The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 28

microtubules. The polar microtubules overlap near the equatorial plane of the spin-
dle. The kinetochore microtubules attach to the kinetochore of chromosomes. The
kinetochore of a chromosme is formed at the centromere by the assembly of sev-
eral proteins and is partly responsible for chromosome movement. Cellular orga-
nelles like mitochondria are replicated by fission and passed on to the daughter cells.
Similarly, the Golgi apparatus is broken into smaller vesicles and partitioned into the
daughter cells during cell division.
Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase,
and telophase.
Prophase
The chromosomes are already replicated and consist of two sister chromatids joined
together at the centromere. The chromosomes start condensing, and the spindle fiber
formation initiates outside the nuclear region. The two replicated centrosomes move
away from each other and start the nucleation of microtubules. The spindle fibers
are rapidly growing and shrinking in prophase, and the polar spindles also come into
existence ( Fig. 1.1 A). In the late prophase, kinetochores assemble on each centro-
mere. Hence, there are two kinetochores on each chromosome.
Prometaphase
The phopshorylation of nuclear lamina occurs so that the nuclear envelope disinte-
grates. The kinetochores on the chromosomes start their interaction with the spindle
fibers because of access to chromosomes due to breakdown of the nuclear membrane.
The kinetochores on the chromosomes interact with the spindles and are captured
by them. The two sister chromatids are captured by the free ends of microtubules
from opposite poles ( Fig. 1.1 B). The kinetochores are pulled toward the pole, but
the chromosome arms are being pushed away by the rapidly nucleating and growing
spindles from the spindle pole; this is termed the astral exclusion force.
Metaphase
The chromosomes align in the equatorial plane of the spindle fibers and form the
metaphase plate. This constitutes a significant proportion of the mitosis phase dura-
tion. The opposite kinetochores are attached to spindles originating from the opposite
poles through kinetochore microtubules ( Fig. 1.1 C).
Anaphase
Anaphase witnesses an abrupt separation of chromatids toward the spindle poles. The
kinetochore on the chromosomes separate, and the kinetochore microtubules shorten,
pulling the chromatid toward the spindle pole it is facing ( Fig. 1.1 D). Anaphase is
also further distinguished as anaphase A and B. Anaphase A is characterized by the
shortening kinetochore microtubules, and in anaphase B movement the polar micro-
tubules elongate. This causes the two spindle poles to move farther apart. Anaphase
A and B contribute to the movement of chromatids toward the spindle pole.