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Chapter 13: Meiosis and Sexual Life Cycles

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Title: Chapter 13: Meiosis and Sexual Life Cycles


1
Chapter 13 Meiosis and Sexual Life Cycles
2
Resourses
PBS Advantage of sexual reproduction http//www.p
bs.org/wgbh/evolution/sex/advantage/
meiosis tutorial (U of Arizona) http//www.biology
.arizona.edu/cell_bio/tutorials/meiosis/main.html
Meiosis tutorial 2 http//www.emc.maricopa.edu/fac
ulty/farabee/BIOBK/BioBookmeiosis.html meiosis
tuorial 3 http//www.meiosistutorial.net
3
Can you pick out the kids for each couple?
4
Heredity and Genetics
  • Heredity
  • The transmission of traits from parents to
    offspring.
  • Gene The DNA for a trait.
  • Locus - the physical location of a gene in a
    chromosome
  • Comment - Humans have been aware of heredity for
    thousands of years.
  • Genetics
  • The scientific study of heredity.
  • Comment - Genetics is only about 150 years old.

5
Reproduction
  • A method of copying genes to pass them on to
    offspring.
  • Two main types
  • Asexual reproduction
  • Sexual reproduction

6
Asexual Reproduction
  • Parent passes all of its genes to its offspring.
  • Uses mitosis/Also known as cloning.
  • Comment - many organisms reproduce this way.

Asexual Bud
7
Asexual Reproduction
  • Advantages
  • Only need 1 parent.
  • Offspring are identical to the parent.
  • Good genetic traits are conserved and reproduced.
  • Disadvantages
  • No new DNA combinations for evolution to work on.
  • Clones may become extinct if attacked by a
    disease or pest.

8
Sexual Reproduction
  • Two parents contribute DNA to an offspring.
  • Comment - most organisms reproduce this way, but
    it hasnt been proven in some fungi and a few
    others.

9
Sexual Reproduction
  • Advantages
  • Offspring has a unique combination of DNA which
    may be an improvement over both parents.
  • New combination of DNA for evolution to work
    with.
  • Disadvantages
  • Need two parents.
  • Good gene combinations can be lost.
  • Offspring may not be an improvement over the
    parents.

10
Question ?
  • Do parents give their whole DNA copy to each
    offspring?
  • What would happen to chromosome number if they
    did?

11
Life Cycle - if Mitosis
  • Female 46 Male 46
  • egg 46 sperm 46
  • Zygote 92

Mitosis
mitosis
mitosis
12
Life Cycle - if Meiosis
  • Female 46 Male 46
  • egg 23 sperm 23
  • Zygote 46

Meiosis
Chromosome number will remain the same with each
sexual reproduction event.
mitosis
  • mitosis

13
Chromosome Number
  • Is usually constant for a species.
  • Examples
  • Humans - 46
  • Corn - 20
  • Onions - 16
  • Dogs - 72

14
Sexual Reproduction
  • Meiosis - Purpose
  • To produce haploid gametes or sex cells.
  • Prevents doubling of chromosome numbers during
    sexual reproduction.
  • Sexual Life Cycle
  • Has alternation of meiosis and fertilization to
    keep the chromosome numbers constant for a
    species.

15
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16
Life Cycle
Variations
17
Chromosome Number
  • Ploidy
  • Number of chromosomes in a "set" for an organism,
    or how many different kinds of chromosomes the
    species has.
  • Usually shown as N
  • Humans N 23
  • Diploid
  • 2 sets of chromosomes.
  • Most common number in body or somatic cells.
  • Humans 2N 46
  • Corn 2N 20
  • Fruit Flies 2N 8

18
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19
Human Karyotype
20
Chromosome Number
  • Haploid
  • 1 set of chromosomes.
  • Number in the gametes or sex cells.
  • Humans N 23
  • Corn N 10
  • Fruit Flies N 4
  • Polyploids
  • Multiple sets of chromosomes.
  • Examples
  • 3N triploid
  • 4N tetraploid
  • Common in plants, but usually fatal in animals.

21
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22
Interphase
23
Prophase I
24
Prophase I
  • Basic steps same as in prophase of Mitosis.
  • But also, synapsis occurs as the chromosomes
    condense.
  • Synapsis - homologous chromosomes form bivalents
    or tetrads.
  • Crossing over occur (the exchange of sister
    chromatid material during synapsis) may occur
    only during this phase.
  • The point of contact where two chromosomes are
    crossing-over is called Chiasmata.
  • Longest phase of division.

25
Metaphase I
26
Metaphase I
  • Tetrads or bivalents align on the metaphase
    plate.
  • Centromeres of homologous pairs point toward
    opposite poles.

27
Anaphase I
28
Anaphase I
  • Homologous PAIRS separate.
  • Duplicate chromosomes are still attached at the
    centromeres.
  • Maternal and Paternal chromosomes are now
    separated randomly. (Independent Assortment)
  • The chance to inherit a single chromosome
    (maternal or paternal) of each pair is 1/2.

29
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30
Telophase I
31
Telophase I
  • Similar to Mitosis.
  • Chromosomes may or may not unwind to chromatin.
  • Cytokinesis separates cytoplasm and 2 cells are
    formed.

32
Interkinesis
  • No DNA synthesis occurs.
  • May last for years, or the cell may go
    immediately into Meiosis II.
  • May appear similar to Interphase of Mitosis.

33
Meiosis II
  • Steps are the same as in Mitosis.
  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II

34
Meiosis II
35
Meiosis - Results
  • 4 cells produced.
  • Chromosome number halved.
  • Gametes or sex cells made.
  • Genetic variation increased. How?
  • 1. Independent Assortment of Chromosomes during
    Meiosis.
  • 2. Random Fertilization.
  • 3. Crossing Over.

36
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37
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38
1. Independent Assortment
  • Gamete Possibilities
  • With 23 pairs of chromosomes, the number of
    combinations of chromosome types (paternal and
    maternal) are
  • 223 or 8,388,608

39
2. Random Fertilization
  • The choice of which sperm fuses with which egg is
    random.
  • Therefore, with 8,388,608 kinds of sperms and
    8,388,608 kinds of eggs, the number of possible
    combinations of offspring is over 64 million
    kinds.
  • Result two offspring from the same human parents
    only resemble each other (except identical
    twins).

40
3. Crossing-over
  • Very common during meiosis.
  • In fact, even multiple cross-overs are common,
    especially on large chromosomes.
  • Breaks old linkage groups.
  • Creates new linkage groups increases genetic
    variation.
  • Frequency can be used to map the position of
    genes on chromosomes.
  • Genes near the centromere do not cross-over very
    often.

41
Crossing-over
42
Summary
  • Know how the chromosomes separate during Meiosis.
  • Know how Meiosis differs from Mitosis.
  • Know how sexual reproduction increases genetic
    variation.
  • Chapter 46 will examine the differences in
    Meiosis between human males and females.
  • AP Lab 3
  • http//www.phschool.com/science/biology_place/labb
    ench/lab3/intro.html
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