Results in the formation of two identical cells, consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis)
Process visible under microscope
Nuclear Division (Mitosis) has four stages
i.
Prophase:
chromatin fibres condense and shorten into chromosomes preventing entangling of DNA; each
prophase chromosomes consists of a pair of identical strands called chromatids; a region called the
centromere
holds the chromatids together; at the outside of the chromatid is the
kinetochore,
a protein
complex; a
mitotic spindle
is formed by tubules in pericentriolar material of the centrosomes and connects
to the kinetochore of the the chromatids; as the microtubules lengthen the push the centrosomes to the poles
of the cell making the spindle extend from pole to pole of the cell thus the spindle is responsible for
separating chromatids to opposite poles; at this point the nucleus disappears and the nuclear envelope
breaks down
ii.
Metaphase:
microtubules of the spindle align the centromeres of the chromatid pairs at the exact centre of
the spindle; the midpoint region is known as the
metaphase plate
iii.
Anaphase:
centromeres split; separated chromatid pairs move towards poles of cell; chromatids now
separated and referred to as chromosomes pull the microtubules of the spindle causing them to appear V-
shaped as the centromeres lead the way
iv.
Telophase:
begins after chromosomes stop moving; the chromosomes uncoil and revert to threadlike
chromatin form; nuclear envelope forms around each chromatin mass; the nucleoli reappear in the identical
nuclei and the mitotic spindle breaks up
Cytoplasmic Division (Cytokinesis)
o
Begins during anaphase with the formation of a
cleavage furrow,
a slight indentation of the plasma membrane
o
It is completed after telophase
o
Actin microfilaments in the plasma membrane form a contractile ring that pulls the membrane inward and
ultimately pinches it in two
o
The furrow is always perpendicular to the mitotic spindle ensuring the chromosomes end up in separate cells
o
When cytokinesis is complete interphase begins
Control of Cell Destiny
Cell has 3 possible destinies:
1.
Remain alive and function without dividing
2.
Grow and divide
3.
Die
Homeostasis is maintained when these destinies are balanced
Within cells there are enzymes called
cyclin-dependent protein kinases
that transfer a phosphate group from ATP to a protein to
activate the protein while other enzymes remove the phosphate group to deactivate it
-
this activation and de-activation at the appropriate time are crucial to cell division
Cellular proteins called
cyclins
are responsible for switching on and off the cdk, cyclin is important for determining the timing and
sequence of events
Necrosis
is a pathological type of cell death that results from tissue injury
Reproductive Cell Division
The mechanism that produces gametes and consists of a special two-step division called
meiosis
in which a number of chromosomes
in the nucleus is reduced by half
Gametes contain a single set of 23 chromosomes and thus are
haploid cells,
only fertilization restores the diploid number of
chromosomes
Meiosis occurs in two successive stages:
meiosis I
and
