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Mitosis
Department of AGB
Veterinary College
Bangalore
Mitotic Cell Division
Functions:
• Growth, maintenance, repair of body
tissues
• Forms the basis of
Asexual Reproduction
What is mitosis?
• Mitosis is nuclear division plus cytokinesis,
and produces two identical daughter cells
during prophase, prometaphase,
metaphase, anaphase, and telophase.
• Interphase is often included in discussions
of mitosis, but interphase is technically not
part of mitosis, but rather encompasses
stages G1, S, and G2 of the cell cycle.
Mitosis
• Somatic cells divide by mitosis – Involves 1 cell cycle / division
• Parental and 2 daughter
cells are genetically identical
• Parental cells are diploid
(46 chromosomes)
• 2 daughter cells are diploid
(46 chromosomes)
Interphase
The cell is engaged in metabolic activity and
prepare for mitosis (the next four phases
that lead up to and include nuclear
division).
Chromosomes are not clearly discerned in
the nucleus, although a dark spot called
the nucleolus may be visible.
Mitosis
4 phases:
1st – Prophase
2nd – Metaphase
3rd – Anaphase
4th – Telophase
and Cytokinesis
Prophase
i) chromosomes condense
• 3 major events
ii) spindle fibers form
iii) chromosomes are captured by
spindle
Chromosomes Condense
• Each chromosome consists
of 2 sister chromatids
attached to each other at the
centromere
Mitotic Spindle Forms
• Spindle fibers are specialized
microtubules
• Spindle fibers radiate out from
centrioles, forming the “aster”
• Centrioles occur in pairs, and are
duplicated during interphase
• One pair of centrioles migrates to
one pole of cell, the other pair
migrates to opposite pole of cell
chromatin
nucleolus
nucleus
centrioles
condensing
chromosomes
Spindle Captures Chromosomes
• When spindle fibers are fully formed
nuclear envelope disintegrates and
nucleolus disappears
• Spindle fibers attach to chromosomes
at the kinetochore, a structure located
at the centromere
• Function of spindle fibers is to
organize division of sister chromatids
into daughter cells
chromatin
nucleolus
nucleus
centrioles
condensing
chromosomes
Prometaphase
• The nuclear membrane dissolves, marking
the beginning of prometaphase. Proteins
attach to the centromeres creating the
kinetochores. Microtubules attach at the
kinetochores and the chromosomes begin
moving.
Metaphase
• Chromosomes align along
equator of the cell, with one
kinetochore facing each pole
centrioles
spindle fibers
chromosomes
•Spindle fibers align the chromosomes along the
middle of the cell nucleus.
•This line is referred to as the metaphase plate.
•This organization helps to ensure that in the next
phase, when the chromosomes are separated, each
new nucleus will receive one copy of each
chromosome.
Metaphase
Anaphase
• Sister chromatids separate
• Spindle fibers attached to
kinetochores shorten and pull
chromatids poleward
• Free spindle fibers lengthen and push
poles of cell apart
free spindle fibers
V-shaped chromatid
Anaphase
Anaphase
• The paired chromosomes separate at the kinetochores and move to opposite sides of the cell.
• Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.
Telophase
• Spindle fibers disintegrate
• Nuclear envelopes form around both
groups of chromosomes
• Chromosomes revert to their extended
state
• Nucleoli reappear
Telophase
Chromatids arrive at opposite poles of cell, and
new membranes form around the daughter
nuclei.
The chromosomes disperse and are no longer
visible under the light microscope.
The spindle fibers disperse, and cytokinesis or the
partitioning of the cell may also begin during this
stage.
• Cytokinesis occurs, enclosing each
daughter nucleus into a separate cell
chromosomes
decondensing
nuclear envelope
reforming
nucleolus reappears
pinching of cell
membrane at equator
cytokinesis
Cytokinesis
In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus.
In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.
Cytokinesis
• Animal cells:
– microfilaments attached to plasma
membrane form a ring around
equator of cell
– ring contracts, like a drawstring,
dividing the cytoplasm