Meiosis and Sexual Life Cycles

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Meiosis and Sexual Life Cycles

• Chapter 13
• A. P. Biology
• Mr. Knowles
• Liberty Senior High School

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Mitosis- Review
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Mutations
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Comparison of Asexual and Sexual Reproduction

• In asexual reproduction
• One parent produces genetically identical
  offspring by mitosis

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• In sexual reproduction
• Two parents give rise to offspring that have
  unique combinations of genes inherited from the
  two parents.

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Leewenhoeks Early Ideas of Sperm
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Types of Cells

• Somatic (nonreproductive cells) are diploid.
  Body cells. Human somatic cells have 46
  chromosomes.
• Gametes (reproductive cells) are haploid.
  Human sperm and ova have 23 chromosomes.

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Gametes Fuse to Form a Zygote
n
2n
Syngamy
n
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• During fertilization
• These gametes, sperm and ovum, fuse, forming a
  diploid zygote.
• The zygote
• Develops into an adult organism.

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Fertilization or Syngamy
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Zygote Undergoes Mitosis
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Zygote is Diploid (2n)
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Why is the number of chromosomes in gametes ½
that of somatic cells?

• If not, after 10 generations the 46 chromosomes
  in human cells would increase to over 46 X 210.

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Meiosis

• A process of cell division in which the number of
  chromosomes in certain cells is halved during
  gamete formation.
• A reduction division.

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• The Human Life Cycle

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The Variety of Sexual Life Cycles

• The three main types of sexual life cycles
• Differ in the timing of meiosis and fertilization

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• In animals
• Meiosis occurs during gamete formation
• Gametes are the only haploid cells

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• Plants and some algae
• Exhibit an alternation of generations.
• The life cycle includes both diploid and haploid
  multicellular stages.

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• In most fungi and some protists
• Meiosis produces haploid cells that give rise to
  a haploid multicellular adult organism
• The haploid adult carries out mitosis, producing
  cells that will become gametes

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Sexual Life Cycle

• Reproduction that alternates between
  fertilization (a diploid state) and meiosis (a
  haploid state) sexual reproduction.
• Sexual reproduction provides DNA from both
  parents.

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Some Sexual Life Cycles

• Unicellular Protists individual cells function
  as gametes?undergo meiosis and fuse with others.
• Plants- may produce haploid cells ?undergo
  mitosis and produce a multicellular haploid
  organism.
• Animals- set aside germ cells? undergo meiosis
  and become haploid gametes.

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The Stages of Meiosis

• An overview of meiosis

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• Interphase and Meiosis I

Figure 13.8
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• Telophase I, Cytokinesis, and Meiosis II

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Meiosis

• Two nuclear divisions.
• Labeled PMAT I and PMAT II.
• Prophase I is very different in meiosis than in
  mitosis.

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Interphase I
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Prophase I
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Synapsis in Prophase I
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Prophase I

• The duplicated homologous chromosomes condense
  and the ends of chromatids attach to the nuclear
  envelope.
• Homologous pairs align next to each other.
• The chromatids of one homologue align in precise
  register with the chromatids of the other in a
  process called synapsis (zipping up a zipper).

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Prophase I

• The DNA of chromatids unwind and base pair with
  the complementary strand on the the other
  homologue.
• The crossed-over sister chromatids form an
  X-shaped structure chiasma.
• DNA from one homologue sister chromatid is
  exchanged with the other Crossing Over.

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Crossing Over During Prophase I
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Prophase I

• The chiasmata hold the two pairs of sister
  chromatids together at the ends Terminal
  Chiasmata.
• The homologous chromatids have exchanged DNA.

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Prophase I
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Metaphase I
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Metaphase I

• Terminal chiasmata are still formed.
• The nuclear membrane breaks down and spindle
  fibers are forming.
• Only one side of each centromere faces outward
  toward the growing spindle fiber.
• The fibers can attach to kinetochore proteins on
  only one side of each centromere.

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Metaphase I

• Alignment of each pair on the metaphase plate is
  random.
• Either homologue may be situated toward a given
  pole.

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Metaphase I
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Anaphase I
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Anaphase I

• Spindle fibers contract and break the chiasmata
  and pull the centromeres toward the poles.
• The entire centromere of the homologue proceeds
  to one pole. (Different than mitosis).
• Each pole receives a haploid number of
  chromosomes, one member of each homologous pair.

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Anaphase I
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Anaphase I

• Because the orientation of the homologous
  chromosomes on the metaphase plate is random,
  genes on separate chromosomes are inherited
  independently Independent Assortment.

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Telophase 1
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Telophase I

• Individual chromosomes cluster at the poles.
• Each chromosome is made of two chromatids that
  are not identical because of crossing over that
  occurred in Prophase I.
• Cytokinesis may or may not occur after Telophase
  I. Nuclei may not reform.

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Telophase I
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The Second Meiotic Division

• No DNA replication before this division
  (Different than mitosis).
• Undergo a simple mitotic division with the
  products from Telophase I.
• Results are four haploid cells.
• Nuclei are reorganized.

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Prophase II
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Metaphase II
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Metaphase II
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Anaphase II
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Anaphase II
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Telophase II
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Telophase II
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• Comparison of Mitosis and Meiosis

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A Comparison of Mitosis and Meiosis

• Meiosis and mitosis can be distinguished from
  mitosis by three events in Meiosis l
• Synapsis and crossing over- Homologous
  chromosomes physically connect and exchange
  genetic information.
• Tetrads on the metaphase plate-
• At metaphase I of meiosis, paired homologous
  chromosomes (tetrads) are positioned on the
  metaphase plates
• Separation of homologues
• At anaphase I of meiosis, homologous pairs move
  toward opposite poles of the cell
• In anaphase II of meiosis, the sister chromatids
  separate

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• Concept 13.4 Genetic variation produced in
  sexual life cycles contributes to evolution
• 3 Sources of Genetic Variation
• 1. Reshuffling of genetic material in
  meiosis-Crossing Over.
• 2. Homologous pairs of chromosomes are oriented
  randomly at metaphase - Independent Assortment.
• 3. Random Fertilization of Gametes- Will produce
  a zygote with any of about 64 trillion diploid
  combinations

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Crossing Over

• Crossing over
• Produces recombinant chromosomes that carry genes
  derived from two different parents

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• Independent Assortment
• Each pair of chromosomes sorts its maternal and
  paternal homologues into daughter cells
  independently of the other pairs

Figure 13.10
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Evolutionary Significance of Genetic Variation
Within Populations

• Genetic variation
• -Is the raw material for evolution by natural
  selection
• Mutations
• Are the original source of genetic variation
• Sexual reproduction
• Produces new combinations of variant genes,
  adding more genetic diversity

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The Fate of Haploid Cells
n
Undergo Mitosis? Multicellular
Gametes
sperm
ova
n
n
Haploid Plants
Haploid Fungi
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Gametogenesis

• Meiotic divisions that result in gametes.

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Oogenesis Spermatogenesis
Oogonium (female germ cell, 2n)
Spermatogonium (male germ cell, 2n)
Germ cells committed to meiosis
Primary Oocyte (2n)
Primary Spermatocyte (2n)
First Meiotic Division
___Before Ovulation____
Secondary Spermatocyte (n)
Secondary Spermatocyte (n)
Secondary Ooctye (n)
First Polar Body (n)
Second Meiotic Division
___After Fertilization____
Four Spermatids (n)
Second Polar Body (n)
Ovum (n)
Four Spermatozoa (n)
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Primary Spermatocytes
Seminiferous Tubule
Spermatids
Mature Sperm
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Rat Sperm
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Fertilization or Syngamy
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Zygote is Diploid (2n)
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Cloning to Save an Endangered Species
Enculeated Cow Egg
Bateng Nucleus
Surrogate Cow
Frozen Bateng Cells
Cloned Bateng
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Microtubules and Motor Proteins-Spindle Fibers
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Cloned Animals as of 4-04
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What animal group hasnt been clonedyet?
Primates Why?
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Can haploid cells develop into functioning
animals or plants?
Bees Do It!
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How to make a bee colony?
Fertilized Egg, (2n)
Female Worker, (2n)
Meiosis
Mitosis w/o Cytokinesis
Unfertilized Egg, (1n)
Queen, (2n)
Male Drone, (1n)