As we discussed in
cell cycle, before
cells are allowed to enter M phase they must meet certain cellular
requirements. Among these requirements are appropriate cell size and cellular
environment. Following
DNA
replication
in
S phase, cells contain twice their normal number of
chromosomes. Because cells
that undergo mitosis are diploid, their
number of chromosomes can be represented as 2
N, where
N equals the
number of distinct chromosomes in the cell. Cells about to enter M phase, which
have passed through S phase and replicated their DNA, have 4
N
chromosomes. Entry into M phase is allowed by the formation of the mitotic
cyclin-Cdk complex known as M phase-promoting factor that occurs as
a
cell cycle regulatory mechanism in the G2
phase.
The first phase of mitosis within M phase is called prophase. It follows
G2, the final phase of
interphase. A cell entering M phase
manifests a number of physicsl signs. Among these are condensation, or
thickening, of chromosomes. Chromosome condensation is visible through a
microscope and is required for subsequent chromosome separation during later
stages of mitosis. Another physical characteristic of cells beginning mitosis
is the sprouting of
microtubules from replicated
centrosomes. Microtubles are
protein filaments on which chromosomes migrate during mitosis.
Prophase
Figure 1.1: Prophase
As we discussed, prophase is marked by very thick and dense chromosomes. At
this phase, the chromosomes are still enclosed in the cell
nucleus within the nuclear
envelope. The chromosomes also contain a
centromere, which is
necessary in later phases for attachment to microtubules for migration. Late in
prophase, kinetochores assemble on the centromeres. Specialized microtubules,
called kinetochore microtubules later attach to these sites.
Duplicated centrosomes, which are the organizing centers of microtubules, begin
to separate towards opposite poles of the cell. The network of cytoskeletal
components begins to break down and the mitotic spindle forms. The mitotic
spindle is an arrangement of microtubules that is responsible for aligning
duplicated chromosomes in later phases.
Prometaphase
Figure 1.2: Prometaphase
The major event marking a cell's entry to prometaphase is the breakdown of
the nuclear envelope into small vesicles. Kinetochores also become fully
matured on the centromeres of the chromosomes. The disruption of the nuclear
envelope allows for the mitotic spindles to gain access to the mature
kinetochores. As the microtubles of the mitotic spindle enter the nuclear
region, some attach to the kinetochores making them kinetochore microtubules.
The remaining microtubules are called non-kinetochore microtubules. Sister
chromatids are captured by microtubules
stemming from centrosomes on opposite ends of the cell. Once they have captured
chromosomes, the kinetochore mictrotubles begin to exert force on the
chromosomes, moving them.
