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Chapter 13
Meiosis and Sexual Life Cycles
AP Biology
Overview: Variations on a Theme
• Living organisms are distinguished by their ability to reproduce
their own kind
•
Genetics is the scientific study of heredity and variation
• Heredity is the transmission of traits from one
generation to the next
• Variation is demonstrated by the differences in
appearance that offspring show from parents and
siblings
Concept 13.1:
Offspring acquire genes from
parents by inheriting chromosomes
Genes
• Children do not inherit particular physical traits from their
parents in a literal sense
•
It is genes that are actually inherited
• Genes are units of heredity made up of segments of
DNA
•
Most genes program cells to make specific enzymes and
proteins
• Cumulative action of these molecules produce an
organism’s inherited traits
Inheritance of Genes
•
Genes are passed to the next generation through reproductive cells called gametes
•
Include sperm and eggs
•
Male and female gametes unite during fertilization and thus pass on genes
of both parents to their offspring
•
Most of a eukaryotic cell’s DNA is packaged into chromosomes in the nucleus
•
One chromosome includes 100s-1000s of genes
•
Each gene has a specific location, called a locus, on a certain
chromosome
•
One set of chromosomes is inherited from each parent
Comparison of Asexual and Sexual Reproduction
•
In asexual reproduction, one parent produces genetically identical
offspring by mitosis
•
Gives rise to clones, or groups of genetically identical individuals from
the same parent
• Ex) Budding hydra
•
Fig. 13-2a
0.5 mm
In sexual reproduction, two parents give rise to
offspring that have unique combinations of genes
inherited from the two parents
•
Parent
Offspring vary genetically from both siblings and
Bud
parents
(a) Hydra
Concept Check 13.1
• 1) How are the traits of parents (such as hair color) transmitted
to their offspring?
• 2) Explain how asexually reproducing organisms produce
offspring that are genetically identical to each other and to their
parents.
• 3) A horticulturalist breeds orchids, trying to obtain a plant with
a unique combination of desirable traits. After many years, she
finally succeeds. To produce more plants like this one, should
she breed it or clone it? Why?
Concept 13.2:
Fertilization and meiosis alternate in
sexual life cycles
Homologous Chromosomes
•
Human somatic (body) cells have 23 pairs (46 total) of chromosomes
•
Chromosomes can be distinguished from one another using a microscope due
to differences in:
•
•
Size
•
Position of centromeres
•
Pattern of colored bands produced by certain stains
A karyotype is an ordered display of the pairs of chromosomes from a cell
•
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
•
Ex) Genes specifying eye color have equivalent loci on homologous
chromosomes
Fig. 13-3b
TECHNIQUE
5 µm
Pair of homologous
replicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
Autosomes and Sex Chromosomes
•
The sex chromosomes are called X and Y
•
Human females have a homologous pair of X chromosomes (XX)
•
Human males have one X and one Y chromosome
• Only small parts of X and Y are homologous
• Most genes on X chromosome do not have counterparts on Y
chromosome
• Y chromosome also contains genes lacking on X chromosome
•
The 22 pairs of chromosomes that do not determine sex are called
autosomes
Diploid Cells
•
Each pair of homologous chromosomes includes one chromosome from
each parent
•
The 46 chromosomes in a human somatic cell are two sets of 23
chromosomes
• One set is inherited from the mother and one from the father
•
The number of chromosomes in a single set is represented by n
•
A diploid cell (2n) has two sets of chromosomes
• For humans, the diploid number is 46 (2n = 46)
Describing Chromosomes
• In a cell in which DNA synthesis has occurred, each
chromosome is replicated
•
Fig. 13-4
Each replicated chromosome consists of two identical
sister chromatids
2n = 6
Key
Maternal set of
chromosomes (n = 3)
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Centromere
Pair of homologous
chromosomes
(one from each set)
Haploid Cells
•
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
•
Each sexually reproducing species has a characteristic haploid and diploid
number
•
Ex) Dogs: n = 39
Behavior of Chromosome Sets in the Human Life Cycle
•
Fertilization is the union of haploid gametes (the sperm and the egg)
•
The fertilized egg is called a zygote
• Zygote contains one set of chromosomes from each parent and is
therefore diploid
Fig. 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
• Mitosis of the zygote and
its descendants produces
Sperm (n)
all somatic cells during
MEIOSIS
FERTILIZATION
development into an adult
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Behavior of Chromosome Sets in the Human Life Cycle
•
At sexual maturity, the ovaries and testes produce haploid gametes
•
Develop from specialized cells called germ cells in the gonads
•
Gametes are the only types of human cells produced by
Fig. 13-5
Key
meiosis, rather than mitosis
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
• Meiosis reduces chromosomes to one
set in each gamete
Sperm (n)
MEIOSIS
FERTILIZATION
• Fertilization and meiosis
alternate in sexual life cycles to
Ovary
Testis
Diploid
zygote
(2n = 46)
maintain chromosome number
Mitosis and
development
Multicellular diploid
adults (2n = 46)
The Variety of Sexual Life Cycles
• The alternation of meiosis and fertilization is common to all
organisms that reproduce sexually
•
Timing of meiosis and fertilization varies, depending on
species
• These variations can be grouped into 3 main types of
life cycles
• Life cycle: generation-to generation sequence of
stages in the reproductive history of an organism,
from conception to reproduction
The Variety of Sexual Life Cycles: Diploid Life Cycle
•
In animals (including humans), meiosis produces gametes
•
These gametes undergo no further cell division before fertilization
• Gametes are the only haploid cells in animals
•
Fig. 13-6a
Gametes fuse to form a diploid zygote
Key
Haploid (n)
Diploid (2n)
• This zygote divides by mitosis to
n
develop into a multicellular (diploid)
organism
Gametes
n
n
MEIOSIS
2n
Diploid
multicellular
organism
(a) Animals
FERTILIZATION
Zygote
2n
Mitosis
The Variety of Sexual Life Cycles: Alternation of
Generations
•
Plants and some algae exhibit a second type of life cycle called alternation of
generations
•
This life cycle includes both diploid and haploid stages that are multicellular
•
Multicellular diploid phase is called the sporophyte
Fig. 13-6b
•
Meiosis in sporophyte produces haploid
cells called spores
•
These spores divide using mitosis,
generating a multicellular haploid
stage called the gametophyte
Key
Haploid (n)
Diploid (2n)
Mitosis
n
Haploid multicellular organism
(gametophyte)
•
A gametophyte makes haploid gametes
using mitosis
Fusion of 2 haploid gametes at
fertilization results in a diploid
zygote that gives rise to the next
sporophyte generation
n
n
n
Spores
•
Mitosis
n
MEIOSIS
Gametes
FERTILIZATION
2n
Diploid
multicellular
organism
(sporophyte)
2n
Mitosis
(b) Plants and some algae
Zygote
The Variety of Sexual Life Cycles: The Haploid Life
Cycle
•
In most fungi and some protists, the only diploid stage is the single-celled
zygote
•
Fig. 13-6c
Formed by fusion of haploid gametes
Key
Haploid (n)
•
•
There is no multicellular diploid stage
The zygote produces haploid cells by meiosis
•
Each haploid cell grows by mitosis into
Haploid unicellular or
multicellular organism
Diploid (2n)
Mitosis
n
n
n
Gametes
a haploid multicellular adult organism
•
The haploid adult produces gametes
by mitosis
Mitosis
n
MEIOSIS
n
FERTILIZATION
2n
Zygote
(c) Most fungi and some protists
The Variety of Sexual Life Cycles: A Comparison
• Depending on the type of life cycle, either haploid or diploid
cells can divide by mitosis
•
Only diploid cells can undergo meiosis
• Haploid cells cannot be further reduced since they only
have 1 set of chromosomes
• In all three life cycles, the halving and doubling of
chromosomes contributes to genetic variation in offspring
Concept Check 13.2
•
1) How does the karyotype of a human female differ from that of a human
male?
•
2) How does the alternation of meiosis and fertilization in the life cycles of
sexually reproducing organisms maintain the normal chromosome count for
each species?
•
3) Each sperm of a pea plant contains 7 chromosomes. What are the
haploid and diploid numbers for peas?
•
4) An certain eukaryote lives as a unicellular organism, but during
environmental stress, its cells produce gametes. The gametes fuse, and the
resulting zygote undergoes meiosis, generating new single cells. What type
of organism could this be?
Concept 13.3:
Meiosis reduces the number of
chromosome sets from diploid to
haploid
An Introduction to Meiosis
•
Many steps of meiosis closely resemble corresponding steps in mitosis
•
Like mitosis, meiosis is preceded by the replication of chromosomes
• This replication, however, is followed by TWO consecutive
divisions in meiosis
• Meiosis I and meiosis II
• These two cell divisions result in four daughter cells, rather than the
two daughter cells in mitosis
• Each daughter cell has only half as many chromosomes as the
parent cell
The Stages of Meiosis
•
In meiosis, both members of a single homologous pair of chromosomes in a diploid
cell are replicated
•
Fig. 13-7-3
Interphase
Homologous pair of chromosomes
in diploid parent cell
These copies are then sorted into 4 haploid
daughter cells
•
In the first cell division (meiosis I), homologous
chromosomes separate
•
Meiosis I results in two haploid daughter cells with
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
replicated chromosomes
Meiosis I
•
It is called the reductional division
1 Homologous
•
chromosomes
separate
In the second cell division (meiosis II), sister chromatids
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
•
Meiosis II results in four haploid daughter cells with
separate
unreplicated chromosomes
Haploid cells with unreplicated chromosomes
•
It is called the equational division
Interphase Prior to Meiosis I
• Like mitosis, meiosis I is preceded by interphase
•
During interphase, chromosomes are replicated to form
sister chromatids
• The sister chromatids are genetically identical and
joined at the centromere
• The single centrosome replicates, forming two
centrosomes
Phases of Meiosis I
• Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
Meiosis I: Prophase I
Prophase I - typically occupies more than 90% of the time required for meiosis
•
Chromosomes begin to condense
•
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene,
forming tetrads
•
Exchange of corresponding segments of DNA (crossing over) occurs during
synapsis
•
•
Chiasmata: X-shaped regions where
crossing over has occurred and homologs
are still associated due to sister chromatid
cohesion
Fig. 13-8b
Synapsis ends in mid-prophase and
chromosomes move slightly apart
•
Centrosome movement, spindle formation, and
nuclear envelope breakdown occur as in mitosis
•
In late prophase, microtubules of spindle fiber attach
to the kinetochores of each homolog
•
Homologous pairs then move toward
metaphase plate
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Meiosis I: Metaphase I
Metaphase I
•
In metaphase I, tetrads line up at the metaphase plate, with one
Fig. 13-8b
chromosome in each pair facing each pole
Prophase I
•
Microtubules from one pole are attached to
the kinetochore of one chromosome of
each tetrad
•
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Microtubules from the other pole are
attached to the kinetochore of the
other chromosome
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Meiosis I: Anaphase I
Anaphase I
•
In anaphase I, pairs of homologous chromosomes separate
Fig. 13-8c
•
Homologs move toward opposite poles,
Telophase I and
Cytokinesis
Anaphase I
guided by the spindle apparatus
•
Sister chromatids remain attached at the
Sister chromatids
remain attached
centromere and move as one unit
toward the pole
Homologous
chromosomes
separate
Cleavage
furrow
Meiosis I: Telophase I and Cytokinesis
Telophase I and Cytokinesis
•
In the beginning of telophase I, each half of the cell has a haploid set of
chromosomes
•
Fig. 13-8c
Each chromosome still consists of two
Telophase I and
Cytokinesis
Anaphase I
sister chromatids
•
Sister chromatids
remain attached
Cytokinesis usually occurs simultaneously
•
Forms two haploid daughter cells
•
In animal cells, a cleavage furrow
forms
•
•
In plant cells, a cell plate forms
No replication occurs between meiosis I and II
Homologous
chromosomes
separate
Cleavage
furrow
Phases of Meiosis II
•
Division in meiosis II also occurs in four phases:
–
Prophase II
–
Metaphase II
–
Anaphase II
–
Telophase II and
cytokinesis
•
Meiosis II is very similar to
mitosis
Meiosis II: Prophase II and Metaphase II
Prophase II
•
In prophase II, a spindle apparatus forms
•
In late prophase II, chromosomes (each still composed of two chromatids) move
toward the metaphase plate
Metaphase II
Fig. 13-8e
•
In metaphase II, the sister chromatids are arranged at
the metaphase plate
•
Due to crossing over in meiosis I, the two sister
chromatids of each chromosome are no longer
genetically identical
•
The kinetochores of sister chromatids attach to
microtubules extending from opposite poles
Prophase II
Metaphase II
Meiosis II: Anaphase II, Telophase II, and Cytokinesis
Anaphase II
•
In anaphase II, the sister chromatids separate
•
The sister chromatids of each chromosome now
move as two newly individual chromosomes toward
opposite poles
Telophase II and Cytokinesis
•
In telophase II, the chromosomes arrive at opposite
poles
•
Nuclei form, and the chromosomes begin
decondensing
•
Cytokinesis separates the cytoplasm
•
At the end of meiosis, there are four daughter cells,
each with a haploid set of unreplicated chromosomes
•
Each daughter cell is genetically distinct
from the others and from the parent cell
A Comparison of Mitosis and Meiosis
•
Mitosis conserves the number of chromosome sets
•
•
Produces cells that are genetically identical to the parent cell
Meiosis reduces the number
Fig. 13-9aof chromosomes sets from two (diploid) to
MITOSIS
one (haploid)
MEIOSIS
Parent cell
•
Produces cells that
differ genetically
Chromosome
replication
Prophase
Chiasma
Chromosome
replication
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
MEIOSIS I
from each other and
from the parent cell
•
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Haploid
n=3
The mechanism for
separating sister
chromatids is virtually
identical in meiosis II
and mitosis
Daughter
cells of
meiosis I
2n
Daughter cells
of mitosis
2n
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Unique Events of Meiosis
•
Three events unique to meiosis occur in meiosis l:
•
Synapsis and crossing over in prophase I:
•
Homologous chromosomes physically connect and exchange
genetic information
•
Homologs on metaphase plate during metaphase I:
•
There are paired homologous chromosomes (tetrads), instead of
individual replicated chromosomes, on the metaphase plate
•
Separation of homologs at anaphase I:
•
Homologous chromosomes, rather than sister chromatids,
separate
Sister Chromatid Cohesion
•
Sister chromatid cohesion allows sister chromatids of a single chromosome
to stay together through meiosis I
•
Protein complexes called cohesins are responsible for this cohesion
•
In mitosis, cohesins are cleaved at the end of metaphase
•
In meiosis, sister chromatic cohesion is released in 2 steps:
•
Cohesins are cleaved along the chromosome arms in anaphase I
(separation of homologs)
•
Remaining cohesins are cleaved at the centromeres in anaphase II
(separation of sister chromatids)
Concept Check 13.3
• 1) How are the chromosomes in a cell at metaphase
of mitosis similar to and different from the
chromosomes in a cell at metaphase at meiosis II?
• 2) Given that the synaptonemal complex disappears
by the end of prophase, how would the two homologs
be associated if crossing over did not occur? What
effect might this ultimately have on gamete formation?
Concept 13.4:
Genetic variation produced in sexual
life cycles contributes to evolution
Mutations and Genetic Variation
• Mutations (changes in an organism’s DNA) are
the original source of genetic diversity
• Mutations create different versions of genes
called alleles
• Reshuffling of alleles during sexual
reproduction produces genetic variation
Origins of Genetic Variation Among Offspring
• The behavior of chromosomes during meiosis and fertilization
is responsible for most of the variation that arises in each
generation
•
Three mechanisms contribute to genetic variation:
• Independent assortment of chromosomes
• Crossing over
• Random fertilization
Independent Assortment of Chromosomes
•
Random orientation of homologous chromosomes at metaphase I contributes to
genetic variation
•
At metaphase I, homologous chromosomes are situated on metaphase plate
•
Each pair may orient with either its maternal or paternal homolog
•
Fig. 13-11-3
There is a 50/50 chance
that daughter cell of meiosis I will get either
homolog
•
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Each homologous pair of
chromosomes is also positioned
independently of other pairs at
metaphase I
•
Metaphase II
Known as independent
Daughter
cells
assortment
Combination 1 Combination 2
Combination 3 Combination 4
Independent Assortment of Chromosomes
• The number of combinations possible when chromosomes
assort independently into gametes is 2n
•
n is the haploid number
• For humans (n = 23), there are more than 8 million (223)
Fig. 13-11-3
possible
combinations of
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
chromosomes
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
Crossing Over
•
Crossing over further contributes to genetic variation
by producing recombinant chromosomes
Fig. 13-12-5
•
Combine genes inherited from each parent
•
•
Prophase I
of meiosis
Pair of
homologs
During meiosis in humans, an average of
1-3 crossover events occur per
chromosome pair
Crossing over begins very early in prophase I
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
•
During this time, homologous
chromosomes pair up gene by gene
Anaphase II
•
In a single crossover event, corresponding
segments of 2 nonsister chromatids
(one maternal, one paternal) are
exchanged
•
Daughter
cells
Recombinant chromosomes
Crossing over thus produces chromosomes with new
combinations of maternal and paternal alleles
Crossing Over
•
1) In prophase I, synapsis and crossing over occur
Fig. 13-12-5
•
•
•
•
Chiasma forms as a result of crossover
occurring while sister chromatid cohesion
is still present along the arms
2) Chiasmata and attachments between sister
chromatids hold homologs together as they move to
metaphase I plate
3) Breakdown of proteins holding sister chromatid
arms together allows homologs with recombinant
chromatids to separate
•
•
Homologs then move slightly apart
Occurs during anaphase I
4) During anaphase II, release of sister chromatid
cohesion at centromeres allows sister chromatids to
separate
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Random Fertilization
• Random fertilization adds to genetic variation because
any sperm can fuse with any ovum (unfertilized egg)
• In humans, each respective gamete represents 1
of ~8.4 million (223) possible chromosome
combinations due to independent assortment
• Fusion of two gametes produces a zygote with
any of about 70 trillion (223 x 223) diploid
combinations
The Evolutionary Significance of Genetic Variation
Within Populations
• Populations evolve through differential reproductive success of
its genetically different members
•
Sexual reproduction contributes to the genetic variation that
originally results from mutation in a population
• Those individuals best suited to local environment leave the
most offspring
•
Results in an increased frequency of the alleles coding for
these favored variations in a population
Concept Check 13.4
• 1) What is the original source of all the different alleles of a
gene?
• 2) The diploid number for fruit flies is 8, while that for
grasshoppers is 46. If no crossing over took place, would the
genetic variation among offspring from a given pair of parents
be greater in fruit flies or grasshoppers? Explain.
• 3) Under what circumstances would crossing over during
meiosis not contribute to genetic variation among daughter
cells?
You should now be able to:
1. Distinguish between the following terms: somatic
cell and gamete; autosome and sex
chromosomes; haploid and diploid
2. Describe the events that characterize each phase
of meiosis
3. Describe three events that occur during meiosis I
but not mitosis
4. Name and explain the three events that contribute
to genetic variation in sexually reproducing
organisms