Chapter 13 Meiosis and
Sexual Life Cycles
Question?
• Does Like really beget Like?
• The offspring will “resemble” the parents, but
they may not be “exactly” like them.
• 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.
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
Male 46
Mitosis
Zygote
mitosis
sperm 46
92
mitosis
Result
• Chromosome number would double each
generation.
• Need a method to reduce the chromosome
number.
Life Cycle - if Meiosis
Female 46
egg 23
Male 46
Meiosis
Zygote
mitosis
sperm 23
46
mitosis
Result
• Chromosome number will remain the same
with each sexual reproduction event.
• Meiosis is used to produce the gametes or sex
cells.
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
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 often fatal in
animals.
Life Cycle Variations
Meiosis/Mitosis Preview of differences
• Two cell divisions, not one.
• Four cells produced, not two.
• Synapsis and Chiasmata will be observed in
Meiosis
Meiosis/Mitosis Preview of differences
• 1st division separates PAIRS of chromosomes,
not duplicate chromosomes (sister
chromatids).
• 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:
2N = 223 = 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
offspring is over
70 trillion kinds.
possible 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 non-sister chromatid material
during synapsis.
• Occurs ONLY in Prophase I.
Chiasmata
• The point of contact where two chromosomes
are crossing-over.
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
Comment
• 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.