cell cycle

1. MEIOSIS
The form of cell division by which
gametes, with half the number of
chromosomes, are produced.

2. • Meiosis in males is called spermatogenesis
and produces sperms.
• Meiosis in females is called oogenesis and
produces ova.
•Two divisions (meiosis I and meiosis II).
Meiosis

3. 2n
n
n
1 cell division:
2 daughter cells
2 cell divisions:
4 cells product
of meiosis
2n
2n
2n
n
n n
n
n=chromosome number
Somatic cells
Cells in sexual cycle
MITOSIS
MEIOSIS

4. • Why must Meiosis take place
to produce sperm and egg?
Meiosis creates cells that are
destined to become gametes (or
reproductive cells), this reduction in
chromosome number is critical —
without it, the union of two gametes
during fertilization would result in
offspring with twice the normal
number of chromosomes.

5. Meiosis involves two divisions, Meiosis I
and Meiosis II. Each follows similar stages
as mitosis (prophase, metaphase,
anaphase, and telophase). Before meiosis,
the reproductive cell is in the interphase
stage whereby DNA replicates to produce
chromosomes having two sister
chromatids. Then, the cell will undergo
second growth phase called interkinesis.
This stage happens between Meiosis I and
II, however, DNA does not replicate in this
stage.

6. Meiosis involves two divisions, Meiosis I
and Meiosis II. Each follows similar stages
as mitosis (prophase, metaphase,
anaphase, and telophase). Before meiosis,
the reproductive cell is in the interphase
stage whereby DNA replicates to produce
chromosomes having two sister
chromatids. Then, the cell will undergo
second growth phase called interkinesis.
This stage happens between Meiosis I and
II, however, DNA does not replicate in this
stage.

7. The first meiotic division, also known as
Meiosis I, is a reduction division phase
(diploid - haploid). There are two daughter
cells produced after Meiosis 1, each daughter
cell is carrying haploid number of
chromosomes. This consists of four stages,
namely, prophase I, metaphase I, anaphase I,
and telophase I.
MEIOSIS I

8. • Prophase I
• The longest phase.
This phase can be
divided into 5 stages;
a) Leptotene
b) Zygotene
c) Pachytene
d) Diplotene
e) Diakinesis

9. 1. Leptotene 2.Zygotene
3.Pachytene 4.Diplotene 5.Diakinesis
Prophase I

10. • Substage 1: Leptotene
Each chromosome is made up of sister
chromatids. These are long threadlike
structures which result from the
replication of DNA during the
Synthesis or S phase of the cell cycle.

11. • Substage 2: Zygotene
The homologous chromosomes
start to pair off through the process
known as synapsis. Pairs of
chromosomes that are similar in
size and shape are called
homologous chromosomes or
tetrads.

12. definitions
• Homologous
chromosomes: the
members of a
chromosome pair
that is identical in
the arrangement of
genes

13. • Synapsis: the
intimate
association of
homologous
chromosomes…
Homologous chromosomes
definitions

14. • Substage 3: Pachytene
The repeated coiling of chromosomes occurs resulting
to its contraction and thickening making the
homologous pair of chromosomes to be very close to
each other. At this stage, the process called crossing
over happens. Here, the exchange of segments between
the sister chromatids of the homologous chromosomes
occurs. The exchanging process form a cross-linkage
called a chiasma. After crossing over, the sister
chromatids of each chromosome may not be identical
with each other based on the genetic material they
contain. Crossing-over is a complicated process that
results to genetic variability.

15. definitions
• Crossing-over:
a term describing
the process of
reciprocal
chromosomal
interchange by
which
recombinants
arise

16. Crossing over
Pairing of
homologous
chromosomes
Chiasma
formation
Chromosomes
Breakage
and joining
Recombinant
chromosomes

17. • Substage 4: Diplotene
The two homologous
chromosomes forming a tetrad
begin to repel one another and
move apart. They are held only by
the chiasma.

18. • Substage 5: Diakinesis
• This is the last stage of meiosis prophase 1.
Diakinesis stage is characterized by chiasmata
terminalization. After diakinesis, the dividing cell
enters metaphase.
• At this stage, bivalents or homologous pair
chromosomes distribute them evenly in the
nucleus. The nuclear membrane breaks down and
the nucleolus disappears. Chiasma moves towards
the end, which is called terminalization.
Chromatids remain attached only at the terminal
chiasmata and enter the metaphase stage.

19. Metaphase I Anaphase I
Telophase I
Prophase II
Prophase I

20. • Metaphase 1
Stage
Spindle fibers from the
centrosomes of each pole
connect to bivalents or
tetrads through the
kinetochores. Homologous
chromosomes line up at the
equatorial plane. There is
double alignment of the
chromosomes.

21. • Anaphase 1
Stage
• Spindle fibers begin to
contract and separate
the bivalent or tetrads.
• Homologous
chromosomes separate
and migrate to each pole
of the cell.

22. • Telophase 1
Stage
• Chromosomes decondense.
• Nuclear membranes reform.
• Cytokinesis or the
cytoplasmic division occurs
and two haploid (N)
daughter cells with
chromosomes with two
sister chromatids are
formed.

23. MEIOSIS II
The second meiotic division forms
four daughter cells, each carrying
haploid number of chromosomes.
This consists of the following stages,
namely, prophase II, metaphase II,
Anaphase II, and telophase II.

24. • Prophase II
Stage
• Chromosomes (chromatids)
condense to form
metaphase chromosomes.
• Nuclear membrane
dissolves and nucleolus
disappears.
• Centrosomes move towards
each pole of the cell.

25. • Metaphase II
Stage
• Spindle fibers attach to
chromatids at the
kinetochores.
• Chromosomes line up at
the equatorial plane.
(Single alignment of
chromosomes).

26. • Anaphase II
Stage
• Spindle fibers shortened
and separated the sister
chromatids.
• Chromatids that are now
called chromosomes
move towards each pole
of the cell.

27. • Telophase II
Stage
• Single-stranded
chromosomes decondense.
• Nuclear membrane and
nucleolus reforms.
• Cytoplasm divides
(cytokinesis).
• Four haploid (n) daughter
cells are formed.

28. 4 Haploid cells Gametes

30. Mitosis vs Meiosis
Mitosis Meiosis
Results in 2 Diploid Cells
(2N)
4 Haploid Cells
(N)
Cells are Genetically
Identical
Genetically
Different
Occurs in Somatic (Body)
Cells
Sex Cells

31. Final result
Meiosis :
Four genetically different haploid cells
from one diploid parent cell
Mitosis :
two genetically identical diploid cells
from one diploid parent cell