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2. medicine
3. genetics

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
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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
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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
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Asexual Reproduction
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In asexual reproduction, one parent
produces genetically identical offspring by
mitosis
Parent
Bud
Figure 13.2
0.5 mm
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Sexual Reproduction - The Human Life Cycle
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A life cycle is the generation-togeneration sequence of stages
in the reproductive history of an
organism
Fertilization and meiosis
alternate in sexual life cycles
At sexual maturity the ovaries
and testes produce haploid
gametes by meiosis
Unlike somatic cells, sperm and
egg cells are haploid cells,
containing only one set of
chromosomes
During fertilization, sperm and
ovum fuse forming a diploid
zygote
The zygote develops into an
adult organism
Haploid gametes (n = 23)
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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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.
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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
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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
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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
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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
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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
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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
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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
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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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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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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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Spermatocytes to Spermatids
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Primary spermatocytes undergo meiosis I, forming
two haploid cells called secondary spermatocytes
Secondary spermatocytes undergo meiosis II and
their daughter cells are called spermatids
Spermatids are small round cells seen close to the
lumen of the tubule
Late in spermatogenesis, spermatids are nonmotile
Spermiogenesis – spermatids lose excess
cytoplasm and form a tail, becoming motile sperm
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Spermatogenesis
Figure 27.8b, c
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Oogenesis
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Production of female sex cells by meiosis
In the fetal period, oogonia (2n ovarian stem cells)
multiply by mitosis and store nutrients
Primordial follicles appear as oogonia are transformed
into primary oocytes
Primary oocytes begin meiosis but stall in prophase I
From puberty, each month one activated primary oocyte
completes meiosis one to produce two haploid cells
– The first polar body
– The secondary oocyte
The secondary oocyte arrests in metaphase II and is
ovulated
If penetrated by sperm the second oocyte completes
meiosis II, yielding:
– One large ovum (the functional gamete)
– A tiny second polar body
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Oogenesis
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