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Big Ideas 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
CHAPTER 13
MEIOSIS AND SEXUAL LIFE
CYCLES
Learning Objectives
LO 3.7 The student can make predictions about natural phenomena occurring
during the cell cycle. [See SP 6.4]
LO 3.8 The student can describe the events that occur in the cell cycle. [See SP 1.2]
LO 3.9 The student is able to construct an explanation, using visual representations
or narratives, as to how DNA in chromosomes is transmitted to the next
generation via mitosis, or meiosis followed by fertilization. [See SP 6.2]
LO 3.10 The student is able to represent the connection between meiosis and
increased genetic diversity necessary for evolution. [See SP 7.1]
LO 3.11 The student is able to evaluate evidence provided by data sets to support the claim that heritable information is passed from one generation to another generation through mitosis, or meiosis followed by fertilization. [See SP 5.3]
– During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles.
– Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and paternal chromosomes.
– During meiosis, homologous chromatids exchange genetic material via a process called “crossing over,” which increases genetic variation in the resultant gametes. [See also 3.C.2]
– Fertilization involves the fusion of two gametes, increases genetic variation in populations by providing for new combinations of genetic information in the zygote, and restores the diploid number of chromosomes.
Concept 13.1 Offspring acquire genes from parents by inheriting chromosomes
• In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes
 What is a clone?
• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents
 What would be the advantages of asexual reproduction? Sexual reproduction?
Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
• A karyotype is an ordered display of the pairs of chromosomes from a cell  What are homologous chromosomes?
 How many chromosomes are in human somatic cells?  How many autosomes are present in a karyotype?
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The Variety of Sexual Life Cycles
• A diploid cell (2n) has two sets of chromosomes
What is the diploid number for humans?
Which cells in the human body are diploid?
What is the haploid number for humans?
Which cells in the human body are haploid?
• The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
 What is alteration of generations?
– This life cycle includes both a diploid and haploid multicellular stage
Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
• After chromosomes duplicate, two divisions follow:
– Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes
– Meiosis II (equational division) sister chromatids separate
• The result is four haploid daughter cells
• No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
The Stages of Meiosis
• Meiosis 1
Chiasmata
Spindle
2nd division of meiosis separates sister chromatids
– prophase 2
– metaphase 2
– anaphase 2
– telophase 2
(1n  1n)
* just like mitosis *
Figure 13.8b
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
(2n  1n)
“reduction division”
• Meiosis 2
Figure 13.8a
Prophase I
1st division of meiosis separates
homologous pairs
– interphase
– prophase 1
– metaphase 1
– anaphase 1
– telophase 1
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Fragments
Homologous
chromosomes of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Cleavage
furrow
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Haploid daughter
cells forming
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
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Events Unique to Meiosis
Crossing Over
There are three unique events that all occur in meiosis l
1. Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
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
A Comparison of Mitosis and Meiosis
MITOSIS
MEIOSIS
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One
Two
Number of daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Role in the animal body
Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes
Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
 What is the original source of genetic diversity?
• What are the three mechanisms that contribute to genetic variation?
1. Independent assortment of chromosomes
2. Crossing over
3. Random fertilization
Figure 13.10-1
1. Independent Assortment of Chromosomes
• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
– Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs
– The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
– For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
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Figure 13.10-2
Figure 13.10-3
Possibility 2
Possibility 1
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Metaphase II
Daughter
cells
Combination 1 Combination 2
Prophase I
of meiosis
2. Crossing Over
Combination 3 Combination 4
Nonsister chromatids
held together
during synapsis
Pair of homologs
• Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Prophase I
of meiosis
Pair of homologs
Chiasma
Chiasma
Centromere
TEM
Nonsister chromatids
held together
during synapsis
Centromere
TEM
Anaphase I
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Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Prophase I
of meiosis
Pair of homologs
Chiasma
Chiasma
Centromere
TEM
Nonsister chromatids
held together
during synapsis
Centromere
TEM
Anaphase I
Anaphase I
Anaphase II
Anaphase II
Daughter
cells
Recombinant chromosomes
3. Random Fertilization
Sexual reproduction allows us to maintain both genetic similarity & differences.
• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
• The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
Jonas
Brothers
Baldwin brothers
The Evolutionary Significance of Genetic Variation Within Populations
Martin & Charlie Sheen, Emilio Estevez
REVIEW…..
• Natural selection results in the accumulation of genetic variations favored by the environment
 Why is it rare for animals to reproduce asexually? 5
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1. Which of the following would need to be
present to produce a realistic signal to control
the cell cycle? (More than one may be
required.)
a) a set of molecules that are sensitive to fluctuations in the abundance of regulators
b) protein kinases
c) molecules that activate kinases
d) molecules that degrade activators
e) All of the above.
3. Fertilization is to zygote as meiosis is to
which of the following?
a)
b)
c)
d)
e)
mitosis
diploid
chromosome
replication
gamete
2. Which of the following transmits genes
from one generation of a family to another?
a)
b)
c)
d)
e)
DNA
gametes
somatic cells
mitosis
nucleotides
4. Privet shrubs and humans each have a
diploid number of 46 chromosomes per cell.
Why are the two species so dissimilar?
a) Privet chromosomes undergo only mitosis.
b) Privet chromosomes are shaped differently.
c) Human chromosomes have genes grouped together differently.
d) The two species have appreciably different genes.
e) Privets do not have sex chromosomes.
5. How and at what stage do chromosomes
undergo independent assortment?
6. In this cell, what phase is represented?
a)
b)
c)
d)
e)
a)
b)
c)
d)
e)
meiosis I pairing of homologs
anaphase I separation of homologs
meiosis II separation of homologs
meiosis I metaphase alignment
meiosis I telophase separation
mitotic metaphase
meiosis I anaphase
meiosis I metaphase
meiosis II anaphase
meiosis II metaphase
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