Cellcycle-Mitosis-Meiosis | Notes-by-UK-Sir | Cell-Bio- 16
CELL Cycle (Cell Biology)
https://uksirnotes.blogspot.com/2021/08/Eukaryotic-Nucleus-chromatin-uksir.html
CELL CYCLE
• Cell cycle
is a series of events by which a cell duplicates its genome, synthesises other
constituents of the cell and eventually divides into two daughter cells.
• In a
typical eukaryotic somatic cell, cell cycle has two main phases-
-
a
long non-dividing interphase (I-phase) and
-
a
short nucleus dividing mitotic phase (M-phase).
 |
| Cell-cycle |
• Interphase (~ 95% time)
* It is
metabolically active phase.
* It is
divided into first gap phase or G1-phase, synthetic phase or S-phase and Second
gap phase or G2-phase.
• G1 phase :(~ 50% time)
* This phase
is in between the end of M-phase of previous cell cycle and initiation of
S-phase.
* It is the
longest phase of interphase.
* In this
phase, cell grows in size to its larger size.
*Necessary
RNA, Ribosome, Lipid and proteins are synthesized.
* Cell
organelles also increase in number.
* From G1 phase, cells may enter into S-phase or sometimes in G0 phase.
• S-phase
: (~25-30% time)
* During
this phase DNA synthesis takes place.
* DNA
content becomes double, though the ploidy level remains same. (If 2n=4. Then
2n= 8).
* Histone
proteins are synthesised during S-phase.
• G2 phase : (~15-20% time)
* In this
phase, synthesis of DNA stops.
* However,
formation of RNAs and proteins continues which are required for multiplication
of cell organelles, spindle formation, aster formation and cell growth.
* In animal
cells, centrosome begins to divide and there is an extensive synthesis of
tubulin protein to form microtubules.
*Chromatin
starts to condense, mitochondria like organelles duplicate.
• M-phase : (~ 4-5% time)
*It is
dividing phase or Mitotic phage.
*It is
divided into -karyokinesis (division of nucleus) & cytokinesis (division of
cytoplasm).
*
Karyokinesis in again divisible into - prophase, metaphase, anaphase and
telophase.
• G0
phase:
- is a period in the cell cycle in
which cells exists in a quiescent state.
* G0 phase
is viewed either as an extended G1 phase, where the cell is neither dividing
nor preparing to divide.
* or a
distinct quiescent stage that occurs outside the cell cycle. The cell
metabolism continues.
Cell Cycle Checkpoints :
• Two key classes
of regulatory molecules, Cyclins and Cyclin dependent protein kinases (CDKs)
determine a cell’s progress through the cell cycle.
These CdKs
are enzymes which add phosphate group to other molecules by phosphorylation
process, by signaling the cell.
• Cell cycle
checkpoints are the control mechanisms that ensure the accuracy of cell
division.
There are
two regulatory mechanisms which takes decision about cell division.
 |
| Check point |
– First checkpoint is called G1 cyclin checkpoint (CG1 or G1/S checkpoint)
* which is
present in between G1 and S phase.
Regulated by
CdK4/ CdK6/ Cyclin-D.
Controlled
by cell size, growth factor, cell environment etc.
-Second checkpoint is present between G2 and M-phase.
Regulate by
CdK2/cyclin B.
* Control by
completion of DNA replication, DNA damage, Cell Size etc.
* It causes
transition from G2 to M-phase.
-A Third check point is present between Metaphase and Anaphase of cell division.
*Regulated
by cyclin- B degradation.
* It ensures
the proper spindle fiber attachment.
Colchicine (Mitotic poison): an alkaloid/ poison- stop spindle fiber attachment and help in doubling the chromosome number.
CELL DIVISION
* Cell
division is the process of formation of new or daughter cells from pre-existing
or parent cells.
* It is of
three types-amitosis, mitosis and meiosis.
Amitosis
or Direct Cell Division
* Amitosis
is characterized by the splitting of nucleus followed by that of cytoplasm.
* It was
described by Flemming (1882).
* In this
method, the nucleus elongates and constricts in the middle to form two daughter
nuclei without the formation of spindle fiber or the appearance of chromosomes.
* It does
not divide the nuclear material equitably.
e.g., meganucleus of Paramecium, internodal cells of Chara, etc.
 |
| Division in Paramecium |
 |
| Amitosis |
Mitosis or Equational Division (2n --> 2n+2n)
* Mitosis or
mitotic cell division is meant for multiplication of cells.
* It
generally takes place in vegetative or somatic cells.
* In this
process, one parent cell divides into two daughter cells, but the chromosome
number remains the same as in parent cell.
*Daughter
cells exactly resemble with parent cell both quantitatively as well as
qualitatively hence, it is called equational division.
Stages of mitosis:
 |
| Mitosis |
1.Karyokinesis (Division of Nucleus)
It consists
of 4 Phase or Steps.
A.Prophase :
* It is the
first stage of mitosis.
* During
this stage viscosity of cytoplasm increases.
*Chromatin
fibers become shorter and thicker due to the coiling and condensation.
* Get
condensed into distinct thread- like chromosomes in late prophase.
* Each
chromosome consists of two coiled sister chromatids joined by a centromere that
appears like a ball of wool called spireme stage.
* Spindle
fibers form at each side ofthe cell and nuclear membrane breaks down.
*
Disappearance of nucleolus and other cellular organelles occur.
*
Disappearance of nuclear membrane also seen.
B.Metaphase :
* The
complete disintegration of the nuclear envelope marks the start of the second
phase of mitosis i.e., metaphase.
*
Condensation of chromosomes is completed by this time.
* At this
stage, the number and morphology of chromosomes can be easily studied and
counted under microscope.
* The chromosomes
line up along the center of the cell (called auto orientation).
*The spindle
fibers attach to each chromosome at the centromere forming the metaphase plate.
* Small
disc-shaped structures at the surface of the centromeres called kinetochores
serve at the site of attachment of spindle fibers.
C.Anaphase :
* This is
the shortest stage of mitosis.
*
Chromosomes divide at the point of centromere and thus, two sister chromatids
get separated.
* Chromatids
move to the opposite poles of the cells due to contraction of spindle fibers.
(Chromosomes look V,L,U,J shape)
* Nucleolus,
Golgi complex and ER re-form.
D.Telophase
:
* The
separated chromatids or newly formed chromosomes reach to the opposite poles.
* The
nuclear envelop re-form and the chromatin de-condenses.
* In
general, the events of prophase occur in reverse sequence during this phase.
*RNA
synthesis restarts causing nucleolus to reappear.
2.Cytokinesis:
 |
| cytokinesis |
* During
this, mitosis ends with division of cytoplasm by a process known as cleavage.
* It starts
towards the middle of anaphase and is completed with the telophase.
* It differs
in plants and animal cells.
* In plants,
it usually occurs by cell plate method
*Whereas in
animals it takes place by cleavage or cell furrow method.
Significance of mitosis
-
Growth
and development: A full grown organism is developed from a single cell zygote,
by repeated mitotic divisions.
-
Genetic
Restore: It re store the exact genetic form by forming clone.
-
Genetic
stability: All the daughter cells of a multi cellular organism have the same
number and type of chromosomes as parent cells due to equitable distribution of
all the chromosomes.
-
Repairing and healing: The mechanism for
replacing old or worn out cells and healing of a wound by new cell production
depend on mitosis.
-
Regeneration: Some organisms are able to
regenerate their missing parts of body or whole organism through mitosis.
Meiosis or Reductional Cell Division:
* Meiosis is
a kind of cell division that occurs in a diploid cell and reduces the chromosome
number by half, which results in the production of haploid daughter cells.
* The term
was coined by Farmer and Moore.
* It
involves two successive nuclear divisions- meiosis I and meiosis II.
* But there
is no DNA replication prior to second division.
* There is a
short inter kinesis between meiosis I and meiosis II.
 |
| mitosis vs meiosis |
 |
| Meiosis stages |
Meiosis I:
* It is the
heterotypic or reductional division.
*Because it brings about changes from diploid to
haploid state. (2n--> n)
* It can be
studied under four stages:
 |
| Meiosis-1 stages |
Prophase -
I:
Metaphase -
I
Anaphase - I :
Telophase -
I :
Prophase - I:
* It is the
most complicated and longest phase of meiotic division.
* It is
further divided into five stages:
 |
| Prophase 1 |
A.Leptotene :
* The size
of nucleus is increased, chromosomes become more apparent and show bead -like
thickenings called chromomeres.
*
Chromosomes also show a typical arrangement and often referred to as “bouquet”
stage.
B.Zygotene
:
*
Chromosomes become shorter and thicker.
* Pairs of
homologous chromosomes called bivalents or tetrad are seen.
* The
process of attachment of the homologous chromosomes due to the development of
nucleoprotein is known as synapsis.
* It produces
a complex called synaptonemal complex.
* Here gene
to gene pairing occurs.
C.Pachytene :
* It is the
longest phase of prophase - I
*Characterized
by the appearance of recombination nodules found.
* Here
crossing over (exchange of genetic material) between non-sister chromatids of
the homologous pair takes place.
D.Diplotene
:
*
Dissolution of synaptonemal complex occurs partially
*Therefore,
the homologous chromosomes separate except in the region of crossing over.
* The points
of attachment between homologous chromosomes after the partial dissolution of
nucleoprotein complex are called chiasmata.
* Lampbrush
chromosomes are actually condensed diplotene chromosomes.
E.Diakinesis :
* Chiasmata
shift towards the ends of the chromosomes.
* It is
termed as terminalisation.
* The
nucleolus degenerates, nuclear envelope disappears, centrioles migrate to poles
and spindle fibers begin to form.
• Metaphase - I :
* The
bivalents (both homologous chromosome) arrange themselves on the equator of the
bipolar.
* Spindle
fiber Attach to the homologous chromosome of metaphase plate.
*
Chromosomes are in maximum condensed state and are ready to separate.
• Anaphase
- I :
* Bivalent
chromosomes (Maternal and paternal) get separated.
*
Microtubules of the spindle fibers begin to shorten .
* Each
separated chromosome with two chromatids start moving towards the opposite
poles.
* Because of
the random orientation of homologous chromosomes on the metaphase plate, a pole
may receive either the maternal or the paternal homologue from each chromosome
pair.
• Telophase - I :
* The
haploid set of chromosomes elongate or decondense.
* nucleolus
is formed and nuclear envelope reappears.
* It is
generally followed by cytokinesis.
• Interkinesis :
* It is the
metabolic stage between meiosis - I and meiosis - II.
* Protein
and RNA synthesis may occur but there is no DNA synthesis.
* It is
short lived and plays important role in bringing true haploidy.
• Meiosis - II :
* It is also
called homotypic or equational division as it maintains the number of
chromosomes produced at the end of reduction division.
* Meiosis - II is divisible into prophase - II, metaphase - II, anaphase - II, telophase - II and cytokinesis.
 |
| Meiosis 2 |
* All these
phases are similar to mitosis.
* But meiosis II is not mitosis
* as it
always occurs in haploid cells .
daughter
cells formed after meiosis II are neither similar to each other not to the
parent cell.
Cytokinesis:
*Just like
the Mitotic cytokinesis.
*1st
cytokinesis may occur immediately after the meiosis 1
*in most of
the cases both Cytokinesis occurs in
same time, i.e. after meiosis 2
Significance of meiosis -
* Meiosis is
essential for gamete formation and maintenance of chromosome number in sexually
reproducing organisms.
* Paternal
and maternal chromosomes assort independently during meiosis, that causes
reshuffling of chromosomes and the traits controlled by them.
* Crossing
over facilitates new combination of genes thus, helps in producing variations.
* Meiosis
also provides evidences for the basic similarity and relationship among
organisms.
* Meiosis II
plays important function in separating the chromatids of univalent chromosomes
that differ from each other in their linkage groups due to crossing over.
* Meiosis
also helps in evolution.
Want to Know about Nucleus, chromatin :
https://uksirnotes.blogspot.com/2021/08/Eukaryotic-Nucleus-chromatin-uksir.html
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