Meiosis

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Meiosis

• Chapter 13 Meiosis and Sexual Life Cycles

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Variation

• Living organisms are distinguished by their
  ability to reproduce their own kind
• Genetics is the scientific study of heredity and
  variation
• Heredity is the transmission of traits from one
  generation to the next
• Variation is demonstrated by the differences in
  appearance that offspring show from parents and
  siblings

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Comparison of Reproductions

• In asexual reproduction, one parent produces
  genetically identical offspring by mitosis
• A clone is a group of genetically identical
  individuals from the same parent
• In sexual reproduction, two parents give rise to
  offspring that have unique combinations of genes
  inherited from the two parents
• A life cycle is the generation-to-generation
  sequence of stages in the reproductive history of
  an organism

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Genes

• Genes are the units of heredity, and are made up
  of segments of DNA
• Genes are passed to the next generation through
  reproductive cells called gametes (sperm and
  eggs)
• Each gene has a specific location called a locus
  on a certain chromosome
• Most DNA is packaged into chromosomes
• One set of chromosomes is inherited from each
  parent

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Chromosomes

• Every organism has its own chromosome number
• Somatic (body) cell chromosomes come in pairs
• Called diploid (2n) number of chromosomes
• The two chromosomes in each pair are called
  homologous chromosomes, or homologs
• They are the same length and carry genes
  controlling the same inherited characters
• 1 from mom
• 1 from dad
• In humans, somatic cells have 46 chromosomes

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Chromosomes

• Gametes have only 1 of each chromosome
• Called haploid (n) number of chromosomes
• In humans, sex cells are haploid n23
• A karyotype is an ordered display of the pairs of
  chromosomes from a cell

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Homologous Chromosomes Exception

• Sex chromosomes are called X and Y
• Human females have a homologous pair of X
  chromosomes (XX)
• Human males have one X and one Y chromosome
• The 22 pairs of chromosomes that do not determine
  sex are called autosomes (found in homologous
  pairs)

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Fertilization

• Each set of 23 chromosomes consists of 22
  autosomes and a single sex chromosome
• In an unfertilized egg (ovum), the sex chromosome
  is X
• In a sperm cell, the sex chromosome may be either
  X or Y
• Fertilization is the union of gametes (the sperm
  and the egg)
• The fertilized egg is called a zygote and has one
  set of chromosomes from each parent
• The zygote produces somatic cells by mitosis and
  develops into an adult

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Meiosis

• Meiosis is a type of cell division used to make
  gametes (sex cells)
• 2 nuclear divisions
• Meiosis I
• Meiosis II
• Begins with 1 diploid (2n) cell
• Ends with 4 haploid (n) cells

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Meiosis

• Gametes are the only types of human cells
  produced by meiosis
• Meiosis results in one set of chromosomes in each
  gamete (23)
• Gametes are the only haploid cells in animals
• Gametes fuse (23 23) to form a diploid zygote
  (46) that divides by mitosis to develop into a
  multicellular organism

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Cell Cycle Review

• G1, S, G2, M
• Mitosis has 4 main phases
• Prophase
• Metaphase
• Anaphase
• Telophase
• Cells split by cytokinesis
• Produce 2 identical cells
• Growth and repair

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Meiosis I

• Division in meiosis I occurs in four phases
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I and cytokinesis
• Meiosis I results in two haploid daughter cells
  with replicated chromosomes
• Focus is on splitting homologous chromosomes

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Interphase

• Meiosis I is preceded by interphase, in which
  chromosomes are replicated
• Each replicated chromosome consists of two
  identical sister chromatids
• Sister chromatids held together by centromere
• The sister chromatids are genetically identical

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Prophase I

• Chromosomes begin to condense
• Nuclear membrane nucleoli dissolve
• Homologous chromosomes pair up (align gene by
  gene) this is called synapsis
• Each pair of chromosomes forms a tetrad, a group
  of four chromatids
• Each tetrad usually has one or more chiasmata,
  X-shaped regions where crossing over occurs
• In crossing over, nonsister chromatids exchange
  DNA segments

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Prophase I Tetrads
Homologous chromosomes (each with sister
chromatids)
A Tetrad
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Metaphase I

• In metaphase I, tetrads independently line up
  across from each other sandwiching the equator
  (metaphase plate)
• In mitosis the homologs made one single line on
  the equator
• Microtubules from the poles are attached to the
  kinetochores of each chromosome of each tetrad

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Anaphase I

• Pairs of homologous chromosomes separate
• One chromosome moves toward each pole, guided by
  the spindle fibers
• Sister chromatids remain attached at the
  centromere and move as one unit toward the pole

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Telophase I

• Reverse of prophase I
• Spindle fibers breaks down
• Chromosomes uncoil
• Nuclear envelope reforms
• In the beginning of telophase I, each half of the
  cell has a haploid set of chromosomes
• Each chromosome still consists of two sister
  chromatids

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Cytokinesis Interkinesis

• Cytokinesis forms two haploid daughter cells
• In animal cells, a cleavage furrow forms
• In plant cells, a cell plate forms
• Each new cell has ½ the genetic information as
  the original
• 1 chromosome from each pair
• Need a second division for sister chromatids to
  split
• Each chromosome from the pair is still doubled
• No chromosome replication occurs between the end
  of meiosis I and the beginning of meiosis II
  because the chromosomes are already replicated

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Meiosis I
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Meiosis II

• Division in meiosis II also occurs in four
  phases
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II and cytokinesis
• Meiosis II is very similar to mitosis
• Focus is on splitting sister chromatids

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Prophase II Metaphase II

• Prophase II
• Spindle apparatus forms nuclear membrane and
  nuclelous dissolve
• Chromosomes condense (each still composed of two
  chromatids) and move toward the metaphase plate
• Metaphase II
• Sister chromatids are arranged at the metaphase
  plate
• The two sister chromatids of each chromosome are
  no longer genetically identical
• The kinetochores of sister chromatids attach to
  microtubules extending from opposite poles

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Anaphase II Telophase II

• Anaphase II
• Sister chromatids separate and move to opposite
  poles
• Each chromatid is now its own chromosome
• Telophase II
• Nuclei reform
• Spindles break down
• Chromosomes uncoil
• Cytokinesis again separates the cytoplasm

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Final Products

• At the end of meiosis, there are four haploid
  daughter cells
• Each daughter cell is genetically distinct from
  the others and from the parent cell
• Each has 1 chromosome from each homologous pair
• Each will mature into eggs or sperm
  (gametogenesis)

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Meiosis II
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Meiosis Distinctions

• Three events are unique to meiosis, and all three
  occur in meiosis I
• Synapsis and crossing over in prophase I
• In metaphase I, paired homologous chromosomes
  (tetrads) independently arrange on either side of
  the equator
• In anaphase I, homologous chromosomes, instead of
  sister chromatids, separate

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Meiosis Variation

• Three mechanisms contribute to genetic variation
• Independent assortment of chromosomes
• Homologous pairs of chromosomes orient randomly
  at metaphase I of meiosis
• Crossing over
• Nonsister chromatids of a tetrad exchange genetic
  information
• Random fertilization
• Over 8 million different gametes possible (223)
• 70 trillion chromosome combinations possible for
  zygotes! (223)x(223)

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Mitosis vs. Meiosis
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Mitosis vs. Meiosis

• Mitosis
• 1 division
• 2 daughter cells
• Exact copies of parent cells
• Diploid to diploid
• Purpose
• Growth
• Repair
• Asexual reproduction

• Meiosis
• 2 divisions
• 1st separates pairs
• 4 daughter cells
• Each unique
• Diploid to haploid
• Purpose
• Make gametes/ sex cells
• Leads to genetic variation

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Review Questions

1. Define genetics and differentiate between
   heredity and variation.
2. Differentiate between asexual and sexual forms of
   reproduction in regards to the life cycles of
   various organisms.
3. Define the following vocabulary associated with
   meiosis gene, gamete, locus, somatic cell,
   karyotype, homologous chromosomes, zygote.
4. Differentiate between autosomes and sex
   chromosomes.
5. Describe the process of fertilization.
6. Define meiosis and explain why there must be 2
   divisions.
7. Define the 4 major phases of meiosis I, along
   with the important events that occur during those
   phases and how they are unique from those phases
   of mitosis.
8. Explain the relationship between synapsis,
   tetrads, chiasmata, and crossing over.
9. Define the 4 major phases of meiosis II, along
   with the important events that occur during those
   phases and how they are unique from those phases
   of mitosis.
10. Describe the 3 events that are unique to meiosis.
11. Name and describe 3 mechanisms that contribute to
    genetic variation.
12. Describe 5 major differences between mitosis and
    meiosis as processes.