CHAPTER 6: MITOSIS
M
itosis is a form of asexual reproduction that replicates and transfers DNA from a parent cell
to its two daughter cells. An integral part of the cellular life cycle, this process allows the
replacement of destroyed cells, the regeneration of damaged organs, and the growth of an organism
as a whole. While the exact nuances vary depending on the type of animal, this particular text is
concerned with mitosis in mammals. This pathway has seven stages: interphase, prophase,
prometaphase, metaphase, anaphase, telophase, and cytokinesis.
INTERPHASE
Interphase is the period during which a cell metabolizes nutrients and executes its normal
functions, as determined by the encoding of its DNA. While not technically a part of mitosis, it is
necessary to set up the cellular machinery used during division. As shown in Figure 1, this phase
encapsulates a majority of a cell’s life and can be broken down into three steps:
1. G1: A period of growth where proteins are synthesized and organelles such as vacuoles
or ribosomes are produced.
2. S: DNA is replicated and repaired in preparation for division, resulting in twice the
normal amount of genetic material. The enzyme DNA polymerase performs this task
using free nucleotides in accordance with the complementary base pair rule.
3. G2: The final period of growth that occurs right before division. During this time, the
cell checks to ensure everything is replicated without error.
Figure 1. Entire life cycle of a
mammalian cell
PROPHASE
Prophase is the first stage of mitosis and prepares DNA
for transfer. At this time, chromatin, a complex of DNA,
proteins, and RNA, condenses. In its natural state, DNA
is very diffuse in order to faciliate transcription.
Condensing these strands not only allows for their
efficient transport, but also prevents any damage to the
nucleotides from shear forces. During this time, the
microtubule organizing centers, centrosomes, begin
moving to the extreme ends of the cell in preparation for
the later stages. This is depicted in Figure 2.
1
2
Figure 2. Condensed chromatin (1) and
centrosomes (2) in early prophase
PROMETAPHASE
During prometaphase, the nuclear envelope begins to
break down allowing microtubules to form a network that
guides chromosomal movement, the mitotic spindle.
Two different types of microtubules, made in the
centrosomes, make up this construct:
1. Kinetochore microtubules: These attach to a
protein on the centromere of each sister
chromatid, the kinetochore, and will be
responsible for pulling it to the end of the cell.
This can be seen in Figures 3 and 4.
2. Polar microtubules: These interact with
corresponding polar microtubules from the
opposite centrosome, forming an arrangement
that steers chromosomes along a set path.
This process is very dynamic, with microtubules shooting
out in every direction from the centrosomes in search of
complementary kinetochores, akin to a fisherman
repeatedly casting out his line. The attachment of these
microtubules begins moving the chromosomes to the
center of the cell.
Figure 3. Kinetochore microtubules attaching
to kinetochores during prometaphase
Figure 4. Kinetochore tubules and
centromere depicted on a pair of sister
chromatids
METAPHASE
In metaphase, the chromosomes are lined up end-to-end
equidistant from the two poles, on a plane otherwise
known as the metaphase plate (Figure 5). Additionally,
during this stage, the two kinetochores per chromosome
are attached to microtubules from the opposite poles. At
the end, the cell checks to ensure all chromatids are
appropriately oriented and secured to the mitotic spindle.
This is done to ensure an even split of chromosomes
between both daughter cells. If there are any defects or
faulty attachments, division is stopped until the issue is
resolved. Due to its importance, metaphase is often the
longest stage.
1
Figure 5. Chromosomes lined along
hypothetical metaphase plate (1)
ANAPHASE
In anaphase, the sister chromatids finally separate,
becoming daughter chromosomes, and are pulled to
opposite ends of the cell. To facilitate this, the protein
that holds them together at the centromere, cohesin,
degrades. While this occurs, the kinetochore microtubules
begin to get shorter, pulling the daughter chromosomes
to the poles. The resultant V-shape seen in Figure 6
shows the trailing ends of the molecule as it is pulled at its
center. At the same time, the polar microtubules lengthen,
separating the poles and elongating the cell.
Figure 6. Sister chromatids being pulled
apart and moved to the opposite ends of the
cell
TELOPHASE
During telophase, internal structures begin to reform in
each of the two daughter cells. First, a nuclear membrane
begins to form around each set of chromosomes, forming
two new nucleoli. This is done to separate the nuclear
DNA from the cytoplasm. Additionally, the mitotic
spindle decomposes into its monomeric subunits. Finally,
the chromosomes begin to decondense (Figure 7),
becoming no longer viewable under a light microscope.
This stage of mitosis takes roughly ten minutes and at its
end results in two identical sets of parent chromosomes
in each newly formed nucleus.
Figure 7. Decondensed chromosomes at each
end of the cell
CYTOKINESIS
The final stage of mitosis, cytokinesis, results in the
separation of the cytoplasm to ultimately create two
daughter cells, seen in Figure 8. To accomplish this, an
actin ring forms around the middle of the cell-cell
complex at the metaphase plate, creating a cleavage
furrow. The ring slowly begins to contract, pinching until
two completely separate cells form. The end result is two
genetically identical daughter cells that will now enter
interphase, and possibly undergo mitosis at some point in
the future.
Figure 8. Formation of cleavage furrow and
complete separation of cytoplasm
The entire process of mitosis takes roughly one hour, yielding two daughter cells with the same exact
genome. First, during interphase, the cell performs its designated function while duplicating its
genetic material during the S stage. When the cell is ready to divide, it enters prophase where
chromatin condenses and a guiding mitotic spindle forms. Next, in prometaphase, the kinetochore
microtubules attach to the chromosomes, beginning to orient them along the metaphase plate.
During metaphase, the sister chromatids are precisely aligned along this hypothetical plane. Their
actual separation occurs during anaphase, with the mitotic spindle pulling each newly formed
daughter chromosome towards the ends of the cell. In telophase, the nucleus begins to reform
around the nuclear DNA with final separation of the two daughter cells occurring during
cytokinesis.
Figure 1 courtesy of http://english.eagetutor.com/content/cell-reproduction-sp-1347352237
Figures 2, 3, 5, 6, 7 courtesy of https://study-biology.wikispaces.com/Mitosis
Figure 4 courtesy of http://www.counterbalance.org/media/chrom-body.html
Figure 8 courtesy of https://sites.google.com/site/mrsebiology97/cell-reproduction/3-objective-3juno-activities-mitosis