Meiosis
Reproduction of Sex Cells
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IB ASSESSMENT STATEMENT
• 4.2.1 State that meiosis is a reduction
division of a diploid nucleus to form
haploid nuclei
Meiosis Introduction
• Meiosis is part of the lifecycle of every organism
that reproduces
sexually.
• Meiosis is cell division
that produces sex cells (
sperm and eggs), also
called gametes
Gametes Introduction
• Each gamete contains
half the number of
chromosomes of the
parent.
• During fertilization the
male gamete fuses with
the female gamete to
produce a zygote with
two sets of
chromosomes.
LE 13-3
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
Human Chromosomes
• The number of chromosomes in a single set is
represented by n
• A cell with two sets is called diploid (2n)
• For humans, the diploid number is 46 (2n = 46)
Human Sex Cells
• Gametes are haploid
cells, containing only
one set of
chromosomes
• For humans, the
haploid number is 23
(n = 23)
Sexual Reproduction -- Fertilization
• At sexual maturity, the ovaries and testes
produce haploid gametes
• Gametes are the only types of human cells
produced by meiosis, rather than mitosis
• Fertilization, the fusing of gametes, restores the
diploid condition, forming a zygote
LE 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Sperm
cell (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Process of Meiosis.
• The number of chromosomes are reduced
from diploid to haploid in the process of
meiosis.
• Therefore, meiosis is describe as a
reduction division.
Overview of Meiosis
• Meiosis involves two divisions,
meiosis I and meiosis II.
• By the end of meiosis II, the diploid
cell that entered meiosis has become 4
haploid cells.
IB ASSESSMENT STATEMET
• 4.2.2 Define homologous chromosomes.
Review of Chromosome terms.
• Homologous
Chromosomes–
Chromosomes with
the same genes, but
different alleles
Review of Chromosome terms.
• Sister Chromatids–
are replicated
chromosome held
together by a
centromere.
• During meiosis each
homologous
chromosome will
have an identical
chromatid.
Assessment Statement
• 4.2.3 Outline the process of meiosis,
including pairing of homologous
chromosomes and crossing over, followed
by two divisions, which results in four
haploid cells.
• Limit crossing over to the exchange of genetic material between
non-sister chromatids during prophase I. Names of the stages are
required.
HL Meiosis Summary
• Meiosis consists of two rounds of cell division.
– M1 : Reduction division which separates the
chromosomes of a homologous pairs.
• Each gametic cell will contain one of the two chromosomes
from every homologous pair.
• This has great genetic significance since it separates the
alleles for every gene.
– M2: Separation of the 'sister chromatids'.
• After M1 the gametic cells contain a pairs of 'sister
chromatids'.
• The separation of the 'sister chromatids' also produces
variation in gametes since this will isolate the 'recombinants
alleles' due to cross-over in a gamete cell.
Meiosis I
Meiosis I a closer look
• In the first cell division (meiosis I),
homologous chromosomes separate
• Meiosis I results in two haploid daughter
cells with replicated chromosomes
Interphase I
Meiosis I
Prophase
Prophase II
Metaphase
Metaphase II
Anaphase I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
Meiosis I
• Meiosis I is preceded by interphase, in
which chromosomes are replicated to form
sister chromatids.
Phase of Meiosis.
•
Division in meiosis I occurs in four
phases:
1.
2.
3.
4.
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase 1
• Homologous
chromosomes will pair up.
• Spindle tubules grow from
each pole to the equator as
in mitosis.
Propase 1
• Crossing over occurs in prophase 1
• In crossing over, nonsister chromatids
exchange DNA segments
Prophase 1
• Each pair of chromosomes forms a tetrad,
a group of four chromatids
• Each tetrad usually has one or more
chiasmata, X-shaped regions where
crossing over occurred
Importance of Crossing Over
• Allows for a recombination of linked
genes.
• Linked genes are genes that are on the
same chromosome (example A, E are on
the pink chromesome)
Prophase 1
Metaphase 1
• The pairs of
chromosomes line
up in the center.
• Spindle
microtubules
attach to each
chromosome in
each pair.
Anaphase 1
• Homologous
chromosomes are
pulled to opposite
poles.
• Each chromosome
still consists of two
chromatids.
Telophase & Cytokinesis
• 2 Nuclear membranes
form.
• The cell separates into
two cells.
• The two cells produced by
meiosis 1 have
chromosomes and alleles
that are different from
each other and from the
diploid cell that entered
meiosis I.
Meiosis 2
Meiosis 2 – Prophase 2
• The cell has divided
forming two haploid cells
• New microtubules grow
and from the poles to the
equator.
Meiosis 2 Metaphase 2
• The chromosomes line up in
the center of cell.
• Because of crossing over in
meiosis I, the two sister
chromatids of each
chromosome are no longer
genetically identical
Meiosis 2 Anaphase 2
• The sister chromatids
separate
• The sister chromatids of
each chromosome now
move as two newly
individual chromosomes
toward opposite poles
Meiosis 2 Anaphase 2
• Sometimes chromosomes fail
to separate during both
anaphase I & II, this results in
non-disjunction.
• Non-disjunction results in
gamete with too few or too
many chromosomes.
• If a gamete with non
disjunction becomes fertilized,
genetic disorders could result,
i.e. down syndrome
Meiosis 2: Telophase 2 &
Cytokinesis
• In telophase II, the chromosomes
arrive at opposite poles
• Nuclear membrane re-formed
• Cytokinesis separates the
cytoplasm
• Meiosis II results in four haploid (N)
daughter cells.
Phases of
Meiosis
• Meiosis II
Telophase I and
Cytokinesis I
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
Formation of sperm by Meiosis
Formation of Egg Cells by Meiosis
In many female animals, only
one egg results from meiosis.
The other three cells, called
polar bodies, are usually not
involved in reproduction
Meiosis Animations
• http://www.biostudio.com/d_%20Meiosis.h
tm
Variation
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:
1. Independent assortment of chromosomes
2. Crossing over
3. Random fertilization
Crossing over revisited
• 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
Crossing over revisted
• 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
Independent Assortment of
Chromosomes.
• 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
Independent Assortment
Random Fertilization
• 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