1. AP Biology The Cell Cycle Part 2
Important concepts from previous units:
1) Centrioles are components of the cytoskeleton that are composed of microtubules.
2) Microtubules also compose the spindle fibers of mitosis and meiosis.
3) Proteins are the work horses of cells. They help move things within cells.

2. Centrioles
1) Centrioles are components of the cytoskeleton that are composed of microtubules

3. 2) Microtubules also compose the spindle fibers of mitosis and meiosis.

4. Interphase The Cell Cycle Phases for Eukaryotic Cells are: Interphase
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”.
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.)

5. Before and after the S phase

6. Interphase cell (Look at the chromatin in the blue nucleus and the yellow cytoskeleton.)

7. Start of Mitosis
Mitosis - means “nucleus division” (First divide the DNA; then secondly the cytoplasm.)
This process has four parts:

8. Cell in Prophase 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.)

9. Mitosis “Division of the nucleus”

10. Cell in Metaphase 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”?)

11. Cell in Anaphase 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.

12. Cell in Telophase and starting Cytokinesis
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.
Cytokinesis (Cleavage means “split”) This is the division of the cytoplasm.
The cytoplasm and cell organelles are separated to produce two daughter cells.

13. One cell becoming two G0 (Zero growth phase)
The cells are tired and take a brief break and rest or they stop adult development.

14. Motor Protein and the Spindle Fibers
Chromosome
movement
Kinetochore
Tubulin
subunits
Motor
protein
Microtubule
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 phosphorylation.
Non-kinetochore spindles are used to “push” the poles farther apart to help produce the cleavage furrow.
Chromosome

15. Animal vs. Plant 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 furrow

16. LE 12-10 Chromatin condensing Nucleus Chromosomes Cell plate 10 µm
Nucleolus
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 fragment.
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 micro-
tubules shorten.
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 cell.

17. Microscopic view of Mitosis in Onion root tips
Microscopic view of Mitosis in Onion root tips. Can you identify the stages?

18. Checkpoints (Is all going according to plan?)
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 phase.)
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?)

19. E-Coli (bacterium) has Circular or Theta Replication (Theta Chromosome)
A common bacteria that colonizes intestines serves as a excellent example of a bacterium with a circular chromosome. Replication of the Escherichia coli chromosome proceeds in stages, which can be divided into three major headings; initiation, elongation and termination. Bacteria initiate DNA replication at a specific site on the chromosome, the replication origin oriC, from which replication proceeds bidirectionally to the terminus. Initiation proceeds in a series of well defined biochemical steps, and is the only phase of DNA replication that is known to be regulated, but is regulated such that replication occurs only once in each cell cycle. During the elongation phase of replication, two distinct but related events occurs; that is the simultaneous synthesis of the leading and lagging strands. Several enzymes at the replication fork are essential to the synthesis of both strands.
What is the difference between a theta chromosome and Eukaryotic chromosomes:
More genes are present on Eukaryotic chromosomes than on theta chromosomes.
This means there exists more genetic variation exists in Eukaryotic organisms.
There also exist more possible genetic combinations that can be inherited.
There exists more genetic stability in eukaryotic chromosomes.
Offspring receive the same number of chromosomes most of the time.
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.