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Cell Cycle & Division
Introduction ● Rudolf Virchow proposed the “Cell lineage theory” which states “omnis cellula e cellula” i.e new cells arise
from preexisting cells. However he failed to prove the theory.
● Karl Nageli proposed that cells arise by division of preexisting cells.
● Strasburger proposed that new nuclei arise by division of preexisting nuclei.
Cell division ● Cell division is the process by which a parent cell divides into two or more daughter cells.1
● There are 3 main types of cell division
○ Mitosis
○ Meiosis
○ Amitosis
Mitosis It is a type of cell division that results in two daughter cells each having the same number and kind of
chromosomes as the parent nucleus.
● Mitosis name was proposed by Fleming and it’s detailed study was given by A. Schneider.
Causes of mitosis 1. Kern Plasm theory
a. Hertwig proposed this theory.
b. According to this theory mitosis is due to disturbance in karyoplasmic index of cell.
Vn
olume of nucleus
K I = V c−V n = VVolume of Cytoplasm
;Vn=Volume of nucleus, VC=Volume of cell
Karyoplasmic index of small cell is high as they have less cytoplasm. Nucleus efficiently controls the activity
of cytoplasm in small cells. In large cells nucleus fails to control the activity of cytoplasm. To attain the
control of nucleus on metabolism a large cell divides into two cells.
2. Surface-Volume Ratio
a. According to this surface-volume ratio of a cell plays an important role in starting cell division.
b. A cell draws all the materials needed for its maintenance & growth from its surface. When a cell grows
in size its volume increases more than its surface. So a stage will reach when the surface area becomes
insufficient to draw the material. At such critical stage, division starts.
Mitotic Cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and
duplication (replication) that produces two daughter cells.2
● A typical eukaryotic cell is illustrated by human cells in culture. These cells divide once in every 24 hours3 .
● Yeast takes only 90 minutes to complete one cycle4 .
● Cell cycle involves two stages
○ Interphase
○ Division Phase/M-phase
Interphase/Growth Phase/Resting Phase
● This is the middle stage of one cycle and occurs between two division phase.
● Cells grow in size and prepares itself for next division.
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"Cell division - Wikipedia, the free encyclopedia." 2003. 18 Jun. 2014 <http://en.wikipedia.org/wiki/Cell_division>
"Cell cycle - Wikipedia, the free encyclopedia." 2003. 18 Jun. 2014 <http://en.wikipedia.org/wiki/Cell_cycle>
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Cooper, GM. "The Eukaryotic Cell Cycle - The Cell - NCBI Bookshelf." 2000. <http://www.ncbi.nlm.nih.gov/books/NBK9876/>
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Cooper, GM. "The Eukaryotic Cell Cycle - The Cell - NCBI Bookshelf." 2000. <http://www.ncbi.nlm.nih.gov/books/NBK9876/>
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● Interphase lasts more than 95% of the duration of
the cell cycle.
● It is the most active phase of cell cycle though it’s
called as Resting phase.
● In interphase, metabolism of cell increases. A series
of metabolic changes occurs during interphase in
cell. These changes are not visible under
microscope, so some scientist termed interphase as
resting phase. But know it’s known that it is the
most active phase of cell cycle.
● Howard and Pelc classified interphase into three
sub stages
○ G1-Phase (1st Gap phase)/Pre DNA synthesis
phase
■ Longest sub stage of cell cycle (12hr).
■ Number of cell organelle increases in cell.
■ Different types of RNA and proteins are
synthesised rapidly by the cell.
■ Synthesis of new protoplasm takes place in
the cell and it starts growing in size.
■ Cells grows maximum in G1 stage.
○ S-Phase (DNA synthesis Phase.)
■ It takes approx 6-10 hrs to complete in humans.
■ Replication of nuclear DNA and synthesis of histone protein takes place in S-phase.
■ New DNA molecules formed are not distinct but intertwined.
■ Replication of cytoplasmic DNA may occur in any stage of cell cycle.
■ Replication of centriole takes place in late S-phase
○ G2-Phase (2nd Gap phase)/Post DNA synthesis Phase/Pre-Mitosis Phase
■ It takes approx 3-12 hrs to complete.
■ Number of cell organelles increase in this phase.
■ Special materials required for cell division like tubulin proteins(required for spindle fibre formation)
are synthesized in this phase.
■ Cell division involves enormous amount of energy, thus cells store ATP in this phase.
M-phase/Division phase/Mitotic phase
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●
●
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It is the shortest phase of cycle.
Lasts upto 1 hr.
Division of nucleus in mitosis unlike in amitosis is indirect5
Mitotic phase is divided into 4 sub-phases
○ Prophase
■ Longest stage of M-phase.
■ Metabolism of cell decreases.
■ Cytoplasm becomes viscous, refractive and pale.
■ Chromatin thread condenses to form
chromosomes.
■ Centrioles start moving towards the opposite
poles.
■ Astral rays starts forming in the animal cells due to
gelation of proteins around centrioles. This is
"I. Embryology. 1. The Animal Cell - Gray’s Anatomy-Bartleby.com." 19 Jun. 2014 <http://www.bartleby.com/107/2.html>
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called as amphiastral or astral mitosis. In plants centrioles are absent and no asters are formed. This is
called as anastral mitosis.
■ Nuclear membrane and nucleolus disappears. This marks the onset of Late-Prophase/Prometaphase.
★ Cells when viewed under microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus
and nuclear envelope.
○ Metaphase
■ Chromosomes arrange on equator to form metaphase plate (process is called as metakinesis) or
equatorial plate.
■ The spindle fibre is now fully formed and the
microtubules attach to each sister chromatid at
kinetochores.
■ Spindle fibres are composed of 97% tubulin
proteins and 3% RNA.
■ In metaphase plate, large chromosomes lie
towards the periphery and small chromosomes
towards the centre. Centromeres lies at equator
and arms remain directed toward poles.
■ In metaphase each chromosome splits lengthwise
upto the centromere (division of matrix of chromosome).
■ Chromosomal fibres have polarity i.e (+ve) end at equator and (-ve) end at the pole, this causes two
chromatids of the chromosome repulse each other and the arms of chromosomes are directed
towards the opposite poles.
■ Condensation of chromosomes is completed. Replicated chromatids are clearly visible under
microscope in this stage.
○ Anaphase
■ Interzonal fibres (small & contracted) appears at
equator of cell in early anaphase.
■ Its is the smallest stage of division phase.
■ Centromere of each chromosome splits lengthwise
and number of chromosomes become double.
■ Chromosomes start moving away from each other
along its spindle fiber.
■ Interzonal fibres expands and they push
chromosomes towards the opposite poles.
(Pushing)
■ Chromosomal fibres contract and they pull chromosomes towards opposite poles. (Pulling)
■ This is called as Pulling-Pushing mechanism, it helps in rapid movement of chromosomes towards the
opposite poles. Apprx 30 ATP are required to carry a chromosome to pole. Chromosomes reach at
poles in late anaphase.
○ Telophase
■ It is also called as (Reverse Prophase).
■ Metabolism of cell increases.
■ Nuclear membrane , golgi complex, ER and nucleolus reappears.
■ Chromosomes decondense to form chromatin net.
Cytokinesis/C-phase
● It stands for the division of cytoplasm.
● It starts in late anaphase.
○ In animals it cytokinesis occurs by Constriction and furrow formation.
■ Microtubules and microfilaments arrange on equator to form midbody.
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■ Contraction occurs in midbody and plasma
membrane starts constricting to form
contractile ring.
■ Thus a furrow forms from the outside to inside
in
cell. This furrow deepens continuously and
ultimately a cell divides into two daughter
cells.
■ In animals cytokinesis occurs in centripetal
order i.e from periphery to centre.
○ In plants cytokinesis takes place by cell plate
formation because constriction is not possible due
to
presence of rigid cell wall.
■ Many golgi vesicles and spindle microtubules arrange themselves on equator to form phragmoplast.
■ Fragments of ER may also deposit in phragmoplast.
■ Membrane of golgi vesicles fuse to form a plate like structure called cell plate.
■ In plants the cytokinesis occurs in centrifugal order.
Regulation of Cell Cycle ● Activity of enzymes, known as cyclin dependent kinases
(Cdk’s). Kinase is an enzyme that removes a phosphate group
from ATP & add to another protein. These are called cyclin
dependent because they are activated only when combined
with key protein called cyclin.
● There are checkpoints G1→S & G2→M where a kinase enzyme
combines with cyclin. This moves the cell cycle forwardly.
○ S-Kinase () is capable of starting the replication of DNA
after is combined with S-cyclin (at G1→S checkpoint). After
the completion of S-stage S-cyclin is destroyed & S-kinase
is no longer active.
○ M-Kinase is capable of turning on mitosis after it is
combined with M-cyclin (at G2→M checkpoint).
Modification of mitosis Cryptomitosis or Promitosis ● In this type of division, nuclear membrane does not disappear but remain intact throughout the division.
● All changes of karyokinesis occurs inside nucleus even the formation of spindle fibre, called as intranuclear
spindle.
● Such division is found in some protozoans (amoeba) during binary fission.
Dinomitosis ● Found in dinoflagellates, which are mesokaryotes.
● In mesokaryotic cells histone protein is absent because of this, the chromosomes fail to condense properly
and hence are not distinctly visible during cell division.
● Nuclear membrane persists throughout the cell division and spindle formed is intranuclear in nature.
Free nuclear division ● Karyokinesis is not followed by cytokinesis and thus leads to coenocytic condition.
● Well known examples are endosperm, fungi of phycomycetes group.
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Endomitosis ● This is duplication of chromosomes without division of nucleus.
● This leads to polyploidy i.e increase in the number of sets of chromosome in the nucleus.
★ Colchicine induces polyploidy in plants. It is a mitotic poison as it arrests the formation and arrangement of
spindle fibres.
★ Mustard gas Ribonucleases are also mitotic poisons.
Endoreduplication ● Here the chromonema replicate but do not get separated. THis leads to the formation of oversized
chromosome called as Polytene Chromosome.
● This process is also called as Polyteny.