MOLECULAR OVERVIEW OF MITOSIS
- nuclear division leading to equal sets of
chromosomes in the daughter nuclei. Characteristic
of somatic cells.
- nuclear division leading to a reduction in
chromosome number of the parent cell to produce hapliod
In a nutshell: As we mentioned above,
the extended, loosely-coiled structure of chromosomes that is
required for gene expression is incompatible with an orderly
division of chromosomes between daughter cells. Therefore,
the chromosomes coil into dense rods that tend to retain stains,
which makes them visible in light microscopy. When we can see the
chromosomes, we call it prophase. The nuclear envelope is also
disassembled during prophase. In metaphase, the chromosomes,
attached to the spindle fibers, are pulled to the center of the
cell in a kind of 'tug of war' between centrosomes on opposite
poles of the cell. During anaphase, chromosomes are pulled along
the spindle fibers toward the poles. Finally, in telophase, the
nuclear envelope reassembles, the chromosomes decondense, and the
nucleus reorganizes, allowing normal cellular functions to resume.
Whitefish Mitosis -- Review
It is important to remember that interphase is defined
by what we can see under the microscope with
conventional staining techniques. While it may
look like nothing is happening, we have already seen
that the term "Interphase"
includes G 1, S and G 2 phases
of the cell cycle.
Onion root tip cells in interphase, stained with aceto
carmine. Nuclei stain red, nucleoli appear white.
Image from Yaping Wang and Brian Fristensky,
University of Manitoba.
Organization of spindle
fibers prior to mitosis
|Centrosomes are the site of
spindle fiber organization, and thus provide a polarity
to the dividing cell. They also serve as
"basal bodies" in cells with flagellae. In animal cells,
discrete centrioles can be seen in centrosomes .
Centrioles are not visible in plant cells.
|Spindle fibers are microtubules
which originate at the centrosomes. Microtubules are
tubes formed from polymers of alternating α- and
β-tubulin heterodimers. As illustrated above, bundles of
microtubules also form the centrioles. Centrosomes are
also referred to as "microtubule organizing centers"
Displayed from http://www.cryst.bbk.ac.uk/pps97/course/section11/mtube3.gif
The role of centrosomes in the
cell cycle are illustrated in the figure.
(from http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.figgrp.2472 ) Prior to the visible onset of
prophase, the following steps occur:
1. Centrosome replication preceeds the onset
of DNA synthesis
double in size. A substantial amount of the proteins in
centrosomes appear to be tubulins.
2. DNA synthesis occurs. This is the S phase
of the cell cycle
Note that the
significance of this point is that by the time you can see the
chromosomes at prophase, the DNA has already replicated. (ie.
S phase of cell cycle) DNA has already replicated (2n ->
3. Centrosomes divide and begin migration to
As the centrosomes
migrate to opposite poles, they play out polar spindle fibers,
such that fibers from one pole are interdigitated with fibers
from the opposite pole. These polar fibers will be used in
anaphase B to push the poles of daughter cells in opposite
To view a GIF animation file
(137K) illustrating the centrosomes in the cell cycle
click here. Centrosomes
are shown in black, chromosomes in brown and orange, and
spindle fibers in green. [taken from http://www.stanford.edu/group/Urchin/mitosis.html]
initiation of chromatin condensation coincides with the
disassembly of the nuclear membrane.
|Prophase in onion
root tips with aceto-carmine staining. A - C show the
progression of chromosome condensation. Note that as
chromosomes condense, they become thicker and less
Images from Yaping
Wang and Brian Fristensky, University of Manitoba.
2. Switch in microtubule
stability accompanies formation of spindle
Spindle fibers extend
randomly in all directions. The length of the
fibers varies due to polymerization and
depolymerization of the microtubule from tubulin dimers.
Polymerization occurs at both ends, but is relatively
slow at the (-) end, which is anchored at the
centrosome. The (+) end grows more rapidly, as it
extends into the cytoplasm. However, when a microtubule
encounters the kinetochore at the centromere of
a chromosome, it's (+) end is stabilized
by the kinetochore. The kinetochore is a protein matrix,
consisting of proteins that attach to centromeric DNA
sequences (inner layer) and proteins that attach to the
spindle fibers (outer layer).
The kinetochore is a
protein/DNA complex which forms at the centromere. It is
the site of attachment of kinetochore microtubules to
the mitotic chromosome.
fibers are shown in green. DNA in chromosomes is
counterstained in blue flurorscence.
3. By end
of prophase, sister chromatids have separated but
remain joined at centromeres.
Despite the fact that DNA
replication occurs prior to prophase, the two sister
chromatids cannot be distinguished microscopically
during early prophase. This is referred to as relational
As the chromosomes condense during proplase, the
relational coils are unwound, and each chromatid coils
tightly with itself. Relational coiling disappears
as prophase continues and the chromatids disengage to
lie side by side.
|In metaphase the chromosomes are
fully condensed and arranged in a plane equidistant from
the poles of the spindle. This is the highest level of
coiling, and the chromosomes are shorter and thicker
than any other stage and therefore ideal for cytogenetic
study. There is no longer much relational coiling
present, and the chromatids lie side by side, not
twisted around each other.
onion root tips.
Yaping Wang and Brian Fristensky, University of
Experimental evidence indicates
that centrosomes appear to pull the spindle fibers taut, in a
kind of "tug of war". Thus, the chromosomes migrate toward
equator of the cell and become aligned.