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Cell Division
The Cell Cycle
-Cycle means the repetition of a sequence
of events during a given time period.
-The cell cycle is divided into:
1. Mitosis: ordinary cell division.
2. Interphase.
The Cell Cycle
DIVISION
M
Mitosis
G2
G1
S
INTERPHASE
Cell Theory and Cell Division
Cell Theory
• Def: All organisms consist of cells and arise
from pre-existing cells
• Cell cycle: predictable sequence of cell life
events
– Chromosomes: thread-like bodies visible during
cell division
The interphase is divided into three phases:
1-Gap I or G phase: The one following mitosis
2-Gap S phase:
Synthesis phase where DNA is
duplicated
3-Gap 2 or G 2 phase:
-So, it is in the G1 or G2 phase where the cell is
performing its specialized function while in the S
phase it is only directed to duplicate its DNA..
-In the interphase the cells become specialized
working cells, so we may get prolonged G1 or rarely
G2 phase.
Functions of the nucleus
1-It directs the cell division.
2-It carries all the genetic information and
hereditary characters.
3-It controls all the cell functions including
protein synthesis.
4-It is responsible for the formation of
RNA.
The cells of the body may be categorized
into 3 types:
1- Very highly specialized cells
-They never divide i.e. we posses a certain
number of these cells since birth.
-If they die or wear out, they never regenerate.
e.g. nerve cells, and cardiac muscle cells.
2- Highly specialized cells
-They can keep their members from other cells of
the same family type from a stem or a mother
cell.
e.g. cells lining the stomach and intestine.
3- Specialized cells
-They can go into cycle only under extreme
conditions.
e.g. liver cells (hepatocytes).
-In case of removal of up to 2/3 of the liver in
experimental animals, these cells are able to
restore the normal size of the liver.
•
Cell Theory and Cell Division
Gap2 phase: a period of time between S and
beginning of mitosis
– Synthesis of materials used in mitosis
– Microtubule proteins, ATP, other organelles
– 4-5 hours duration
• Duration times based upon one organism, others
may differ
The Cell Cycle
DIVISION
M
Mitosis
G2
G1
S
INTERPHASE
Cell Division
-In the human body, two types of
cell division are known:
I-Mitosis.
II-Meiosis.
Cell Theory and Cell Division
Set of chromosomes
• Ploidy: a term referring to the numbers of sets
of chromosomes in a given cell
• Def: one of each kind of chromosome that is
characteristic of a species
– Human set has 23 chromosomes
• Haploid (n): one set
• Diploid (2n): two sets
I-Mitosis
-It is the process by which the cell divides.
-In animal cells, the whole process takes
from 1-15 hours.
-Before going into mitosis, duplication of
the chromosomes occurs during the
stage of the interphase.
s-
-At the G2 stage of the interphase, we have 46 dchromosomes (double-stranded chromosomes)
and also double pairs of centrioles.
-During mitosis, the 46 d-chromosomes separate
giving us 92 s-chormosomes, where each half of
them is directed towards the corresponding pole
of the cell.
-Thus, two nuclei with the typical46 s-chromosome
arrangement of a nucleus in the g1 stage of the
interphase are formed, and two similar daughter
cells result.
• -Another important feature of mitosis, is
the formation of mitotic spindle.
• -The cells in the G1 stage have 2
centrioles which are perpendicular to
each other. If this cell starts to go to the
s-phase and G2 phase, these 2 centrioles
become 4 in number.
Phases of Mitosis
1-Prophase
• 2-Metaphase
• 3-Anaphase
• 4-Telephase
•
Mitosis (M Phase)
Prophase Events
• Chromosomes condense, become visible
– Replicated chromosomes (=chromatids) joined
at the centromere
• Spindle apparatus forms and attaches to
kinetochore (structure associated with the
centromere)
• Nuclear membrane begins to disintegrate
1-Prophase:
-Disapearance of nucleolus and nuclear
envelop.
-Formation of the mitotic spindle.
-Direction of each divided cenrtiole to
one of the cell pole.
-Chromosomes condensed to 2
chromatid
Mitosis (M Phase)
Metaphase Events
• Chromosomes align along the equator of the
cell
• Spindle appratus anchored (in animals) at the
centriole (site of spindle formation)
Stages of Mitosis
PRIOR TO MITOSIS
Chromosomes replicate,
forming sibling
chromatids
MITOSIS
Sibling chromatids separate
Centrosomes
Centrioles
Early mitotic spindle
1. Chromosomes
replicate in
parent cell.
2. Prophase: Chromosomes
condense and mitotic
spindle begins to form.
Nuclear envelope breaks
down.
Interphase
Prophase
Kinetochore
3. Metaphase:
Chromosomes
migrate to
middle of cell
Metaphase
2-Metaphase:
-The
chromosomes are aligned in the
equatorial plane.
-The chromosomes are attached to the
spindles at the kinetochore
Mitosis (M Phase)
Anaphase Events
• Centromere divides allowing spindle apparatus
to shorten and separate sibling chromatids
• Separated chromatids called chromosomes
• Chromosomes move to opposite poles of the
cell
Stages of Mitosis
Anaphase
4. Anaphase: Sister
chromatids separate.
Chromosomes are pulled
to opposite poles of the
cell.
Telophase
5. Telophase: The nuclear
envelope reforms.
6. Cytokinesis: The cell
divides.
4-Telophase:
*Reappearance of the nuclear memb.
and constriction in the cytoplasm to
form two new cells
* Formation of the chromatin network
Mitosis (M Phase)
Telophase and Cytokinesis Events
• Nuclear envelope begins to form around
chromosomes
• Chromosomes become diffuse
• Spindle apparatus disappears
• Cytokinesis begins
Mitosis (M Phase)
• Cytokinesis in animals
characterized by a cleavage furrow
• Cytokinesis in plants
characterized by the formation of a new wall
between newly forming nuclei
Mitosis in Progress
Animal Cell Mitosis
Interphase
17 m
Animal Cell Mitosis
Prophase
17 m
Animal Cell Mitosis
Metaphase
17 m
Animal Cell Mitosis
Anaphase
17 m
Animal Cell Mitosis
Telophase
17 m
Animal Cell Mitosis
Cytokinesis
17 m
MEIOSIS
Meiosis
• The form of cell division by which gametes, with half the
number of chromosomes, are produced.
• Diploid (2n)
 haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).
Meiosis
• Sex cells divide to produce gametes (sperm or egg).
• Gametes have half the number of chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some chromosomal
differences.
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
II-Meiosis
-It is the division which occur in the gamete cells
in the ovary and the seminiferous tubules.
-The mother cell in the female ovary is the
primary oocyte.
-At the time of birth, the baby girl has two
million primary oocytes in her two ovaries.
-The oocyte passes to the s-stage where it
contains 46 d-chromosomes then through the
G2 to start the division.
Meiosis I (four phases)
• Cell division that reduces the chromosome number by
one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come together to
form a tetrad.
• Tetrad is two chromosomes or four chromatids (sister and
nonsister chromatids).
First meiotic division
-It consists of prophase, metaphase, anaphase
and telophase.
-In the prophase there are 5 stages:
1-Leptotene
2-Zygotene
3- Pachytene
4-Diplotene
5-Diakinesis
1-Leptotene
The 46 d-chromosomes appear as ribbons or
in a thread-like form.
2-Zygotene
Each member of the pair of d-chromosomes
lies beside each other
.thus, a bivalent is formed, one of a maternal
origin and the other from a paternal origin.
3- Pachytene
The chromosomes become thicker and shorter.
4-Diplotene
Each bivalent consists of 2 d-chromosomes and
every d-chromosome is composed of 2 strands or
2 s-chromosomes.
5-Diakinesis
-The 2 arms of the bivalent may cross one
another and this is called a chiasma.
-This causes detachment and the fragment
reunits with the other stump.
-This results in the exchange of paternally
derived chromosome portions with parts of
maternally derived chromosomes and viceversa.
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are similar in shape
and size.
• Homologous pairs (tetrads) carry genes controlling the same inherited
traits.
• Each locus (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes.
a.
b.
22 pairs of autosomes
01 pair of sex chromosomes
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Crossing Over
• Crossing over (variation) may occur between nonsister
chromatids at the chiasmata.
• Crossing over: segments of nonsister chromatids break and
reattach to the other chromatid.
• Chiasmata (chiasma) are the sites of crossing over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Sex Chromosomes
XX chromosome - female
XY chromosome - male
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two identical sister
chromatids attached at their centromeres.
• Centriole pairs also replicate.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example:
2n = 4
then
n=2
thus
22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Anaphase I
• Homologous chromosomes separate and move towards
the poles.
• Sister chromatids remain attached at their centromeres.
Anaphase I
Telophase I
Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase plate
metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember:
produced.
four haploid daughter cells
gametes = sperm or egg
Telophase II
Meiosis
sex cell
n=2
sperm
n=2
n=2
2n=4
haploid (n)
n=2
diploid (2n)
n=2
n=2
meiosis I
meiosis II
Karyotype
• A method of organizing the chromosomes of a cell in relation to
number, size, and type.
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote