes with ½
  the number of chromosomes

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Fig. 9-12, p.150
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Meiosis

• During meiosis there are two rounds of division
• Meiosis I
• Separates homologs
• Meiosis II
• Separates sister chromatids

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Each homologue in the cell pairs with its partner,
then the partners separate.
p.141c
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Meiosis

• The process of meiosis is very similar to
  mitosis, however there are some key differences
  that account for producing genetically variable,
  haploid gametes
• The following slides will give an overview of
  meiosis especially pointing out the key
  differences from mitosis
• However, the actual processes of each phase will
  not be presented as they are the same as in
  mitosis
• Study figure 12.5

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Meiosis

• Meiosis I
• Prophase I
• Homologs pair up along their length
• The event is called synapsis or pairing of
  homologous chromosomes
• The structure of the two chromosomes is called a
  tetrad
• There are four strands of DNA, two sets of sister
  chromatids
• Crossing over
• While the homologs are closely associated they
  can swap segments of DNA
• This creates novel combinations of gene traits in
  both strands
• Figure 12.7

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Fig. 9-6c, p.144
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Fig. 9-6d, p.144
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Fig. 9-6e, p.144
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Fig. 9-6f, p.144
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Meiosis

• Meiosis I
• Metaphase I
• Homolog pairs line up at the equator (pushed and
  pulled by the microtubules of the bipolar
  spindle, just as in mitosis)
• Random Alignment (fig 12.7)
• Homologs can be attached to either spindle pole
• Each homolog can be packaged into either one of
  the two new nuclei
• Increases the number of potential combinations of
  maternal and paternal alleles in gametes
• There are over 8 million possible combinations of
  the maternal and paternal chromosomes which adds
  to genetic variability of meiosis

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1
2
3
combinations possible
or
or
or
Fig. 9-7, p.145
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Meiosis

• Meiosis I
• Anaphase I
• One of each duplicated chromosome or homolog is
  pulled towards a spindle pole by the microtubules
• Its homolog moves to the opposite pole
• This is the step that creates haploid cells by
  separating the homologous pairs

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p.141c
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Meiosis

• Meiosis I
• Telophase I
• One of each type of chromosome has arrived at a
  spindle pole
• For most species the cytoplasm will divide
• For a few, cytoplasmic division occurs after both
  rounds of meiosis
• Cells often proceed directly into meiosis II
  without completely finishing telophase I

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Meiosis

• Meiosis II (both cells created by meiosis I will
  follow through the steps of meiosis II to divide
  a second time)
• Prophase II
• The centriole pairs (centrosomes) separate and
  create a new bipolar spindle
• Microtubules latch onto separate chromatids
• Chromosomes are
• Still duplicated (two chromatids each), but only
  one of each type of chromosome (haploid)
• Generally still condensed following meiosis I
• There is no DNA replication between meiosis I and
  II

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Meiosis

• Meiosis II
• Metaphase II
• Chromosomes are lined up at the equator
• Anaphase II
• Sister chromatids separate and are pulled towards
  opposite spindle poles
• Telophase II
• New nuclear envelope forms around all four new
  haploid nuclei
• Cytoplasmic division results in four cells each
  containing a haploid number of unduplicated
  chromosomes

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p.141d
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Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
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Meiosis

• Meiosis introduces genetic variations in traits
• Two parents both contribute genes to their
  offspring
• One of each autosomal chromosome and one sex
  chromosome are passed to offspring
• Thus offspring get two of each type of chromosome
• Each pair of chromosomes carries the same genes
• Genes may not be identical
• The differences or traits of the same gene are
  called alleles

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Meiosis

• Meiosis introduces genetic variations (mixes of
  different alleles) in traits
• Crossing over (figure 12.6)
• Prophase I
• Random alignment (figure 12.7)
• Metaphase I
• Remember genetic variation is one of the pros
  of sexual reproduction

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Meiosis

• Results of meiosis
• 1 diploid parent cell ? 4 genetically variable
  haploid daughter cells

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Questions

• When homologs swap DNA this is called?
• T or F Homologs have a predictable pattern they
  use to line up during metaphase I.
• What is separated during Anaphase I?
• What is separated during Anaphase II?
• What processes of meiosis contribute to genetic
  variation?
• How many cells are produced by meiosis?

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From gametes to offspring

• Sexual reproduction
• Meiosis ? four haploid cells
• Gamete formation
• Sperm formation
• Ova/Egg formation
• Fertilization
• Fusion of the haploid nuclei of the ovum and
  sperm
• Creates a diploid zygote
• Adds to variation
• Which sperm fertilizes which egg is a matter of
  chance (see next chapter!)

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Fig. 9-9, p.147
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Fig. 9-10a, p.147
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Fig. 9-12, p.150
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Sexual Reproduction in Animals

• Animals use Gametic Meiosis
• Meiosis produces haploid gametes
• Gametes do not divide (remain uni-cellular)
• Fertilization produces a diploid zygote
• Zygote undergoes mitosis to produce a
  multicellular diploid body (adult animal)

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Sexual Reproduction in Plants

• Plants use Sporic Meiosis (alternation of
  generations)
• Meiosis produces haploid spores
• Spores undergo mitosis to produce a multicellular
  haploid body (gametophyte)
• Fertilization produces a diploid zygote
• Zygote undergoes mitosis to produce a
  multicellular diploid body (sporophyte)
• Alternates between multi-cellular diploid and
  multi-cellular haploid bodies

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mitosis
Stepped Art
Fig, 21.3, p. 326
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Sexual Reproduction in Plants

• Sporophyte (diploid, multicellular)
• Produces flowers which contain the germ cells
• Germ cells are located in the carpel (ova) and
  stamen (pollen)

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Sexual Reproduction in Plants

• Production of the ova
• Occurs in flowers within the pistil
• Made up of 1 or more carpels
• Stigma
• Style
• Ovary

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Sexual Reproduction in Plants

• Production of the ova
• Occurs in flowers within the pistil
• The ovary contains 1 or more ovules which produce
  egg sacs
• Meiosis of the ovules produces spores
• Spores undergo mitosis to become the egg sac
  (gametophyte)

Meiosis
Spores
Egg Sac
Mitosis
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an ovule
ovary wall
megasporocyte
integument
stalk
ovary (cutaway view)
Diploid Stage Haploid Stage
Double Fertilization
Meiosis
Fig. 28.6b, p. 454-5
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Sexual Reproduction in Plants

• Production of pollen
• Occurs in flowers in the stamen
• Anther
• Filament

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Sexual Reproduction in Plants

• Production of pollen
• The anther contains germ cells in pollen sacks
• Meiosis produces spores
• Spores undergo mitosis to become the pollen
  (gametophyte)

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pollen sac
anther (cutaway view)
filament
Germ cell
Diploid Stage Haploid Stage
Meiosis
Spores
Mitosis
Pollen
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Sexual Reproduction in Plants

• Pollination
• The transfer of pollen from an anther to a stigma
  (not fertilization)
• Wind, insects, mammals, etc.
• Released pollen lands on the stigma
• Pollen grows a tube through the style to the
  ovary

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Sexual Reproduction in Plants

• Fertilization
• Two sperm enter the egg sac from the pollen tube
• One fuses with the egg forming the diploid zygote
• Develops into the embryo
• The other fuses with the central cell to form a
  triploid endosperm

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Sexual Reproduction in Plants

• Development
• A seed develops from each mature fertilized ovule
• Fruit develops from the ovary or other tissue
• Embryo grows into a new mature sporophyte

Nutritive tissue
Embryo
Seed
Ovule
Ovary
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Fig. 27-5, p.451
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Fig. 9-8, p.146
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Sexual Reproduction in Fungus

• Fungus use Zygotic Meiosis
• Meiosis of the zygote produces haploid spores
• Spores undergo mitosis to produce a multicellular
  haploid body (mycelium/fungus)
• Mycelia of two fungi fuse and develop into a
  fruiting body (mushroom)
• Fertilization produces a diploid zygote
• Zygote remains uni-cellular
• Zygote is the only diploid cell during the life
  cycle

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Diploid Zygote
a
Diploid Stage
nuclear fusion
meiosis
Haploid Stage
Cells with two nuclei (n n) form on gills
g
spore (n)
at gill margin
gill
Mitosis
cap
stalk
mycelium
cytoplasmic fusion
mycelium
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Summary

• Terms and concepts
• Meiosis I and II
• Pairing of homologs, crossing over, random
  alignment
• 4 haploid daughter cells
• Sexual reproduction
• Meiosis, gamete formation, fertilization
• Animal reproduction
• Plant reproduction
• Fungus reproduction