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Mitosis
What is Mitosis?
Mitosis is the division of the cell’s nucleus
Why Do Cells Need to Divide?
Cells want to divide for several reasons:
1) Large cells can’t control their functions as well
as small cells
-- as a cell gets bigger, it still has the same
amount of DNA to do the same jobs
2) Large cells can’t exchange materials as well
-- the ratio of surface area (cell membrane) to
cell volume decreases as a cell gets
bigger,
making it harder for food to get in
and wastes
to go out
3) As cells age, they aren’t as efficient
The Cell Cycle
Cells aren’t constantly dividing; instead, cells grow, live
their lives and then divide when necessary.
-- this process of growth and division is known as the
cell cycle
The cell cycle consists of 3 parts:
Interphase
Mitosis
Cytokinesis
Image of the Cell Cycle
Interphase
The cell cycle begins when a new daughter cell is formed
and immediately begins interphase. During interphase, the
cell undergoes the following life “phases”
G1 Phase
In G1, cells do most of their growing
Cells also synthesize new proteins and organelles in G1
S Phase
In S, cells replicate their DNA so that they have a copy for
each new daughter cell
Once S phase begins, it is ready for mitosis
Interphase (continued)
G2 Phase
In G2, the cell grows a little bit more
The cell also makes the organelles and molecules
required for mitosis while in G2
Once G2 ends, mitosis begins!
About Chromosomes
Only when the cell is about to begin mitosis (and during
mitosis) do chromosomes become visible.
Chromosomes consist of a chromatid (a strand of DNA)
with a condensed area called the centromere.
-- usually, centromeres are near the middle of a
chromatid
At the end of each chromosome is an area of repetitive DNA
called a telomere
DNA (and therefore chromosomes) were replicated during
the S phase of interphase, so each chromosome actually
consists of two “sister” chromatids, and looks like the
image on the next page
Humans have 23 pairs of chromosomes, for a grand total
of 46 chromosomes
More About Telomeres
When DNA is replicated (during the S phase of the cell
cycle), the enzyme DNA polymerase cannot go all the way to
the end of the strand.
The telomere is a non-coding region of repetitive DNA that
allows the DNA to replicate all the genetic info without
replicating the entire chromosome.
-- this also prevents chromosome fraying and
chromosomes from falling apart and attaching to each other
When a new strand of DNA is constructed, enzymes called
telomerases rebuild the telomeres to their proper length
Image of a Chromosome
Mitosis
As mentioned earlier, mitosis is divided into 4 parts. The
parts of mitosis are:
-- prophase
-- metaphase
-- anaphase
-- telophase
Prophase
Prophase is the longest
phase of mitosis
During prophase:
-- the centrioles line up on
opposite sides of the
nucleus
-- spindle fibers made of microtubules form, and are
organized by the centriole
-- the chromatin condenses into visible chromosomes
-- nucleolus disappears and nuclear envelope breaks
down
Metaphase
Metaphase only last for a few minutes
During metaphase:
-- the chromosomes line up across the center of the
cell
-- microtubules connect to the centromeres
Anaphase
During anaphase:
-- the centromeres that join sister chromatids separate
and the chromatids become individual chromosomes
-- the chromosomes move apart until they reach the
centrioles near the end of the spindle
-- anaphase ends when the centrioles stop moving
Telophase
During telophase:
-- the chromosomes begin to
tangle back into chromatin and
lose their visibility
-- a nuclear envelope begins
to re-form around each set
of chromosomes
-- the spindle breaks apart
-- nuceloli become visible in
each new nucleus
-- this is the end of mitosis!
Cytokinesis
Although cells have split up their genetic material and formed
new nuclei, the cells are not split.
Cytokinesis is the process by which cells split their
cytoplasm into two new daughter cells
In animals, cytokinesis happens when the cell membrane is
drawn in and the cell cleaves into two new cells
In plant, a structure made of wall-building vesicles, called a
cell plate forms between the two new nuclei, and eventually
becomes a dividing membrane
Full Diagram of Mitosis