Molecular
Genetics III
Purpose of
mitosis
Mitosis is the process of cell
division, and occurs only in "somatic" or body cells. When haploid sperm
meets haploid egg, a chain of events that begins with a single diploid cell and
ends with an adult organism made of billions of cells is set in motion. The
single cell divides into 2, and each of those 2 divide again, and this process
continues geometrically along the following progression: 1, 2, 4, 8, 16, 32,
64, 128, and so on into the billions. Therefore, the
first purpose of mitosis is growth. The second function of mitosis is repair.
Cells are constantly wearing out and getting damaged and unless an organism
replaces them at least as fast as they are lost, a gradual deterioration will
occur. Mitosis is also used as a means to reproduce asexually, as seen in the
vegetative reproduction in plants.
View
an animation of mitosis
http://www.cellsalive.com/mitosis.htm
The phases of
mitosis
Mitosis is a dynamic process;
there is a continuum of activity. Scientists have broken the process down to
four phases (much like our life cycle of infancy, childhood, adolescence…). The
stages are Prophase (“pro” means before); Metaphase (“meta” means middle);
Anaphase (“ana” means apart); and Telophase
(“telo” means distant). If followed by cytokinesis, an easy way to remember the order is
Please Make A Telephone Call.
Prophase
At the start of mitosis (prophase),
the chromosomes spiral and coil to produce short,
microscopically visible
structures. If the chromosomes did not condense, their long fibers would become
tangled during subsequent separation. Each chromosome is visible as a pair of
sister chromatids joined together at a region called
the centromere.
The nuclear envelope breaks down so there is no longer a
recognizable nucleus. Some mitotic spindle fibers elongate from the centrioles, and attach to kinetochores.
Other spindle fibers elongate but instead of attaching to chromosomes, overlap
each other at the cell centre. The nucleolus disappears during this phase as
well.
Metaphase
The replicated
chromosomes converge toward the centre of the spindle, and once they get
there, significant movements cease. At several points during metaphase, the chromatid arms may unwind from each other. This unwinding
is especially apparent late in metaphase, just 1 or 2 minutes before the chromatids will separate at their centromeres,
with each replicated chromosome giving rise to two daughter chromosomes.
The daughter chromosomes then begin to separate from each
other, each moving away from the metaphase plate and toward one of the two
spindle pole regions. The mechanisms
that control chromosome separation clearly involve the interactions between
microtubules and components in or near the kinetochore.
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 phase.
Summary
Prophase: The chromatin, diffuse in interphase,
condenses into chromosomes. Each chromosome has duplicated and now consists of
two sister chromatids. At the end of prophase, the
nuclear envelope breaks down into vesicles.
Metaphase: The chromosomes align at the equatorial plate and are held
in place by microtubules attached to the mitotic spindle and to part of the centromere.
Anaphase: The centromeres divide. Sister chromatids separate and move toward the corresponding
poles.
Telophase: Daughter chromosomes
arrive at the poles and the microtubules disappear. The condensed chromatin
expands and the nuclear envelope reappears.
Cytokinesis
The cytoplasm divides; the
cell membrane pinches inward ultimately producing two daughter cells.
There are significant
differences in mitosis between plant and animal cells. First, plant cells lack centrioles. Both types of cells, however, have a centrosome
or microtubule organizing centre (MTOC). During cytokinesis,
animal cells form a cleavage furrow, while plant cells form a cell plate.
