Results of mitosis
Mitosis followed by cytokinesis results in two identical daughter cells.
Without cytokinesis, mitosis multiplies the number of nuclei in each cell. This
process occurs in the formation of human muscle cells that are very large cells
containing numerous nuclei.
The triggering mechanism for cell division varies among different cell types
and tissues, but two common mechanisms deserve consideration. One is the size
of the cell. As the cell churns out proteins and products, its volume
increases. Since volume increases more quickly than surface area, the surface
area to volume (SA:V) ratio declines. Mitosis regains a large SA:V ratio
when two daughter cells are produced from the parent cell. A larger volume is
also more difficult for one nucleus to control, so mitosis helps to maintain an
optimal cytoplasmic volume to nucleus ratio.
interactive video of mitosis
Mistakes in mitosis
There are several "checkpoints" throughout the cell cycle to ensure
that a phase does not occur prematurely or incorrectly. There are times and
conditions, however, when cell division does not proceed smoothly.
Sometimes the microtubules controlling the movement of chromosomes do not
function properly. In this case, the chromosomes will not separate cleanly and
the daughter cells will not receive the correct number and/or types of
chromosomes. This condition is known as non-disjunction. It usually affects
only one duplicated chromosome that ends up with both chromatids in only one
daugher cell. In human somatic cells, non-disjunction may result in a daughter
cell containing 47 chromosomes, while the sister cell contains 45.
Mutations in cell cycle regulators are implicated in cancers where cell growth
becomes disorganized and uncontrollable. Some mutations affect promoters of
cell division (accelerating the process), while others affect inhibitors of
cell division (taking the brakes off). Either way, cell division proliferates
and results in atypical daughter cells. These daughter cells are usually
undifferentiated. This means that they take up a lot of nutrients but
contribute nothing to the well-being of the organism. What is cancer?
video of mitotic
non-disjunction in animal cells
Meiosis
Meiosis is a cell division process that results in a reduction in ploidy. To
this end, only cells that have an even ploidy may undergo meiosis.
Meiosis consists of two cycles of division but only one cycle of DNA
replication (this occurred earlier in the S phase of interphase). In the
first division, (Meiosis I or MI), the homologous pairs of chromosomes are
separated. A good way to envision this is to think of meiosis in human
spermatocytes: The X and Y chromosomes MUST be segregated, and this principle
applies to the other 22 pairs of homologues. In the second cycle of
division, the duplicated chromosomes (now numbering 22 + X in one cell, and 22
+ Y in the other) separate at the centromere and create two new daughter cells.
This second division will result in the formation of four daughter cells each
containing half the number of chromosomes as the parent cell.
videos of meiosis I and II
Phases of Meiosis
Once you understand the objectives of the two cell divisions in meiosis, you
can begin to look at the detailed phases in each cycle. The terms used to
describe the phases are the same as we use in mitosis but we must distinguish
between the two cycles. So the first phase in the first meiotic division is
called prophase I; the first phase in the second meiotic division is called
prophase II and so on.
summary of meiosis
meiosis with crossing over
video
