Transcript document
Lecture delivered at KMC IC
On 6th October 2011
Anaphase
Telophase
Cytokinesis
Functions of non kinetochore microtubules.
Binary Fission
The Evolution of Mitosis
Cancer cells
Significance of Mitosis
Aanaphase is the shortest stage of mitosis
lasting only a few minutes.
It begins when two sister chromatids of
each pair suddenly part..
Centromere divides and chromatids move
to their respective poles
Each chromatid appears as V-shaped
structure.
The two liberated chromosomes begin
moving toward opposite ends of the cell as
their kinetochore microtubules shorten.
Because these microtubules are attached to
centromere region, the chromosomes move
centromere first.
The cell elongates as nonkinetochore
microtubules lengthen.
By the end of anaphase, the two end of the
cells have equivalent and complete
collections of chromosomes
Anaphase commences suddenly when
proteins holding together the sister
chromatids of each chromosome are
inactivated.
Once the chromatids become separate, full
fledged chromosomes, they move toward
opposite ends of the cell.
Possibility one: Chromosomes are “reeled
in” by microtubules that are shortening at
the spindle poles.
Experimental evidence supported
hypothesis: Primary mechanism of
movement involves motor proteins on the
kinetochore that “walk” a chromosome
along the attached microtubules toward the
nearest pole.
Meanwhile, the microtubules shorten by
depolymerizing at their kinetochore ends.
Shortest phase
Two daughter nuclei begin to form in the
cell.
Nuclear envelopes arise from the fragments
of the parent cell’s nuclear envelope and
other portions of the endomembrane cell.
The chromosomes become less condensed.
The chromosomes are at the poles in
telophase and become less distinguishable
as individual strands
It is as long as prophase and is
characterised by reappearance of nuclear
membrane and nucleolus
It is some sort of a reversal of prophase
Mitosis, the division of nucleus into two
genetically identical nuclei, is now
complete.
It is the division of the cytoplasm which begins
in late anaphase.
The division of cytoplasm is usually well
underway by late Telophase, so the two
daughter cells appear shortly after the end of
mitosis.
A contractile belt is formed around the equator
which contracts more and more finally
separating a cell into two daughter cells
In animal cells, cytokinesis involves the
formation of a cleavage furrow which pinches
the cell into two.
In animal cells, cytokinesis occurs by a
process known as cleavage.
First sign of cleavage is the appearance of
cleavage furrow- a shallow groove in the
cell surface near the old metaphase plate.
On the cytoplasmic side, the furrow is a
contractile ring of actin microfilaments
associated with molecules of the protein
myosin.
The actin microfilaments interact with
myosin molecules causing the ring to
contract.
The cleavage furrow deepens until the
parent cell divides into two complete cells,
each with its own nucleus and share of
cytosol and organelles.
Cytokinesis in plant cells, which have cell
walls is markedly different.
There is no cleavage furrow. Instead, during
telophase, vesicles derived from the Golgi
apparatus move along microtubules to the
middle of the cell, where they coalesce,
producing a cell plate.
Cell wall materials carried in the vesicles
collect in the cell plate as it grows.
The cell plate enlarges until its surrounding
membranes fuses with the plasma
membrane along the perimeter of the cell.
Two daughter cells result each with its own
plasma membrane. Meanwhile, a new cell
wall arising from the contents of the cell
plates has formed between the daughter
cells.
In an animal cell cytokinesis occurs by
cleavage, which divides the parent cell in
two with a contractile ring of actin
filaments.
In a plant cell , a cell plate forms in the
middle of the cell and grows until its
membrane fuses with the plasma
membrane of the parent cell.
A new cell wall is also produced from cell
plate resulting in 2 daughter cell.
In a dividing animal cell these microtubules
are responsible for elongating the whole cell
during anaphase.
Non kinetochore microtubules overlap
extensively during metaphase.
During anaphase the region of overlap is
reduced as motor protein attached to
microtubules.
As the microtubules push apart from each
other, their spindle poles are pushed apart,
elongating the cell.
At the end of anaphase duplicate groups of
chromosomes have arrived at opposite ends
of the elongated parent cell.
Nuclei re-form during telophase.
Cytokinesis generally begins during these
later stages of mitosis and the spindle
eventually disassembles.
At metaphase, the centromeres of all
duplicated chromosomes are on a plane
between the spindle’s two poles. This
imaginary plane is called the metaphase
plate of the cell.
Prokaryotes reproduce by a type of cell division
called binary fission.(meaning literally: division
in half)
Most bacterial genes are carried on a single
bacterial chromosome that consists of a circular
DNA molecule and associated proteins.
The problem of replicating genomes in an
orderly fashion and distributing the copies
equally to two daughter cells is still formidable.
Eg. Chromosome of bacterium Escherichia coli
when fully stretched is 500 times longer than
the cell.
In E. coli the process of cell division begins
when the DNA of the bacterial chromosome
begins to replicate at a specific place on the
chromosome called origin of replication
producing two origins.
As the chromosomes continue to replicate,
one origin moves rapidly toward the
opposite end of the cell.
While the chromosome is replicating, the
cell elongates.
When replication is complete and the
bacterium has reached about twice its initial
size the plasma membrane grows inward
dividing the parent E. coli cell into two
daughter cells.
Each cell inherits a complete genome.
Hypothesis: mitosis had its origin in simpler
bacterial mechanisms of cell reproduction.
Evidences: prokaryotes preceded
eukaryotes on earth by billions of years.
Some of the proteins involved in bacterial
binary fission are related to eukaryotic
proteins.
Recent research shows two of the proteins
involved in binary fission are related to
eukaryotic tubulin and actin proteins.
As eukaryotes evolved, along with their larger
genomes and nuclear envelopes, the ancestral
process of binary fission somehow gave rise to
mitosis- hypothesis for step wise evolution of
mitosis.
Two examples of nuclear division where
ancestral mechanisms have remained
relatively unchanged over evolutionary time:
1. Dinoflagellates- replicated chromosomes are
attached to nuclear envelope and separate as
the nucleus elongates prior to cell division.
2. Diatoms- a spindle within the nucleus
separates the chromosomes. In most
eukaryotic cells, the nuclear envelope
breaks down and a spindle separates the
chromosomes.
In both cases the nuclear envelope remains
intact.
Cancer cells elude normal regulations and
divide out of control, forming tumors.
Malignant tumors invade surrounding
tissues and can metasize, exporting cancer
cells to other parts of the body where they
may form secondary tumors.
The two daughter cells formed by mitosis
possess the same number of chromosomes
as the parent cell.
Due to mitosis, all cells of living body
possess equal number of chromosomes.
As mitosis takes place in somatic cells (all
body cells except the reproductive cells) , it
has significance of growth in the body.