MITOSIS and the
CELL CYCLE
Cell Cycle vs. Mitosis
• Cell cycle= life cycle of a cell; growth, division,
etc.
• Mitosis= one step of the cell cycle; division.
Why do our Cells Need to Divide?
• 1. cell survival- remember surface area to volume
ratio determines how quickly nutrients are able
to get through the cell.
• 2. growth- for an organism to grow we need new
cells!
• 3. repair- dead or damaged cells need to be
replaced
• 4. replenishment- some cells get used up when
doing their job, such as red blood cells, skin cells,
intestinal lining cells, etc.
How do our Cells Know when to
Divide?
• Proteins!
– Specifically hormones which send messages to
receptor proteins on our cells which tell them
that it is time to begin to divide.
The Cell Cycle
• The cell cycle is the fancy name for a cell’s life
cycle.
• It includes four (4) phases, one (1) of which is
mitosis.
• Cells will undergo this same cycle over and
over again.
Key terms
• Chromosome- structure made of
highly coiled or condensed DNA.
• Chromatin – the uncoiled form of
DNA.
• Sister chromatids – two identical
sections (copies) of DNA joined
together at the centromere.
• Pictures on next slide
Often confused terms
• 1. Single strand of
chromatin (DNA’s normal
structure in the nucleus)
• 2. Two sister chromatids
joined at centromere (After
replication)
• 3. Replicated or double
stranded chromosome
after condensation. Each
half is still considered a
sister chromatid (During the
cell cycle)
1
Gap1
2
3
Synthesis
Prophase
What is the cell cycle?
• All cells pass through a series of
events called the cell cycle.
• Some cells cycle continually, other
cell types only a limited number of
times before they die.
Cell Cycle
Only one
hour out of
22 is spent in
active division
(Mitosis)
Depending upon the type
of cell undergoing the
cycle.
Relationship of Stages
• Interphase
consists of:
• Gap 1 (G1)
• Synthesis (S)
• Gap 2 (G2)
• Mitosis consists
of:
• Prophase
• Metaphase
• Anaphase
• Telophase
The Four Stages of Cell Cycle
• 1. G1 phase – this phase has a main focus of
cell growth.
– Near the end of G1 the cell is checked to see that it
is large enough to survive its environment.
• 2. S phase – this phase has a main focus of
DNA replication (synthesis).
• 3. G2 phase – this phase prepares the cell for
mitosis.
– At the end of G2 the cell is checked to ensure that
the DNA has been copied properly before
proceeding w/ mitosis.
Interphase
• 1. Interphase is made up of G1, S, and G2
– To the naked eye it appears nothing is happening,
but that isn’t true!
• 2. DNA is in chromatin form (46 pieces)
• 3. Nucleolus is visible
– The nucleolus is part of the nucleus that is
involved in making ribosomes.
• 4. The cell is performing normal cellular
functions.
Interphase
• 5. DNA replicates later in this phase (during
the S portion)
• 6. After replication, DNA remains in chromatin
form, but made up of two chromatids (still 46
total pieces of chromatin, but double the
amount of actual DNA).
Mitosis
• Mitosis is the 4th and final stage of the cell
cycle.
• It, itself is divided into 5 steps (PMAT):
– Prophase
– Metaphase
– Anaphase
– Telophase
– Cytokinesis
Prophase
• Prophase is the first step of mitosis (cell division)…
• Early prophase is the first part of mitosis:
– 1. DNA starts to condense from chromatin to
chromosomes (so it can move!)
– 2. Nuclear membrane begins to disappear– DNA may still
appear to be in nuclear shape, though.
– 3. Nucleolus may or may not be there, but the cell is
slowly breaking it down.
– 4. centrioles begin to move to the opposite ends of the
cell.
• Centrioles are organelles that are found in animal cells that help
to move the “spindle-fibers” and control the movement of
chromosomes and nuclear material.
Prophase
• By the time Prophase is nearing the middle to
end of its time…
– 1. DNA is totally in chromosome form.
– 2. The nuclear membrane is complete gone –
chromosomes are no longer contained and look more
scattered.
– 3. Nucleoli are totally gone (no need for ribosomes)
– 4. Centrioles have moved to the opposite ends of the
cell and begin producing spindle fibers.
– 5. DNA is in chromosome form, made of 2 chromatids
(46 chromosomes, 92 chromatids)
Metaphase
• Metaphase is the second stage of mitosis
• During early metaphase…
– 1. spindle fibers reach out from the centrioles and
connect to the centromeres of the chromosomes to
begin moving them toward the center of the cell.
• Near the end of metaphase…
– 1. Chromosomes are lined up in the middle of the
cell by the spindle fibers.
– 2. At the end of the phase, DNA is still in
chromosome form and made of 2 chromatids (46
chromosomes, 92 chromatids).
Anaphase
• Anaphase is the 3rd stage of mitosis
• During early anaphase…
– 1. Chromosomes are pulled apart at the
centromeres causing the chromatids to be
separated.
– 2. At the end of this phase, DNA is still in
chromosome form, but it is no longer made of 2
chromatids, instead each chromatid is now an
individual chromosome (92 chromosomes, 0
chromatids)
Late Anaphase/Early Telophase
• Before telophase (the 4th stage of mitosis)
really gets going…
– 1. Chromosomes are clearly pulled apart from
each other, heading towards opposite ends of the
cell.
– 2. At this point in time we have 92 chromosomes,
46 going in one direction, 46 in the other.
Telophase
• The 4th stage of mitosis
• During telophase…
– 1. Chromosomes have reached the opposite ends of the
cell
– 2. A nuclear membrane has started to form around the
groups of DNA (creating a new nucleus for each new
cell)
– 3. The cell begins to pinch (animal cell) or a cell plate
begins to form.
• In animals this is called a cleavage furrow
– 4. DNA is starting to uncoil back into chromatin for use
by the new cells.
– 5. Nucleoli are being formed
Cytokinesis
• Occurs at the same time as Telophase
– 1. Cell completely pinches (animal cell) and
becomes 2 new cells. OR Cell plate completely
divides the cell in half (plant cell) to form a new
cell wall, thus dividing the cell into 2 new cells.
DNA has uncoiled back into 46 pieces of chromatin.
There would be 46 pieces in each new cell (just like
we started with!)
Finished Product
• The entire process is one cell dividing into 2
cells.
• The two new cells are genetically identical to
each other and the original cell (which is now
gone).