2. MEIOSIS
OBJECTIVES
At the end of the lesson, students
should be able to:
Explain & compare the processes in
meiosis I & meiosis II.
Explain the position and changes of
the chromosomes at each stages.
3. Objectives
• Define chromatid, synapsis,
bivalent, tetrad, chiasma,
crossing over & centromere
• State the significance of
meiosis
• Compare meiosis & mitosis
4. CONTENT OF MEIOSIS
Meiosis- a form of nuclear division in
which the chromosome number is halved
from the diploid number (2n) to the
haploid number (n).
involves interphase
followed by two cycle of cell division,
known as meiosis I and meiosis II (first
& second meiotic division).
gives rise to four haploid cells.
5. occurs during gametogenesis in
animal and during spores formation
in plants.
a continuous process but is
conveniently divided into prophase,
metaphase, anaphase and telophase.
These stages occur in the first
meiotic division and again in the
second meiotic division.
7. Importance term s in meiosis:
b) Chromatid
- prior to meiosis 1,DNA replication occurs and each
chromosomes has two sister chromatids.
- After crossing over occurs,sister chromatid of a
chromosome are not longer identical
- the chromosomes in the four daughter cells of
meiosis are actually chromatids from the original
parent cell.
b) Synapsis
- during meiosis 1,homologous chromosomes
pair,come together and line up in synapsis
8. c) Bivalent
- during synapsis,the two sets of paired chromosomes
lay alongside each other as a bivalent
d) Tetrad
- is a group of four homologous chromatids synapsed
together during prophase 1 of meiosis
e) Chiasma
- a special structure,chiasma,occurs during crossing
over.
- it is the point at which a chromatid of one
chromosome crosses with a chromatid of the
homologous chromosome
- it can be usually seen in prophase 1
9. f) Crossing over
- chromosomes have equal exchanges of genetic
materials while paired up.
d) Centromere
- specialised region of a chromosome at which siser
chromatids are held together
10. MEIOSIS I
• Prophase I
sister
chromatids
begin to
condense &
homologous
In a process
called synapsis,
visible as a
tetrad
11. Stages in Prophase I
• There’s 5 substages in Prophase I:
leptotene, zygotene, pachytene,
diplotene & diakinesis
• Things that occur during these
stages:
- Chromosomes become visible as
fine thread
- Spindle starts to form
12. • Homologous chromosomes attracted to
each other forming a bivalent or tetrad
(consist of 4 chromatids) in a process
known as synapsis
• Nucleolus disappeared
• chromatids become visible as they
move apart from each other; they
remain in contact at points called
chiasmata.
13. • chromatids continue to move apart as
they shorten and thicken
• shortening and thickening continues;
chiasmata move to ends; nuclear
envelope breaks down
- continue to repel each other
- bivalents assume particular shapes
depending upon the number of
chiasmata.
14. By the end of prophase I:
• chromosomes are fully contracted
and deeply stained;
• centrioles migrated to the poles;
• nucleoli and nuclear envelope
dispersed;
• spindle fibres form.
15. MEIOSIS I
• Metaphase I
The bivalents
become arranged on
the metaphase
plate
Kinetochore
microtubules from
each pole of the
cell attached to
kinetochore of
chromosomes
16. MEIOSIS I
• Anaphase I
Spindle fibers pull
homologous
chromosomes,
centromeres first
This separate the
chromosomes into
two haploid sets
17. MEIOSIS I
• Telophase I
The arrival of
homologous
chromosomes at
opposite poles
marks the ends
of meiosis I.
18. MEIOSIS I
• Telophase I
Cleavage (animals) or cell wall
formation (plants) THEN occurs
as in mitosis.
Halving of chromosome number
has occurred but the
chromosomes are still composed
of two chromatids.
19. MEIOSIS II
• Interphase
- In some species; chromosomes
decondense, nuclear membrane
& nucleoli re-form
- No further DNA replication
occurs.
Meiosis II is similar to mitosis.
20. MEIOSIS II
• Prophase II
absent if interphase
II is absent.
nucleoli and nuclear
envelopes disperse &
the chromatids
shorten, thicken.
Centrioles, move to
opposite poles
new spindle fibers
appear.
21. MEIOSIS II
• Metaphase II
Chromosomes line up
separately on the equator
of the spindle.
Kinetochores pointing
towards opposite poles
• Anaphase II
The centromeres divide
and the spindle fibers pull
the chromatids to
opposites poles
22. MEIOSIS II
• Telophase II
four haploid daughter
cells are formed.
The chromosomes
uncoiled, lengthen and
become very indistinct.
spindle fibers disappear
23. MEIOSIS II
Nuclear envelope
re-form
Subsequent
cleavage (animals)
or cell wall
formation (plants)
will produce four
daughter cells
from the original
single parent cell.
24. SIGNIFICANCE OF MEIOSIS
Halving the chromosome number
ensures that when gametes with the
haploid number fuse to form a zygote
the normal diploid number is
restored.
Meiosis leads to increased variation
because:
When fertilization there is
recombination of parental genes.
25. SIGNIFICANCE OF
MEIOSIS
During metaphase I, homologous
chromosomes are together at the
equator of the spindle, but they
separate into daughter cells
independently of each other.
Chiasmata and crossing-over can
separate and rearrange genes located
on the same chromosome.
26. Three events, unique to meiosis, occur
during the first division cycle.
1. During prophase I, homologous
chromosomes pair up in a process called
synapsis.
A protein zipper, the synaptonemal
complex, holds homologous
chromosomes together tightly.
27. Three events, unique to meiosis,
occur during the first division cycle
Later in prophase I, the joined
homologous chromosomes are visible as a
tetrad.
At X-shaped regions called chiasmata,
sections of nonsister chromatids are
exchanged.
Chiasmata is the physical manifestation of
crossing over, a form of genetic
rearrangement.
28. Three events, unique to meiosis, occur
during the first division cycle
2. At metaphase I homologous pairs
of chromosomes, not individual
chromosomes are aligned along the
metaphase plate.
In humans, you would see 23
tetrads.
29. Three events, unique to meiosis,
occur during the first division cycle
3. At anaphase I, it is homologous
chromosomes, non sister
chromatids are separated and
carried to opposite poles of the
cell.
Sister chromatids remain
attached at the centromere until
anaphase II.
30.
31. COMPARISON BETWEEN MEIOSIS AND
MITOSIS
No. MITOSIS MEIOSIS
1. Conserves Reduces the chromosome
chromosome number number by half (n).
(2n).
32. COMPARISON BETWEEN MEIOSIS AND
MITOSIS
No. MITOSIS MEIOSIS
2. no synapsis occur to Synapsis occurs to form
form bivalent. bivalent at the homologous
chromosomes during
prophase I
3. No chiasma occurs so Some chiasma occurs to
there is no cross – form cross – over.
over.
33. COMPARISON BETWEEN MEIOSIS AND
MITOSIS
No. MITOSIS MEIOSIS
4. genetic composition in Genetic composition in
daughter cells are daughter cells are different
identical to parental to parental cell.
cells.
5. Two daughter cells Four daughter cells each
each diploid (2n) haploid (n)
6. Cytokinesis occurs Cytokinesis occurs once or
once. twice.
7. The daughter cell can The daughter cell can
undergoes mitosis . undergoes mitosis but not
meiosis.
