Download Chapter 13 Meiosis - Perry Local Schools

Chapter 13 Meiosis
and Sexual Life
Cycles
Question?
• Reproduction is a characteristic
of Life
• Does Like really beget Like?
• This chapter deals with
reproduction of life.
Heredity
• The transmission of traits from
parents to offspring.
• 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.
Variation
• Is demonstrated by the
differences in appearance that
offspring show from parents
and siblings.
• Offspring only “resemble” their
parents and siblings.
Genes
• The DNA for a trait.
• Locus - the physical location of
a gene in a chromosome.
Reproduction
• A method of copying genes to
pass them on to offspring.
• Two main types:
• Asexual reproduction
• Sexual reproduction
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
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.
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.
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.
Question ?
• Do parents give their whole
DNA copy to each offspring?
• What would happen to
chromosome number if they
did?
Chromosome Number
• Is usually constant for a
species.
• Examples:
•
•
•
•
Humans - 46
Corn - 20
Onions - 16
Dogs - 72
Life Cycle - if Mitosis
Female 46
egg 46
Mitosis
Male
sperm 46
Zygote
mitosis
46
92
mitosis
Result
• Chromosome number would
double each generation.
• Need a method to reduce the
chromosome number.
Life Cycle - if Meiosis
Female
egg 23
46
Meiosis
Zygote
mitosis
Male 46
sperm 23
46
mitosis
Result
• Chromosome number will
remain the same with each
sexual reproduction event.
• Meiosis is used to produce the
gametes, sex cells or spores.
Meiosis - Purpose
• To reduce the number of
chromosomes by half.
• 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.
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
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.
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.
• 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.
• A diploid cell (2n) has two sets
of chromosomes.
• For humans, the diploid number
is 46 (2n = 46).
Haploid
• Single set of chromosomes.
• Number in the gametes or sex
cells.
• Humans N = 23
• Corn N = 10
• Fruit Flies N = 4
• 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.
• In an unfertilized egg (ovum),
the sex chromosome is X.
• In a sperm cell, the sex
chromosome may be either X
or Y.
Polyploids
• Multiple sets of chromosomes.
• Examples
• 3N = triploid
• 4N = tetraploid
• Common in plants, but often
fatal in animals.
Life Cycle Variations
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.
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.
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.
Fungi
• Each haploid cell grows by
mitosis into a haploid
multicellular organism.
• The haploid adult produces
gametes by mitosis.
Meiosis/Mitosis Preview of
differences
• Two cell divisions, not one.
• Four cells produced, not two.
• Synapsis and Chiasmata will
be observed in Meiosis
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.
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.
Meiosis/Mitosis Preview of
differences
• 1st division separates PAIRS of
chromosomes, not duplicate
chromosomes.
• Interkinesis is present.
Meiosis
• Has two cell divisions.
Steps follow the names for
mitosis, but a “I” or “II” will be
added to label the phase.
Prophase I
• Basic steps same as in
prophase of Mitosis.
• Synapsis occurs as the
chromosomes condense.
• Synapsis - homologous
chromosomes form bivalents or
tetrads.
Prophase I
• Chiasmata observed.
• Longest phase of division.
Metaphase I
• Tetrads or bivalents align on
the metaphase plate.
• Centromeres of homologous
pairs point toward opposite
poles.
Anaphase I
• Homologous PAIRS separate.
• Duplicate chromosomes are
still attached at the
centromeres.
Anaphase I possibilities
Anaphase I
• Maternal and Paternal
chromosomes are now
separated randomly.
Telophase I
• Similar to Mitosis.
• Chromosomes may or may not
unwind to chromatin.
• Cytokinesis separates
cytoplasm and 2 cells are
formed.
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.
Meiosis II
• Steps are the same as in
Mitosis.
•
•
•
•
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis - Results
•
•
•
•
4 cells produced.
Chromosome number halved.
Gametes or sex cells made.
Genetic variation increased.
Sexual Sources of Genetic
Variation
1. Independent Assortment of
Chromosomes during Meiosis.
2. Random Fertilization.
3. Crossing Over.
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.
Gamete Possibilities
• With 23 pairs of chromosomes,
the number of combinations of
chromosome types
(paternal and maternal) are:
23
2
or 8,388,608
Random Fertilization
• The choice of which sperm fuses
with which egg is random.
Random Fertilization
• Therefore, with 8,388,608 kinds
of sperms and 8,388,608 kinds
of eggs, the number of possible
offspring is
over 64 million kinds.
combinations of
Result
• Is it any wonder that two
offspring from the same human
parents only resemble each
other and are not identical
twins?
Crossing-Over
• The exchange of sister
chromatid material during
synapsis.
• Occurs ONLY in Prophase I.
Chiasmata
• The point of contact where two
chromosomes are crossingover.
Importance
• Breaks old linkage groups.
• Creates new linkage groups
increases genetic variation.
Importance
• Very common during meiosis.
• Frequency can be used to map
the position of genes on
chromosomes.
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.
Comments
• Genes near the centromere do
not cross-over very often.
Summary
• Know how the chromosomes
separate during Meiosis.
• Know how Meiosis differs from
Mitosis.
• Know how sexual reproduction
increases genetic variation.