meiosis and sexual life cycles - perry local...
TRANSCRIPT
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Chapter 13 Meiosis
and Sexual Life
Cycles
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Question?
• Reproduction is a characteristic
of Life
• Does Like really beget Like?
• This chapter deals with
reproduction of life.
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Heredity
• The transmission of traits from
parents to offspring.
• Comment - Humans have been
aware of heredity for thousands
of years.
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Genetics
• The scientific study of heredity.
• Comment - Genetics is only
about 150 years old.
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Variation
• Is demonstrated by the
differences in appearance that
offspring show from parents
and siblings.
• Offspring only “resemble” their
parents and siblings.
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Genes
• The DNA for a trait.
• Locus - the physical location of
a gene in a chromosome.
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Reproduction
• A method of copying genes to
pass them on to offspring.
• Two main types: • Asexual reproduction
• Sexual reproduction
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Asexual Reproduction
• Parent passes all of its genes
to its offspring.
• Uses mitosis.
• Also known as cloning.
• Comment - many organisms
reproduce this way.
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Asexual Bud
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Advantages
• Only need 1 parent.
• Offspring are identical to the
parent.
• Good genetic traits are
conserved and reproduced.
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Disadvantages
• No new DNA combinations for
evolution to work on.
• Clones may become extinct if
attacked by a disease or pest.
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Sexual Reproduction
• 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.
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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.
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Disadvantages
• Need two parents.
• Good gene combinations can
be lost.
• Offspring may not be an
improvement over the parents.
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Question ?
• Do parents give their whole
DNA copy to each offspring?
• What would happen to
chromosome number if they
did?
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Chromosome Number
• Is usually constant for a
species.
• Examples: • Humans - 46
• Corn - 20
• Onions - 16
• Dogs - 72
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Life Cycle - if Mitosis
Female 46 Male 46
egg 46 sperm 46
Zygote 92
mitosis mitosis
Mitosis
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Result
• Chromosome number would
double each generation.
• Need a method to reduce the
chromosome number.
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Life Cycle - if Meiosis
Female 46 Male 46
egg 23 sperm 23
Zygote 46
mitosis mitosis
Meiosis
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Result
• Chromosome number will
remain the same with each
sexual reproduction event.
• Meiosis is used to produce the
gametes, sex cells or spores.
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Meiosis - Purpose
• To reduce the number of
chromosomes by half.
• Prevents doubling of
chromosome numbers during
sexual reproduction.
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Sexual Life Cycle
• Has alternation of meiosis and
fertilization to keep the
chromosome numbers constant
for a species.
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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
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Diploid
• 2 sets of chromosomes.
• Most common number in body
or somatic cells. • Humans 2N = 46
• Corn 2N = 20
• Fruit Flies 2N = 8
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Human Chromosomes
• Human somatic cells (any cell
other than a gamete) have 23
pairs of chromosomes.
• A karyotype is an ordered display
of the pairs of chromosomes from
a cell.
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Human Chromosomes • The two chromosomes in each
pair are called homologous
chromosomes, or homologs.
• Chromosomes in a
homologous pair are the same
length and carry genes
controlling the same inherited
characters.
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• Each pair of homologous
chromosomes includes one
chromosome from each parent.
• The 46 chromosomes in a
human somatic cell are two
sets of 23: one from the mother
and one from the father.
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• A diploid cell (2n) has two sets
of chromosomes.
• For humans, the diploid number
is 46 (2n = 46).
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Haploid
• Single set of chromosomes.
• Number in the gametes or sex
cells. • Humans N = 23
• Corn N = 10
• Fruit Flies N = 4
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• A gamete (sperm or egg)
contains a single set of
chromosomes, and is haploid
(N).
• For humans, the haploid number
is 23 (N = 23).
• Each set of 23 consists of 22
autosomes and a single sex
chromosome.
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• In an unfertilized egg (ovum),
the sex chromosome is X.
• In a sperm cell, the sex
chromosome may be either X
or Y.
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Polyploids
• Multiple sets of chromosomes.
• Examples • 3N = triploid
• 4N = tetraploid
• Common in plants, but often
fatal in animals.
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Life Cycle Variations
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Life cycle variation
• Plants and some algae exhibit
an alternation of generations.
• This life cycle includes both a
diploid and haploid multicellular
stage.
• The diploid organism, called
the sporophyte, makes
haploid spores by meiosis.
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Plants
• Each spore grows by mitosis
into a haploid organism called a
gametophyte.
• A gametophyte makes haploid
gametes by mitosis.
• Fertilization of gametes results
in a diploid sporophyte.
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Another variation
• In most fungi and some
protists, the only diploid stage
is the single-celled zygote;
there is no multicellular diploid
stage.
• The zygote produces haploid
cells by meiosis.
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Fungi
• Each haploid cell grows by
mitosis into a haploid
multicellular organism.
• The haploid adult produces
gametes by mitosis.
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Meiosis/Mitosis Preview of
differences
• Two cell divisions, not one.
• Four cells produced, not two.
• Synapsis and Chiasmata will
be observed in Meiosis
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Meiosis - Uniqueness
• Three events are unique to meiosis, and all three occur in meiosis l: 1. Synapsis and crossing over in
prophase I: Homologous chromosomes physically connect and exchange genetic information.
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Continued…
2. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes.
3. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate.
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Meiosis/Mitosis Preview of
differences
• 1st division separates PAIRS of
chromosomes, not duplicate
chromosomes.
• Interkinesis is present.
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Meiosis
• Has two cell divisions.
Steps follow the names for
mitosis, but a “I” or “II” will be
added to label the phase.
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Prophase I
• Basic steps same as in
prophase of Mitosis.
• Synapsis occurs as the
chromosomes condense.
• Synapsis - homologous
chromosomes form bivalents or
tetrads.
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Prophase I
• Chiasmata observed.
• Longest phase of division.
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Metaphase I
• Tetrads or bivalents align on
the metaphase plate.
• Centromeres of homologous
pairs point toward opposite
poles.
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Anaphase I
• Homologous PAIRS separate.
• Duplicate chromosomes are
still attached at the
centromeres.
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Anaphase I possibilities
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Anaphase I
• Maternal and Paternal
chromosomes are now
separated randomly.
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Telophase I
• Similar to Mitosis.
• Chromosomes may or may not
unwind to chromatin.
• Cytokinesis separates
cytoplasm and 2 cells are
formed.
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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.
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Meiosis II
• Steps are the same as in
Mitosis. • Prophase II
• Metaphase II
• Anaphase II
• Telophase II
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Meiosis - Results
• 4 cells produced.
• Chromosome number halved.
• Gametes or sex cells made.
• Genetic variation increased.
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Sexual Sources of Genetic
Variation
1. Independent Assortment of
Chromosomes during Meiosis.
2. Random Fertilization.
3. Crossing Over.
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Independent Assortment
• There are 23 pairs of
chromosomes in humans.
• The chance to inherit a single
chromosome (maternal or
paternal) of each pair is 1/2.
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Gamete Possibilities
• With 23 pairs of chromosomes,
the number of combinations of
chromosome types
(paternal and maternal) are:
223 or 8,388,608
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Random Fertilization
• The choice of which sperm fuses
with which egg is random.
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Random Fertilization
• 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.
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Result
• Is it any wonder that two
offspring from the same human
parents only resemble each
other and are not identical
twins?
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Crossing-Over
• The exchange of sister
chromatid material during
synapsis.
• Occurs ONLY in Prophase I.
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Chiasmata
• The point of contact where two
chromosomes are crossing-
over.
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Importance
• Breaks old linkage groups.
• Creates new linkage groups
increases genetic variation.
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Importance
• Very common during meiosis.
• Frequency can be used to map
the position of genes on
chromosomes.
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Comments
• With crossing over, offspring can never be 100% like a parent if sexual reproduction is used.
• Multiple cross-overs are common, especially on large chromosomes.
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Comments
• Genes near the centromere do
not cross-over very often.
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Summary
• Know how the chromosomes
separate during Meiosis.
• Know how Meiosis differs from
Mitosis.
• Know how sexual reproduction
increases genetic variation.