Download In meiosis

Mitosis & Meiosis
Objectives
1- To understand the cell cycle:
i.e steps of (Mitosis and Meiosis cell
divisions)
2- Differentiate between Mitosis and Meiosis
and their end results.
 Mitosis:
Is the somatic (body) cell
division , for growth and repair.

Meiosis
-Is the division of gametes (sex
cells)
Mitosis
 Mitosis

animations:
http://www.youtube.com/watch?v=2WwIKdyBN_s&feature=relat
ed

http://www.youtube.com/watch?v=VlN7K1-9QB0
Cell Cycle
Stages
1- Interphase
 G1 Phase
 S Phase
 G2 Phase
2- M Phase
 Cytokinesis

Interphase – Resting Stage
 Cells
carrying on normal
activities
 Chromosomes aren’t visible
 Cell metabolism is occurring
 Occurs before mitosis
G1 Phase
 Growth stage (synthesis of
RNA and
proteins)
 Cell
performs its specific functions
 Cell
increases in size
 Cell
prepares to copy its DNA

This is the longest phase of the cell cycle.
Synthesis (S) Phase
 Replication of
DNA
 Chromosomes are
duplicated (each
chromosome is formed of 2 chromatids
formed of double helix of DNA).
G2 Phase
 Time
between DNA synthesis
& mitosis
 It is a non-synthetic period
M Phase
 Cell
growth & protein
production stop
 Cell’s
energy used to make 2
daughter cells
 Called
mitosis or karyokinesis
(nuclear division)
Stages of Mitosis
 Prophase
 Metaphase
 Anaphase
 Telophase

Mitosis animation:
http://www.youtube.com/watch?v=2WwIKdyBN_s&feature=related
Prophase

DNA coils tightly &
becomes visible as
chromosomes

Nuclear membrane
disappears

Nuceolus disappears

Centrioles migrate to
poles

Spindles begin to form
Each Chromosome in prophase is
formed of 2 identical chromatids
joined at the centromere
Metaphase

Spindle fibers from
centrioles attach to
each chromosome

Cell preparing to
separate its
chromosomes

Chromosomes
move to the
equatorial plane of
the cell, become
shorter, deeply
stained and easily
studied.
Anaphase
 Spindle
fibres
contract
 The centromere of
each chromosome
divides
longitudinally
 The 2 chromatids
separate apart
and each moves to
one pole of the cell
Telophase
 Separation
of
chromosomes
completed
 Nucleus &
nucleolus
reform
 Chromosomes
uncoil
Cytokinesis
 Cytokinesis is
the division of
the cytoplasm
of a parent
cell
 Occurs after
chromosomes
separate
 Forms two,
identical
daughter cells
Meiosis
It is cell division occurring in the gonads (testes
and ovaries) to produce gametes (sperms and ova).
 It is formed of :
- Meiosis I (reduction division where a cell
with 46 chromosomes --- 2 cells each has 23
chromosomes only.
- DNA replication takes place.
- Meiosis II which is the same as mitotic
division but there is no DNA replication.

Meiosis animation:
http://www.youtube.com/watch?v=D1_-mQS_FZ0&NR=1
Meiosis I Interphase
The chromosomes
have replicated,
and the chromatin
begins to condense.
http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html
Meiosis I Prophase
-The chromosomes
are completely
condensed.
- In meiosis (unlike
mitosis), the
homologous
chromosomes pair
with one another
http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html
Meiosis I Metaphase

- Chromosome pairs are
arranged in the equatorial
plane.
- Spindles are formed,
each attach to the
centromere of ONE
chromosome.
Cross over occurs in this
step
Meiosis I Anaphase

- spindles contract.
- Each SINGLE
chromosome moves
towards one pole of the
cell.
Meiosis I Telophase

- Formation of nuclear
membrane.
- Division of cytoplasm.
- End result is formation of 2
daughter cells each one has ½
(haploid) number of
chromosomes of the mother
cell (46 = Diploid number).
Meiosis II


There is no DNA replication.

The chromatids of each chromosome
separate and each daughter cell divides.
At the end of Meiosis, there are four daughter
cells from each parent cell. Each daughter cell
has half the number of chromosomes of the
parent (23 chromosomes in human).




1. Reduces the Diploid
number of chromosome to
Haploid.
2. Allows shuffling of
maternal & paternal
chromosomes between the
gametes Segregation
3. Allows Crossing Over of
chromosome segments:
It is the interchange of
chromatid segments
between paired homologus
chromosomes which
redistributes genetic
material.
N.B. It enhances genetic
diversity through cross
over and segregation.
Mitosis





Occurs in somatic and
germ cells
Single division
Chromosome no in daughter cell is 46
One chromatid of a chromosome goes
to each daughter cell
No crossingover
Meiosis
- in germ cells
- 2 divisions (meiosis I, II)
- 23 (haploid no)
- Each chromosome of a
homologous pair goes to
a daughter cell
- Crossing over