Chromosomes, Mitosis, and Meiosis

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Chromosomes, Mitosis,and Meiosis

• Brooker Chapter 15

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All Organisms Reproduce

• All life arises from previous living forms
• Reproduction can be asexual
• Requires ONE parent, which produces offspring
  which are genetically identical to the parent
• Reproduction can be sexual
• Requires TWO parents and their offspring are NOT
  genetically identical to either parent

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Asexual Reproduction

• Advantages
• Reproduction without sex
• Dont have to find a mate
• Yields more of the same more organisms that are
  well adapted for survival and reproduction to the
  existing environment like the parent was.
• Yields many offspring

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Sexual Reproduction

• How do we define SEX?
• Sex is the exchange of genes
• Requires specialized cells
• Gametes (sperm and egg)
• Requires mates
• Takes more time
• Advantage
• produces offspring that are genetically different
  from their parents
• Generates genetic variation within a species
• Genetic variation is the raw material of evolution

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Asexual Reproduction inProkaryotes and Eukaryotes

• Prokaryotes undergo simple fission to produce
  identical daughter cells asexual reproduction.
• No mitotic spindle (they have no cytoskeleton)
• Single circular DNA molecule separates as the
  cell wall separates the new cells
• Eukaryotes use mitosis to produce identical
  daughter cells by means of asexual reproduction
• Complex process involving the cytoskeleton
• Linear DNA strands are separated by cytoskeleton
  motor proteins

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Life of a Cell The Eukaryotic Cell Cycle

• Mitosis is asexual cell division
• Occurs in all body (somatic cells)
• Development
• Wound repair
• Regeneration

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Cell division at the genetic level

• Review the structure of a chromosome
• DNA replication

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Chromosomes
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The 23 Chromosomes of Homo sapiens

• Mitotic (doubled) chromosomes taken from a white
  blood cell at metaphase
• 23 chromosome types, 46 total
• What is the sex of this individual?

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Haploid and Diploid

• Haploid one set of chromosomes
• n chromosomes
• In most animals found in sperm and eggs
• Diploid two sets of chromosomes
• 2n chromosomes
• Somatic cells
• Fertilized egg

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Haploid and Diploid
Diploid 2n?
Diploid 2n8
Haploid n4
Haploid n?
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The Eukaryotic Cell Cycle

• Interphase (blue arrows)
• G1 Gap 1
• Basic cell functions
• S Synthesis of DNA
• DNA replication
• Centrioles duplicate
• G2 Gap 2
• Increased protein synthesis in preparation to
  divide
• Mitosis
• Cytokinesis

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Interphase Chromosomes and Chromatids

• In G1 the chromosomes are single
• In S, DNA replication results in duplicate
  chromosomes, one chromosome with two sister
  chromatids
• Sister chromatids are held together by the
  centromere

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Mitosis Occurs in 6 Stages

• Prophase
• Pro-metaphase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis

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Mitosis The Early Stages

• Prophase
• nuclear envelope degrades
• chromosomes condense
• spindle forms from the centrioles

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2. Pro-Metaphase

• Mitotic spindle has formed completely
• Sister chromatids attach to the spindle at the
  centromere

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Chromosomes, Centrioles and Centromeres

• The centrioles produce microtubules (MTs) which
  form the spindle. MTs bind to the centromere
• Chromatids separate during anaphase
• The spindle pulls chromosomes apart by applying
  pressure at the centromeres

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• 3. Metaphase
• Chromosomes attach to the spindle at the middle
  of the cell
• The metaphase plate

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• 4. Anaphase
• Chromatids separate at centromeres
• Chromosomes move to poles

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5. Telophase

• Nuclear envelope reforms in each of two daughter
  cells

6. Cytokinesis

• New cell membranes form around the new daughter
  cells

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Animal Cell 6. Cytokinesis

• Cytokinesis is the final separation of the two
  daughter cells
• Actin and myosin- dependent (like muscle
  contraction)
• Actin/myosin fibers pull like purse-string, nip
  cells apart

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At the end of Mitosis

• Two cells
• Each new cell has identical chromosomes
• 2n ? 2n

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Meiosis A special kind of cell division

• Sexual eukaryotic organisms combine their DNA to
  produce progeny (offspring)
• To do this they must prepare their DNA for
  combination with the DNA of another individual
• This process used to prepare the DNA is called
  meiosis

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Where does Meiosis Happen?

• Meiosis occurs in the ovaries or testis of
  animals
• It is the process by which eggs and sperm are
  produced.

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Meiosis Overview

• Meiosis means to make smaller
• Meiosis is the process by which haploid cells are
  produced from a cell that was originally diploid
• The two most important results of Meiosis
• A reduction of chromosome number
• Genetic variation

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Meiosis Happens in Two Phases

• Meiosis I
• Genetic variation
• Reduction in chromosome
• Meiosis II
• Almost identical to mitosis
• Both phases have the same stages as mitosis

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Meiosis I

• Interphase DNA centriole replication
• Prophase I Chromosomes condense
• recombination and crossover (gene shuffling)
• Metaphase I Chromosomes attach to spindle and
  line-up at the center of the cell
• Chromosome shuffling
• Anaphase I Separation of homologous chromosomes
• Reduction in chromosome
• Telophase I Reformation of nuclear envelope

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Prophase I

• DNA condenses into chromosomes
• Nuclear envelope disappears
• Homologous chromosomes find each other
• Gene shuffling
• Recombination or crossing over
• This step is important for creating genetic
  variation or diversity

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• Pro-Metaphase I
• Spindle forms
• Chromosome pairs randomly attach to spindle
  fibers (Random Assortment)
• This step is important for creating genetic
  variation or diversity

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• Metaphase I
• Chromosome align randomly in the center of the
  cell (Random Assortment)
• Metaphase plate
• This step is important for creating genetic
  variation or diversity

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Anaphase I and Telophase I
Reduction in chromosome numbers
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The First Meiotic Division

• At the end of meiosis I
• Two genetically unidentical cells
• Each with half the number of original chromosomes
• Diploid ? Haploid
• 2N? N

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Meiosis II

• Meiosis II is just like mitosis except that no
  DNA replication takes place.
• Chromatids separate during anaphase II
• The final results are haploid gametes

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Second Meiotic Division

• The second meiotic division (Meiosis II) proceeds
  exactly as if it were mitosis
• Prophase II
• Chromosomes condense again
• Metaphase II
• Chromosomes move to equator, with centromeres
  lined up on equator
• Anaphase II
• Sister chromatids separate
• Telophase II
• Nuclear envelopes reform
• Cytokinesis
• Haploid (1n) daughter cells gametes
• Now the cells are ready for fertilization process

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After the Second Meiotic Division

• Four genetically unidentical cells
• Each with ½ the number of original chromosomes
• Meiosis I 2N ? N
• Meiosis II N ? N

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Comparison of Meiosis I and II
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Sexual Life Cycles

• Mitosis, meiosis and fertilization allow
  organisms to time their sexual stages differently
• Females produce eggs via oogenesis males produce
  sperm via spermatogenesis, but both are haploid
  gametes.
• Fusion of the 1n gametes is fertilization, always
  results in a 2n zygote, which develops into embryo

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Mitosis, Meiosis, and Fertilization in Animals

• Meiosis gametes are formed sperm and eggs have
  1 set of chromosomes, and are haploid.
• Fertilization When they combine with another
  gamete, the new cell (the zygote) has both sets
  one from the mother, one from the father. It is
  2n or diploid.

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Spermatogenesis
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Oogenesis Meiosis in Females