Part II: Genetic Basis of
Life
 Meiosis
and sexual
reproduction
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Meiosis occurs at different times in
the life cycle of plants, animals, and
fungi, but its phases are the same.
In humans, meiosis is a part of
spermatogenesis and oogenesis.
Halving the
Chromosome Number:
Sexual Reproduction
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Meiosis is nuclear division reducing
chromosome number from diploid (2n)
to haploid (n) number.
Haploid (n) number is half the diploid
number of chromosomes.
Requires gamete formation and then
fusion of gametes to form a zygote.
A zygote always has a full (2n) number
of chromosomes.
Halving the
Chromosome Number:
Homologous Pairs
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Diploid cells=
chromosomes in pairs.
Each set of chromosomes
is a homologous pair;
each member is a
homologous chromosome
or homologue.
A location on one
homologue contains
same type of gene at the
same locus on other
homologue.
Halving the Chromosome
Number: Homologous Pairs
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Chromosomes duplicate just
before nuclear division.
Duplication produces two
identical parts called sister
chromatids, held together at the
centromere.
Non-sister chromatids do not
share the same centromere.
One member of each
homologous pair is inherited
from either male or female
parent.
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Meiosis involves
two nuclear
divisions and
produces four
haploid daughter
cells.
Each daughter
cell has half the
number of
chromosomes in
the diploid
parent nucleus.
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Prior to Meiosis I,
DNA replication
occurs and each
chromosome has
two sister
chromatids.
During Meiosis I,
homologous
chromosomes
come together
and line up in
synapsis.
•During synapsis, the two sets of paired chromosomes
lay alongside each other as bivalents or tetrad.
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Crossing over is an
exchange of
homologous
segments between
non-sister
chromatids of
bivalent during
Meiosis I.
This results in
genetic
recombinations.
After crossing over
occurs, sister
chromatids of a
chromosome are
no longer identical.
Halving the Chromosome
Number: Meiosis II
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No replication of DNA is needed
between Meiosis I and Meiosis II b/c
chromosomes were already doubled.
During Meiosis II, centromeres
divide; sister chromatids separate.
Chromosomes in the four daughter
cells have only one chromatid.
Each daughter cell is haploid, and in
animal life cycle, becomes gametes.
Genetic Recombination
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Due to genetic recombination,
offspring have a different
combination of genes than their
parents.
Without recombination, asexual
organisms must rely on
mutations to generate variation
among offspring.
Genetic Recombination:
Crossing-Over
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Crossing over of nonsister
chromatids results in
exchange of genetic material
between non-sister
chromatids; introduces
variation.
At synapsis, homologous
proteins are held in position
by a nucleoprotein lattice
(the synaptonemal complex).
Genetic Recombination:
Crossing-Over
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As lattice of synaptonemal
complex breaks down at the
beginning of anaphase I,
homologues are temporarily
held together by chiasmata,
regions where non-sister
chromatids are attached due
to crossing-over.
Due to crossing over,
chromatids are no longer
identical.
Genetic Recombination:
Fertilization
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Meiosis
increases
variation.
When gametes
fuse at
fertilization,
chromosomes
donated by
parents
combine.
Phases of Meiosis
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Prophase I:
Nuclear division is about to occur:
nucleolus disappears; nuclear
envelope fragments; centrosomes
migrate away from each other;
spindle fibers assemble.
Homologous chromosomes undergo
synapsis forming bivalents; crossing
over may occur at this time.
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Metaphase I:
During prometaphase I, bivalents
held together by chiasmata have
moved toward the metaphase plate.
Kinetochores are regions just outside
centromeres; they attach to spindle
fibers called kinetochore spindle
fibers.
Bivalents independently align
themselves at the metaphase plate
of the spindle.
Phases of Meiosis
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Anaphase I:
The homologues of each
bivalent separate and move
toward opposite poles.
Each chromosome still has two
chromatids.
Phases of Meiosis
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Telophase I:
Only occurs in some species.
When it occurs, the nuclear envelope
reforms and nucleoli reappear.
Interkinesis:
• This period between meiosis I and II is
similar to interphase in mitosis.
• However, no DNA replication occurs.
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Meiosis II:
During metaphase II, the
haploid number of
chromosomes align at
metaphase plate.
During anaphase II,
centromeres divide and
daughter chromosomes move
toward the poles.
At the end of telophase II and
cytokinesis, there are four
haploid cells.
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Mitosis II continued:
Due to crossing-over, each
gamete can contain
chromosomes with different
genes.
In animals, the haploid cells
mature and develop into
gametes.
In plants, the daughter cells
become spores and divide to
produce a haploid adult
generation.
Comparison of Meiosis
and Mitosis
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Meiosis I differs from Mitosis:
DNA is replicated only once
before both mitosis and
meiosis; in mitosis there is only
one nuclear division, in meiosis
there are 2.
During prophase I of meiosis,
homologous chromosomes pair
and undergo crossing-over.
Comparison of Meiosis
to Mitosis
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During metaphase I, paired homologous
chromo’s align at the metaphase plate:
in mitosis individual chromo’s align.
During anaphase I, homologous
chromo’s with centromeres intact
separate and move to opposite poles; in
mitosis at this stage, sister chromatids
separate and move to poles.
Comparison of Meiosis
to Mitosis
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Meiosis II differs from Mitosis:
Events in Meiosis II are same
stages as in mitosis.
However, the nuclei contain the
haploid number of
chromosomes in meiosis.
Mitosis produces two daughter
cells; meiosis produces four
daughter cells.
The Human Life Cycle
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Life cycle refers to all reproductive
events between one generation and
next.
In animals, adult is diploid and gametes
are haploid.
Mosses are haploid most of their cycle,
oak trees are diploid.
In fungi and algae, organism you see is
haploid and produces haploid gametes.
Spermatogenesis.
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In males, meiosis is part of
spermatogenesis or production of
sperm.
In the testes of males, primary
spermatogenesis with 46 chromo’s
divide meiotically to form two
secondary spermatocytes, each with
23 duplicated chromo’s.
Secondary spermatocytes divide to
produce four spermatids (23
chromo’s).
Spermatogenesis
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Spermatogenesis continued:
Spermatids then differentiate
into sperm.
Meiotic cell division in males
always results in four cells that
become sperm.
Oogenesis
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Oogenesis is in the ovaries of
human females, primary oocytes
with 46 chromosomes divide
meiotically to form two cells, each
with 23 duplicated chromosomes.
One of the cells, secondary oocyte,
receives most cytoplasm.
Other cell, a polar body,
disintegrates or divides again.
Oogenesis
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Secondary oocyte begins in meiosis II
and then stops at metaphase II.
At ovulation, secondary oocyte leaves
ovary and enters an oviduct where it
may meet sperm.
If sperm enters oocyte, it is activated
to continue meiosis II, resulting in
mature egg and another polar body,
each with 23 chromosomes.
The Human Life Cycle:
Oogenesis
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Polar bodies serve to discard
unnecessary chromosomes and
retain most of the cytoplasm in
the egg.
The cytoplasm serves as a
source of nutrients for the
developing embryo.