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AP Biology
The Cell Cycle
Part 2
Important concepts from previous units:
1) Centrioles are components of the
cytoskeleton that are composed of
microtubules. (Pg. 114)
2) Microtubules also compose the spindle
fibers of mitosis and meiosis. (pg. 113)
3) Proteins are the work horses of cells. They
help move things within cells.
The Cell Cycle Phases for Eukaryotic Cells are:
Cells spend 90% of there existence in this phase.
This phase consists of three parts:
G1 (Primary or “first” growth)
This is ordinary, everyday growth, activity, or repair of
the cell.
Organelles begin replicating.
First checkpoint (called “point of no return”) is the
barrier to the rest of the cycle.
S (synthesis)
The DNA replicates or is synthesized during this phase.
In humans, we go from 46 Chromosomes “2n” to 92
chromosomes “4n”. (make correction!!!!!!)
G2 (Secondary or “second” growth)
The organelles mainly enlarge or complete replication.
The newly synthesized DNA is checked for errors.
Second checkpoint occurs after this “part”. (Do we have
everything for TWO cells? If yes, then proceed to
dividing; if no, then make what is missing.)
• Human cells NEVER normally contain 92
– A diploid human cell prior to replicating its DNA has
23 pairs of non-replicated chromosomes (46 total).
– A diploid human cell after replicating its DNA has 23
pairs of replicated chromosomes (46 total).
– The difference is that each replicated chromosome
consists of two DNA molecules called 'sister
– Non-replicated chromosomes consist of only one DNA
• (it is not called a chromatid because chromatids are like
twins--if you only have one you don't say there is one twin.
– Thus, you can say that a human cell following DNA
replication has 92 chromatids, but never 92
Before and after the S phase
Interphase cell
(Look at the chromatin in the blue nucleus and the
yellow cytoskeleton.)
Mitosis - means “nucleus division” (First divide
the DNA; then secondly the cytoplasm.)
This process has four parts:
Prophase (“pro” means “first”)
Nuclear envelope is broken down and
rearranged to make the spindle apparatus.
The chromatin condenses to form “X” shaped
chromosomes. (Two chromatids)
Centrioles move toward the poles. (In animal
cells only…plants use the cell wall.)
Cell in Prophase
Start of Mitosis
Metaphase (“meta” means “middle”)
The replicated chromosomes line up on the
metaphase plate. (Middle of cell.)
The spindle apparatus attaches to the
kinetochore (a part of the centromere) AND
centrioles (the anchors).
Third checkpoint occurs here. (Are all the
chromosomes attached and lined up and ready
to “divide/separate” or “segregate”?)
Cell in Metaphase
Anaphase (“ana” means “separate”)
Replicated chromosomes are pulled apart into sister
chromatids and each chromatid moves toward opposite
poles (ends) of the cell.
The spindle apparatus is being broken down as the two
sister chromatids are “walked” toward the poles by the
motor protein using ATP.
Cell in Anaphase
Telophase (“telo” means “last”)
The nuclear envelope is rebuilt by using broken down
spindle apparatus pieces.
The Chromatids begin to decondense back to their
chromatin state.
A cleavage furrow begins to form using actin and
myosin microfilaments.
Cell in Telophase and starting
“Division of the nucleus”
– Cytokinesis (Cleavage means “split”) This is the
division of the cytoplasm.
– The cytoplasm and cell organelles are separated to produce
two daughter cells.
– G0 (Zero growth phase)
– The cells are tired and take a brief break and rest or they
stop adult development.
One cell becoming two
Spindle Apparatus
These structures are formed from the broken down of
the cytoskeleton and nuclear envelope. (recycled)
The construction starts at the centrosome (where the
centrioles are) and works toward the chromosomes.
They attach to the kinetochore on the centromere of
the replicated chromosomes.
Motor Protein “walks” the sister chromatids toward
the opposite poles (ends) using ATP by
Non-kinetochore spindles are used to “push” the poles
farther apart to help produce the cleavage furrow.
Motor Protein and the
Spindle Fibers
Cell Plate
Remember Plant cells DO NOT have centrioles
because the have cell walls to anchor to.
The NEW cell wall “Plate” develops, using small
segments of cellulose, instead of a cleavage
Animal vs. Plant
Regulation “control” of the Cell Cycle.
Regulation is crucial for normal growth and development.
Regulation varies for each different type of cell.
The regulation is controlled by protein molecules called
Cyclins. (They control the cell CYCLE.)
Three checkpoints exist: (Checkpoints are “stopping points to
make sure everything is correct before going on to the next
First – It is at the end of G1. (Called the Restriction point.) “point
of no return”
Second – It is at the End of G2. (Do we have 2 sets of DNA and 2
sets of organelles?)
Third – It is at the End of Metaphase. (Are all the replicated
chromosomes in the middle of the cell and are they ALL attached
to the spindle fibers?)
(Is all going according to plan?)
What is the difference between a theta
chromosome and Eukaryotic chromosomes:
More genes are present on Eukaryotic chromosomes
than on theta chromosomes.
There exists more genetic stability in eukaryotic
This means there exists more genetic variation exists in Eukaryotic
There also exist more possible genetic combinations that can be
Offspring receive the same number of chromosomes most of the
Important linked genes tend to be inherited together.
Allows for diploid cells to exist, as a result of sexual
reproduction and one half of the DNA to come from
each parent.
Also increases variation among a species.
LE 12-10
Prophase. The
chromatin is condensing.
The nucleolus is
beginning to disappear.
Although not yet visible
in the micrograph, the
mitotic spindle is starting
to form.
Prometaphase. We
now see discrete
chromosomes; each
consists of two identical
sister chromatids. Later
in prometaphase, the
nuclear envelope will
Cell plate
Metaphase. The spindle is
complete, and the
chromosomes, attached
to microtubules at their
kinetochores, are all at
the metaphase plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends of
the cell as their
kinetochore microtubules shorten.
10 µm
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will divide
the cytoplasm in two, is
growing toward the
perimeter of the parent
Microscopic view of Mitosis in Onion root tips.
Can you identify the stages?
Onion root tip mitosis