MEIOSIS
CHAPTER 9
DISCOVERY OF MEIOSIS
• Gametes are reproductive cells (eggs and
sperm) that contain half the complement of
chromosomes found in somatic cells.
• The gametes fuse to form a new cell called a
zygote, which contains two complete copies of
each chromosome.
• The fusion of gametes is called fertilization.
DISCOVERY OF MEIOSIS
• The formation of gametes must involve
some mechanism to halve the number of
chromosomes found in somatic cells.
• If not the number of chromosomes would double
with each fertilization.
• Meiosis is a process of reduction division in
forming gametes.
• This ensures a consistent chromosome number
across generations.
DISCOVERY OF MEIOSIS
• Meiosis and fertilization
constitute a cycle of
sexual reproduction.
• Somatic cells have two
sets of chromosomes
making them diploid.
• Gametes have only
one set of
chromosomes, making
them haploid.
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Maternal
homologue
Paternal
homologue
Haploid sperm
Diploid zygote
Haploid egg
REPRODUCTION
• Some organisms reproduce by mitotic
division and do not involve gametes.
• This is called asexual reproduction.
• An example is binary fission in prokaryotes.
REPRODUCTION
• Other organisms are able to reproduce both
sexually and asexually.
• For example, strawberry plants flower (sexual
reproduction) and send out runners (asexual
reproduction).
REPRODUCTION
• In sexual reproduction, haploid cells or
organisms alternate with diploid cells or
organisms
Haploid cells (n)
Haploid cells (n)
Diploid
individual (2n)
MITOSIS
MITOSIS
n
MITOSIS
MEIOSIS
MEIOSIS
Haploid
individual
(n)
MITOSIS
MITOSIS
FERTILIZATION
Zygote (2n)
(a) Many protists
n
n
2n
Gametes
(n)
Gametes
(n)
2n
MEIOSIS
2n
MITOSIS
Diploid
individual
FERTILIZATION
(2n)
Zygote (2n)
(b) Most animals
Gametes
(n)
MITOSIS
FERTILIZATION
Zygote (2n)
(c) Some plants and some algae
THE SEXUAL LIFE CYCLE
• In animals, the cells that will eventually
undergo meiosis are reserved early on for
the purpose of reproduction.
• These cells are referred to as germ-line cells and
are diploid like somatic cells.
• But only the germ-line cells will undergo meiosis to
produce haploid gametes.
THE STAGES OF MEIOSIS
• Meiosis involves two divisions, meiosis I
and meiosis II.
• Meiosis I separates the homologues in a
homologous pair.
• DNA is replicated only before meiosis I.
• Meiosis II separates the replicate sister
chromatids.
• When completed, meiosis produces four
haploid cells from one diploid cell.
Haploid gametes
PROPHASE
I
Germ-line cell
KEY
BIOLOGICAL
PROCESS:
MEIOSIS
II
TELOPHASE
I
METAPHASE
II
II
ANAPHASE
II
I
I
THE STAGES OF MEIOSIS
• Meiosis I is divided into four stages:
• 1. Prophase I
• Homologues pair up and exchange
segments.
• 2. Metaphase I
• The paired homologous chromosomes align
on a central plane.
• 3. Anaphase I
• Homologues separate from the pairing and
move to opposite poles.
• 4.Telophase I
• Individual chromosomes gather at each of
the two poles.
THE STAGES OF MEIOSIS
• During prophase I,
homologous chromosomes
line up together as a pair.
• Crossing over occurs
between two nonsister
chromatids of homologous
chromosomes.
• The chromatids break in
the same place, and
sections of chromosomes
are swapped.
Centromere
Adjacent
homologues
(nonsister
chromatids)
• The result is a hybrid chromosome.
• The pairing is held together by the cohesion between sister
chromatids and the crossovers.
THE STAGES OF MEIOSIS
• During metaphase I,
the orientation of the
homologous
chromosome pairs is a
matter of chance.
• Each possible orientation
of which homologue
faces which pole results
in gametes with different
combinations of
parental chromosomes.
• This process is called
independent assortment.
THE STAGES OF MEIOSIS
• In anaphase I, the chromosome pairs
separate and individual homologues move
to each pole.
• In telophase I, the chromosomes gather at
their respective poles to form two
chromosome clusters.
Meiosis I
Prophase I
Homologous
chromosomes
further condense
and pair.
Crossing over
occurs. Spindle
fibers form.
Metaphase I
Microtubule
spindle apparatus
attaches to
chromosomes.
Homologous pairs
align along spindle
equator.
Anaphase I
Homologous pairs
of chromosomes
separate and move
to opposite poles.
Telophase I
One set of duplicated
homologues arrives
a teach pole, and
nuclear division
begins.
THE STAGES OF MEIOSIS
• After meiosis I, a brief interphase occurs
where there is no replication of DNA.
• Meiosis II follows and is basically a mitotic
division of the products of meiosis I.
• Except that the sister chromatids are nonidentical
because of crossing over in meiosis I.
THE STAGES OF MEIOSIS
• Meiosis II is also divided into four stages:
• 1. Prophase II
• New spindle forms to attach to chromosome
clusters.
• 2. Metaphase II
• Spindle fibers bind to both sides of the
centromere and individual chromosomes align
along a central plane.
• 3. Anaphase II
• Sister chromatids move to opposite poles.
• 4. Telophase II
• The nuclear envelope is reformed around each
of the four sets of daughter chromosomes.
Meiosis II
Prophase II
Chromosomes
recondense.
Spindle fibers form
between centrioles
(if present).
Metaphase II
Microtubule spindle
apparatus attaches
to chromosomes.
Chromosomes
align along spindle.
Anaphase II
Sister chromatids
separate and move
to opposite poles.
Telophase II
Chromatids arrive
a teach pole, and
cell division begins.
Cell division is
complete. Each
cell ends up with
half the original
number of
chromosomes.
HOW MEIOSIS DIFFERS FROM
MITOSIS
• Meiosis has two unique features not found in
mitosis:
• Synapsis
• This is the process of drawing together
homologous chromosomes down their entire
lengths so that crossing over can occur.
• Reduction division
• Because meiosis involves two nuclear divisions but
only one replication of DNA, the final amount of
genetic material passed to the gametes is
halved.
REDUCTION
DIVISION
Diploid
germ-line
cell
Chromosome
duplication
SYNAPSIS
Sister chromatids
of one duplicated
chromosome
Synapsis
(a)
Sister chromatids
of chromosome’s
homologue
Meiosis I
Haploid gametes
(b)
Meiosis II
MEIOSIS
MITOSIS
Homologous
chromosomes
pair up
Homologous
chromosomes
do not normally
pair up
Crossing over
No crossing over
Two cell divisions
One cell division
Four daughter cells
Two daughter cells
Daughter cells
haploid (n)
Daughter cells
diploid (2n)
MEIOSIS
MITOSIS
Paternal homologue
Maternal homologue
Chromosome
replication
Chromosome
replication
Pairing of
Homologous chromosomes
MEIOSIS I
Synapsis and
crossing over
Cell division
MEIOSIS II
Cell
division
Four daughter cells (n)
Cell
division
EVOLUTIONARY
CONSEQUENCES OF SEX
• Sexual reproduction has an enormous impact
on how species evolve because it rapidly
generates new genetic combinations.
• Three mechanisms help produce this variety:
• 1. Independent assortment
• 2. Crossing over
• 3. Random fertilization
INDEPENDENT ASSORTMENT INCREASES
GENETIC VARIABILITY
Maternal gamete
Paternal gamete
Diploid offspring
Homologous pairs
Potential gametes