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Meiosis

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Title: Meiosis


1
Meiosis
  • Chapter 13 Meiosis and Sexual Life Cycles

2
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

3
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

4
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

5
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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8
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

9
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)

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
Prophase I Tetrads
Homologous chromosomes (each with sister
chromatids)
A Tetrad
18
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

19
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

20
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

21
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

22
Meiosis I
23
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

24
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

25
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

26
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)

27
Meiosis II
28
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

29
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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31
Mitosis vs. Meiosis
32
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

33
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.
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