Cell Division .2345567789999999887887pdf

Notes on cytology for medical students

1. Hermann
Tchakounte,
Medical Student
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2. Defination: Cell division is the process by
which a parent cell divides into two or more
daughter cells. Cell division usually occurs as
part of a larger cell cycle
In cell division, the cell that is dividing is called
the "parent" cell. The parent cell divides into two
"daughter" cells and the process then repeats in
what is called the cell cycle.
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3. Why Do Cells Divide?
Cells divide basically for three reasons. They are:
a)For the growth & development of our body
b)To repair the dead and damaged tissues
c)For reproduction
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4. The cells of higher organisms divide by two
processes. They are :
1) Mitosis: The process that cells use to make
exact replicas of themselves is mitosis. Mitosis is
observed in almost all the body’s cells, including
eyes, skin, hair and muscle cells.
2) Meiosis: In this type of cell division, sperm or
egg cells are produced with haploid set of
chromosomes instead of identical daughter cells
as in mitosis.
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5. CELL DIVISION
Mitosis & Meiosis

6. Cell Cycle
During the cell cycle,
– Cell grows.
– DNA is replicated.
– Mitotic cell division produces daughter cell
identical to the parent.
There are two primary phases in the cell cycle:
Interphase: This phase was thought to represent the resting stage
between subsequent cell divisions, but new research has shown that it
is a very active phase.
M Phase (Mitosis phase): This is where the actual cell division occurs.
There are two key steps in this phase, namely cytokinesis and
karyokinesis.
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7. The interphase comprises three phases:
G0 Phase (Resting Phase): The cell neither divides nor
prepares itself for the division.
G1 Phase (Gap 1): The cell is metabolically active and grows
continuously during this phase.
S phase (Synthesis): The DNA replication or synthesis
occurs during this stage.
G2 phase (Gap 2): Protein synthesis happens in this phase.
Quiescent Stage (G0): The cells that do not undergo further division
exits the G1 phase and enters an inactive stage. This stage is known
as the quiescent stage (G0) of the cell cycle.
Interphase
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8. Interphase
It is actually a Non-dividing state. In
this state….
– cell grows in size
– organelles replicated
– replication of DNA
– synthesis of proteins
associated with DNA
– synthesis of proteins
associated with mitosis
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9. Division of somatic cells in eukaryotic organisms is called
as mitosis
In this process, a single cell divides into two identical
daughter cells.
Daughter cells have same set of chromosomes as does
the parent cell.
Mitosis Cell Division
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10. Mitosis
It has 4 sub-phases:1
st – Prophase
2nd – Metaphase
3rd – Anaphase
4th – Telophase
followed by
Cytokinesis
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11. 1. Prophase
In Prophase 3 Major
Events take place.
1) Chromosomes condense
2) Spindle fibers form
(spindle fibers are specialized
microtubules radiating out from
centrioles)
3) Chromosomes are
captured by spindle
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12. chromatinnucleolusnucleus
centrioles
condensing
chromosomes
Fig; Main events of Prophase
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13. In this phase chromosomes align
along equator of the cell, with one
kinetochore facing each pole
centriol
es
spindle fiberschromosomes
2. Metaphase
Kinetocores not pictured in this illustration.
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14. 3. Anaphase
In anaphase stage the
sister chromatids separate
from each other.
Spindle fibers attached to
kinetochores shorten and
pull the chromatids towards
the opposite poles.
The cell appears almost oval
in shape as it starts becoming
longer.
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15. 4. Telophase
•In the telophase stage, the spindle fibers between
the poles disintegrate.
•The nuclear envelopes start reforming around both
the groups of chromosomes at the poles
•Chromosomes revert to their extended state by
absorbing water from the cytoplasm
•There appears a constriction in the cytoplasm
between the two groups of dividing chromosomes
* Cytokinesis completes the enclosing of each
daughter nucleus into a separate cell
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16. CYTOKINESIS– IN ANIMAL CELL
.
Cytokinesis is the last phase of the
normal cell cycle. In this phase the cell
physically divides into two identical
daughter cells. In animal cells, the cell
membrane pinches together and the
membrane breaks apart where it was
pinched and now it is two daughter
cells. In both new cells the DNA is
identical. In plant cells, a cell plate
forms down the middle of the cell and
the cell breaks apart where the cell
plate was formed. The two daughter
cells will often stay attached to each
other side-by-side
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17. REDUCTION CELL DIVISION
Meiosis is a
process where a
single cell divides
twice to produce
four daughter cells
containing half the
original amount of
genetic information.
These cells are
called as gamates
sperm in males,
eggs in females.
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18. Diploid organisms receive one of each type of
chromosome from female parent and one of each
type of chromosome from male
Ploidy refers to the number of sets of
chromosomes in cells.
● Haploid : * It contains only one copy of chromosome
* It is designated as “n”
● Diploid : * It contains two sets of chromosomes
* It is designated as “2n”
Genetics Terminology: Ploidy
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19. GENETICS TERMINOLOGY: HOMOLOGUES
Chromosomes exist in homologous pairs
in all diploid (2n) cells except in Sex Chromosomes in male (XY)
Apart from the Sex chromosomes, the other chromosomes are
known as autosomes and all they have homologues.
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20. MEIOSIS – THE GAMETE FORMATION
In meiosis, there are 2
nuclear divisions.
They are
Meiosis I
&
Meiosis II
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21. MEIOSIS - I
Meiosis is a reduction division. The salient features of meiotic
division that make it different from mitosis are as follows:-
1) It occurs in two stages of the nuclear and cellular division as
Meiosis I and Meiosis II. DNA replication occurs, however, only
once.
2) It involves the pairing of homologous chromosomes and
recombination between them.
3) Four haploid daughter cells are produced at the end, unlike two
diploid daughter cells in mitosis.
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22. Meiosis-I
has following
four sub stages :
Prophase - I
Metaphase - I
Anaphase - I
Telophase - I
Prophase - I
Prophase - I is longer than the
mitotic prophase and is further
subdivided into 5 sub stages.
They are :
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
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23. LEPTOTENE
Leptotene is the first of five stages of Prophase - I and it
consists of the condensation of the already replicated
chromosomes
The chromosomes become visible and now they can be
distinguished easily.
The chromosomes at this stage are likened to a string
with beads, called chromomeres.
Each sister chromatid is attached to the nuclear envelope
and they are so close together that they can be mistaken
as one chromosome
.
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24. Fig : Leptotene Stage
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25. ZYGOTENE
In this stage the homologous chromosomes begin to pair.
This process is also known as zygonema (Synapsis).
These synapsis can form along the entire length of the
chromosomes allowing numerous points of contact called
'synaptonemal complex‘.
The synaptonemal complex facilitates synapsis by holding
the chromosomes together
. along the entire length.
After the homologous pairs synapse they are either
called bivalents.
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26. Fig: Zygotene stage
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27. PACHYTENE
Once the synapse is formed, now the cell is ready
for crossing over.
In this stage further thickening and shortening of
chromosomes take place.
During this stage, exchanges of chromosome material
between maternal and paternal homologous
chromosomes occur by crossing over.
At the points of crossing over, X–shaped chiasmata are
seen in variable numbers.
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28. Fig : Pachytene stage
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29. DIPLOTENE
In the "Diplotene" stage, the paired chromosomes begin
to get separated from each other.
While getting separated, the homologous chromosomes
remain united at the points of interchange of "Chismata".
Chiasma are formed as the result of crossing over in the
Diplotene stage.
With the progression of diplotene, the nuclear membrane
gets disorganized and the nucleolus disappears.
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30. Fig : Diplotene Stage
under Electron Microscope
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31. In this stage, bivalents get distributed in the nucleus. The
nuclear membrane breaks down and the nucleolus
disappears.
Chiasma moves towards the end, and this process is
called as terminalization.
Chromatids remain attached only at the terminal
chiasmata and enter the metaphase stage.
Recondensation of chromosomes takes place. Tetrad
move towards equatorial metaphase plate and spindle
formation is initiated.
DIAKINESIS
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32. Fig : Diakinesis
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33. METAPHASE - I
In Metaphase-I of meosis the maternal and paternal
chromosomes (homologous chromosomes) align along
the equator of the cell.
A process called independent assortment occurs where
the maternal and paternal chromosomes line up randomly
and align themselves on either side of the equator.
At this stage 50% of the chromosomes start migrating to
the opposite pole of the dividing cell.
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34. Fig : Metaphase-I
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35. ANAPHSE-I
In this stage Kinetochore microtubules shorten and pull
the homologous chromosomes to opposite poles.
The sister chromatids remain tightly bound together at the
centromere.
The chiasmata are broken in as the microtubules attached
to the fused kinetochores pull the homologous
chromosomes apart .
Non-kinetochore microtubules lengthen, pushing the
centrosomes farther apart and the cell starts elongatting.
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36. Fig : Anaphase-I
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37. TELOPHASE-I
This is the end of the First meiotic division.
Each daughter cell now has half the number of chromosomes
but each chromosome consists of a pair of chromatids.
The microtubules that make up the spindle network disappear,
and a new nuclear membrane surrounds each haploid set.
The chromosomes uncoil back into chromatin.
Cytokinesis occurs, completing the creation of two daughter
cells. However, cytokinesis does not fully complete resulting in
"cytoplasmic bridges" which enable the cytoplasm to be shared
between daughter cells until the end of meiosis II
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38. Fig : Telophase-I
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39. MEIOSIS-II
Meiosis-II initiates immediately after cytokinesis, usually before the
chromosomes have fully decondensed.
In contrast to meiosis I, meiosis II resembles a normal mitosis. In some
species, cells enter a brief interphase, or interkinesis, before entering
meiosis II.
However this interphase lacks an S phase, so chromosomes are not
duplicated. The two cells produced in meiosis I go through the events of
meiosis II together.
During meiosis II, the sister chromatids within the two daughter cells
separate, forming four new haploid gametes. The mechanics of meiosis
II is similar to mitosis, except that each dividing cell has only one set of
homologous chromosomes.
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