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Meiosis and Sexual
Life Cycles
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Life is distinguished by the ability of
organisms to reproduce their own kind.
Genetics: the scientific study of heredity
and variation.
Heredity: the transmission of traits from
one generation to the next.
Variation: offspring differ in appearance
from parents and siblings.
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In a literal sense,
children do not inherit
particular physical traits
from their parents.
It is genes that are
actually inherited.
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Genes: segments of DNA that
are the units of heredity.
Each gene has a specific locus
on a certain chromosome.
One set of chromosomes is
inherited from each parent.
Reproductive cells called
gametes (sperm and eggs)
unite, passing genes to the
next generation.
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In asexual reproduction,
one parent produces
genetically identical
offspring by mitosis.
In sexual reproduction,
two parents give rise to
offspring that have
unique combinations of
genes inherited from the
two parents.
Parent
Bud
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Each human somatic cell (any cell other
than a gamete) has 46 chromosomes
arranged in pairs.
The two chromosomes in each pair are
called homologous chromosomes, or
homologues.
Both chromosomes in a pair carry genes
controlling the same inherited
characteristics.
A karyotype is an ordered display of the pairs of
chromosomes from a cell.
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
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Autosomes: the chromosomes that do not
determine sex.
A cell with a single set is called haploid (n).
A cell with two sets is called diploid (2n).
Each pair of homologous chromosomes
includes one chromosome from each parent.
For humans, the haploid number is 23 and
the diploid number is 46.
(n = 23, 2n = 46)
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In humans, 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 (XY).
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In a cell in which DNA
synthesis has occurred, each
chromosome is replicated.
Each replicated chromosome
consists of two identical
sister chromatids.
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
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Gametes are haploid cells, containing only
one set of chromosomes.
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.
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At sexual maturity, the ovaries and testes
produce haploid gametes.
Gametes are the only types of human cells
produced by meiosis, rather than mitosis.
Meiosis results in one set of chromosomes
in each gamete.
Fertilization, the fusing of gametes,
restores the diploid condition, forming a
zygote.
The diploid zygote develops into an adult.
The alternation of meiosis and fertilization is
common to all organisms that reproduce sexually.
 The three main types of sexual life cycles differ
in the timing of meiosis and fertilization.
 Depending on the type of life cycle, either
haploid or diploid cells can divide by mitosis.
 However, only diploid cells can undergo meiosis.
 In all three life cycles, chromosome halving and
doubling contribute to genetic variation in
offspring.
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Key
Haploid
Diploid
n
Gametes
n
Mitosis
n
MEIOSIS
Haploid multicellular
organism (gametophyte)
n
FERTILIZATION
Diploid
multicellular
organism
Animals
Zygote 2n
Mitosis
Mitosis
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
2n
n
Haploid multicellular
organism
n
n
n
Gametes
Diploid
multicellular
organism
(sporophyte)
n
FERTILIZATION
MEIOSIS
2n
Mitosis
n
2n
Mitosis
Plants and some algae
Zygote
FERTILIZATION
2n
Zygote
Most fungi and some protists
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Like mitosis, meiosis is preceded by the
replication of chromosomes.
Meiosis takes place in two sets of cell
divisions, called meiosis I and meiosis II.
The 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.
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Meiosis I is preceded by
interphase, in which
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.
Interphase
Homologous pair
of chromosomes
in diploid parent cell
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In the first cell division
(meiosis I), homologous
chromosomes separate.
Meiosis I results in two haploid
daughter cells with replicated
chromosomes.
In the second cell division
(meiosis II), sister chromatids
separate.
Meiosis II results in four
haploid daughter cells with
unreplicated chromosomes.
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
Sister chromatids
separate
Haploid cells with unreplicated chromosomes
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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MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Propase
Prophase I
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
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrads
positioned at the
metaphase plate
Metaphase I
Anaphase I
Telophase I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
n
Property
Mitosis
Meiosis
DNA
replication
Divisions
During
interphase
One
During
interphase
Two
Synapsis and
crossing over
Daughter cells,
genetic
composition
Do not occur
Role in animal
body
Form tetrads in
prophase I
Two diploid,
Four haploid,
identical to
different from
parent cell
parent cell and
each other
Produces cells
Produces
for growth and
gametes
tissue repair
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Three events are unique to meiosis, and all
three occur in meiosis l:
 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, it is homologous chromosomes,
instead of sister chromatids, that separate and
are carried to opposite poles of the cell.
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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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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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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.
Nonsister
chromatids
Prophase I
of meiosis
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.
Tetrad
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Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes
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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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.