Meiosis and Sexual Life Cycles
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Living organisms are distinguished by their ability to
reproduce their own kind
Heredity
– Is the transmission of traits from one generation to the
next
Variation
– Shows that offspring differ somewhat in appearance
from parents and siblings
1
Inheritance of Genes
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Genes are segments of DNA, units
of heredity
Offspring acquire genes from
parents by inheriting
chromosomes
Genetics is the scientific study of
heredity and hereditary variation
2
Inheritance of Genes
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Each gene in an organism’s DNA has a
specific locus on a certain chromosome
We inherit one set of chromosomes from our
mother and one set from our father
Two parents give rise to offspring that have
unique combinations of genes inherited from
the two parents - sexual reproduction
3
Asexual Reproduction
•
In asexual reproduction, one parent
produces genetically identical offspring by
mitosis
Parent
Bud
Figure 13.2
0.5 mm
4
Sexual Reproduction
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Fertilization and meiosis alternate in sexual
life cycles
A life cycle is the generation-to-generation
sequence of stages in the reproductive
history of an organism
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Sex Cells - Gametes
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Unlike somatic cells, sperm and egg cells
are haploid cells, containing only one set of
chromosomes
At sexual maturity the ovaries and testes
produce haploid gametes by meiosis
6
Meiosis
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Reduces the chromosome number such that
each daughter
Cell has a haploid set of chromosomes
Ensures that the next generation will have:
– Diploid number of chromosome
– Exchange of genetic information
(combination of traits
– that differs from that of either parent)
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Meiosis
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Only diploid cells can divide by meiosis.
Prior to meiosis I, DNA replication occurs.
During meiosis, there will be two nuclear divisions, and the result will be
four haploid nuclei.
No replication of DNA occurs between meiosis I and meiosis II.
8
Sexual Life Cycles
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The three main types of sexual life cycles
– Animals
– Plants and Algae
– Fungi and Protists
Differ in the timing of meiosis and fertilization
9
Plants and Some Algae
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Exhibit an alternation of
generations
The life cycle includes
both diploid and
haploid multicellular
stages
Haploid multicellular
organism (gametophyte)
n
Mitosis
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
Diploid
multicellular
organism
(sporophyte)
2n
FERTILIZATION
2n
Zygote
Mitosis
(b) Plants and some algae
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Most Fungi and Some Protists
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Meiosis produces haploid
cells that give rise to a
haploid multicellular adult
organism
The haploid adult carries out
mitosis, producing cells that
will become gametes
Haploid multicellular
organism
Mitosis
n
Mitosis
n
n
n
Gametes
MEIOSIS
n
FERTILIZATION
2n
Zygote
(c) Most fungi and some protists
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Animals
Key
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Meiosis occurs
during gamete
formation
Gametes are the
only haploid cells
Haploid
Diploid
n
n
Gametes
n
MEIOSIS
FERTILIZATION
Zygote
2n
Diploid
multicellular
organism
2n
Mitosis
(a) Animals
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Meiosis
Interphase
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Meiosis reduces the
number of chromosome
sets from diploid to
haploid
Meiosis takes place in
two sets of divisions
–
–
Meiosis I reduces the
number of chromosomes
from diploid to haploid
Meiosis II produces four
haploid daughter cells
Figure 13.7
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
13
Meiosis Phases
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Meiosis involves the same four phases seen in
mitosis
 prophase
 metaphase
 anaphase
 telophase
They are repeated during both meiosis I and
meiosis II.
The period of time between meiosis I and meiosis
II is called interkinesis.
No replication of DNA occurs during interkinesis
because the DNA is already duplicated.
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Prophase I
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Prophase I occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, the 2 members of each homologous pair of chromosomes
line up side-by-side, aligned gene by gene, to form a tetrad consisting
of 4 chromatids
During synapsis, sometimes there is an exchange of homologous parts
between non-sister chromatids. This exchange is called crossing over
Each tetrad usually has one or more chiasmata, X-shaped regions
where crossing over occurred
Nonsister
chromatids
Prophase I
of meiosis
Tetrad
Chiasma,
site of
crossing
over
15
Metaphase I
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At metaphase I, tetrads line up at the metaphase plate, with one
chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one
chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the
other chromosome
PROPHASE I
Sister
chromatids
Tetrad
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Chiasmata
Metaphase
plate
Spindle
Microtubule
attached to
kinetochore
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
Homologous
chromosomes
separate
Tetrads line up
Pairs of homologous
chromosomes split up
16
Anaphase I
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In anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the
spindle apparatus
Sister chromatids remain attached at the centromere and
move as one unit toward the pole
PROPHASE I
Sister
chromatids
Tetrad
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Chiasmata
Metaphase
plate
Spindle
Microtubule
attached to
kinetochore
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
Homologous
chromosomes
separate
Tetrads line up
Pairs of homologous
chromosomes split up
17
Telophase I and Cytokinesis
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In the beginning of telophase I, each half of the
cell has a haploid set of chromosomes; each
chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming
two haploid daughter cells
In animal cells, a cleavage furrow forms; in plant
cells, a cell plate forms
No chromosome replication occurs between the
end of meiosis I and the beginning of meiosis II
because the chromosomes are already replicated
18
Prophase II
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Meiosis II is very similar to mitosis
In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of
two chromatids) move toward the metaphase plate
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
19
Metaphase II
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At metaphase II, the sister chromatids are at the metaphase plate
Because of 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
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
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Anaphase II
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At anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as
two newly individual chromosomes toward opposite poles
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
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Telophase II and Cytokinesis
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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
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
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Interphase and meiosis I
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Centrosomes
(with centriole pairs)
Sister
chromatids
Chiasmata
Spindle
Metaphase
plate
Nuclear
envelope
Homologous
Microtubule
chromosomes
Tetrad
attached to
Chromatin
separate
kinetochore
Pairs of homologous
Chromosomes duplicate
Tertads line up
chromosomes split up
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
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Telophase I, cytokinesis, and meiosis II
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
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A Comparison of Mitosis and Meiosis
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Mitosis conserves the number of chromosome
sets, producing cells that are genetically identical
to the parent cell
Meiosis reduces the number of chromosomes sets
from two (diploid) to one (haploid), producing cells
that differ genetically from each other and from the
parent cell
The mechanism for separating sister chromatids is
virtually identical in meiosis II and mitosis
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A Comparison of Mitosis and Meiosis
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Three events are unique to meiosis, and all three
occur in meiosis l:
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Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
At the metaphase plate, there are paired homologous
chromosomes (tetrads), instead of individual replicated
chromosomes
At anaphase I of meiosis, homologous pairs move
toward opposite poles of the cell. In anaphase II of
meiosis, the sister chromatids separate
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A Comparison Of Mitosis And Meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Sister chromatids
separate during
anaphase
Anaphase
Telophase
2n
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
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Comparison
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Meiosis
DNA duplication
followed by 2 cell
divisions
Sysnapsis
Crossing-over
One diploid cell
produces 4
haploid cells
Each new cell
has a unique
combination of
genes
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Mitosis
Homologous
chromosomes do not
pair up
No genetic exchange
between homologous
chromosomes
One diploid cell
produces 2 diploid
cells or one haploid
cell produces 2
haploid cells
New cells are
genetically identical to
original cell (except for
mutation)
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Genetic Variation Produced In Sexual Life
Cycles Contributes To Evolution
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Mutations (changes in an organism’s DNA)
are the original source of genetic diversity
Mutations create different versions of genes
Reshuffling of different versions of genes
during sexual reproduction produces genetic
variation
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Origins of Genetic Variation Among Offspring
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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
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Independent Assortment
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Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis
In independent assortment, 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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Independent Assortment of Chromosomes
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Each pair of chromosomes sorts its maternal and paternal homologues
into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
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Crossing Over
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Crossing over produces recombinant
chromosomes, which combine genes inherited
from each parent
Crossing over begins very early in prophase I, as
homologous chromosomes pair up gene by gene
In crossing over, homologous portions of two
nonsister chromatids trade places
Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome
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Crossing Over
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Crossing over produces recombinant chromosomes that
carry genes derived from two different parents
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes
34
Random Fertilization
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Random fertilization adds to genetic
variation because any sperm can fuse with
any ovum (unfertilized egg)
The fusion of gametes produces a zygote
with any of about 64 trillion diploid
combinations
Crossing over adds even more variation
Each zygote has a unique genetic identity
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Evolutionary Significance of Genetic Variation
Within Populations
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Natural selection results in accumulation of
genetic variations favored by the
environment
Sexual reproduction contributes to the
genetic variation in a population, which
ultimately results from mutations
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Sexual Reproduction - The Human Life Cycle
Haploid gametes (n = 23)
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During fertilization,
sperm and ovum fuse
forming a diploid
zygote
The zygote develops
into an adult organism
Haploid (n)
Diploid (2n)
Ovum (n)
Sperm
Cell (n)
FERTILIZATION
MEIOSIS
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
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38
Preparing a Karyotype
APPLICATION A karyotype is a display of
condensed chromosomes arranged in pairs.
Karyotyping can be used to screen for
abnormal numbers of chromosomes or
defective chromosomes associated with
certain congenital disorders, such as Down
syndrome.
TECHNIQUE Karyotypes are prepared from
isolated somatic cells, which are treated with
a drug to stimulate mitosis and then grown in
culture for several days. A slide of cells
arrested in metaphase is stained and then viewed
with a microscope equipped with a digital camera.
A digital photograph of the chromosomes is entered
into a computer, and the chromosomes are
electronically rearranged into pairs according to
size and shape.
RESULTS This karyotype shows the chromosomes
from a normal human male. The patterns of stained
bands help identify specific chromosomes and parts
of chromosomes. Although difficult to discern in the
karyotype, each metaphase chromosome consists of
two, closely attached sister chromatids (see diagram).
Pair of homologous
chromosomes
5 µm
Centromere
Sister
chromatids
39