Transcript Ch. 13 Meiosis

Chapter 13:
Meiosis and
Sexual Life Cycles
Resourses
PBS Advantage of sexual reproduction
http://www.pbs.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/faculty/farabee/BIOBK/BioBookmeiosis.html
meiosis tuorial 3
http://www.meiosistutorial.net
Can you pick out the kids for each couple?
Heredity and Genetics
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Heredity
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Comment - Humans have been aware of heredity for
thousands of years.
Genetics
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The transmission of traits from parents to offspring.
Gene: The DNA for a trait.
Locus - the physical location of a gene in a chromosome
The scientific study of heredity.
Comment - Genetics is only about 150 years old.
Reproduction
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A method of copying genes to pass them on to
offspring.
Two main types:
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Asexual reproduction
Sexual reproduction
Asexual Reproduction
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Parent passes all
of its genes to its
offspring.
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Uses mitosis/Also
known as cloning.
Comment - many
organisms
Asexual Bud
reproduce this
way.
Asexual Reproduction
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Advantages
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Only need 1 parent.
Offspring are identical
to the parent.
Good genetic traits are
conserved and
reproduced.
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Disadvantages
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No new DNA
combinations for
evolution to work on.
Clones may become
extinct if attacked by a
disease or pest.
Sexual Reproduction
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Two parents contribute DNA to an offspring.
Comment - most organisms reproduce this
way, but it hasn’t been proven in some fungi
and a few others.
Sexual Reproduction
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Advantages
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Offspring has a unique
combination of DNA
which may be an
improvement over both
parents.
New combination of
DNA for evolution to
work with.
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Disadvantages
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Need two parents.
Good gene
combinations can be
lost.
Offspring may not be
an improvement over
the parents.
Question ?
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Do parents give their whole DNA copy to
each offspring?
What would happen to chromosome number
if they did?
Life Cycle - if Mitosis
Mitosis
Female 46
Male 46
egg 46
sperm 46
Zygote
mitosis
92
mitosis
Life Cycle - if Meiosis
Meiosis
Female 46
Male 46
egg 23
sperm 23
Zygote
46
Chromosome number will remain the same with each sexual reproduction event.
mitosis
mitosis
Chromosome Number
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Is usually constant for a species.
Examples:
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Humans - 46
Corn - 20
Onions - 16
Dogs - 72
Sexual Reproduction
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Meiosis - Purpose
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To produce haploid gametes or sex cells.
Prevents doubling of chromosome numbers
during sexual reproduction.
Sexual Life Cycle
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Has alternation of meiosis and fertilization to
keep the chromosome numbers constant for a
species.
Chromosome Number
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Ploidy
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Number of chromosomes in a "set" for an organism,
or how many different kinds of chromosomes the
species has.
Usually shown as N = ……
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Humans N = 23
Diploid
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2 sets of chromosomes.
Most common number in body or somatic cells.
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Humans 2N = 46
Corn 2N = 20
Fruit Flies 2N = 8
Human Karyotype
Chromosome Number
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Haploid
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1 set of chromosomes.
Number in the gametes or sex cells.
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Humans N = 23
Corn N = 10
Fruit Flies N = 4
Polyploids
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Multiple sets of chromosomes.
Examples
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3N = triploid
4N = tetraploid
Common in plants, but usually fatal in animals.
Interphase
Prophase I
Prophase I
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Basic steps same as in prophase of Mitosis.
But also, synapsis occurs as the chromosomes
condense.
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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.
Metaphase I
Metaphase I
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Tetrads or bivalents align on the metaphase
plate.
Centromeres of homologous pairs point
toward opposite poles.
Anaphase I
Anaphase I
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Homologous PAIRS separate.
Duplicate chromosomes are still attached at
the centromeres.
Maternal and Paternal chromosomes are now
separated randomly. (Independent
Assortment)
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The chance to inherit a single chromosome
(maternal or paternal) of each pair is 1/2.
Telophase I
Telophase I
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Similar to Mitosis.
Chromosomes may or may not unwind to
chromatin.
Cytokinesis separates cytoplasm and 2 cells
are formed.
Interkinesis
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No DNA synthesis occurs.
May last for years, or the cell may go
immediately into Meiosis II.
May appear similar to Interphase of Mitosis.
Meiosis II
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Steps are the same as in Mitosis.
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Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis II
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.
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1. Independent Assortment
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Gamete Possibilities
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With 23 pairs of chromosomes, the number of
combinations of chromosome types (paternal and
maternal) are:
223 or 8,388,608
2. Random Fertilization
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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).
3. Crossing-over
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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.
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Genes near the centromere do not cross-over very often.
Crossing-over
Summary
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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/lab
bench/lab3/intro.html