Cell biology 4: the cell cycle (Dr. Whitmore)

Question 1

What are the 4 stages of the cell cycle ?

Answer 1

G1 = DNA growth
S = DNA duplication (where errors can occur)
G2 = More growth and prep for mitosis
M = Mitosis (cell division) + cytokinesis

Question 2

When does mitosis occur during the day ?

Answer 2

Just before dawn –> no exposure to UV light –> less likely to introduce mutations/errors that will be passed on to the 2 daughter cells

Question 3

When does DNA replication occur during the day ?

Answer 3

Around 5pm.

Question 4

What is the average rate of cell division in the body ?

Where is this rate biggest ?

Answer 4

96M cell divisions/hour

Especially in gut, skin, liver and olfactory cells

Question 5

At which point of the cell cycle can the cell “lock” ?

Why would it lock ?

Answer 5

If the environment is unstable or if there are not enough nutrients, cells can “lock” in the G1/S phase.

Question 6

Which proteins control the different events of the cell cycle ?

Answer 6

Distinct cyclin-dependent protein kinases (Cdks) associate with different cyclins to trigger the different events of the cell cycle.

Question 7

What are some of the major cyclins and Cdks in vertebrates ?

Answer 7

Cyclin D associates w/ Cdk4 or Cdk6 to form the G1-Cdk dimer/complex
Cyclin E associates w/ Cdk2 to form the G1/S-Cdk dimer
Cyclin A associates w/ Cdk2 to form the S-Cdk dimer
Cyclin B associates w/ Cdk1 to form the M-Cdk dimer

Question 8

Which proteins can degrade cyclins ?
What are examples of ubuiquitins ?
How do these act ?

Answer 8

Ubiquitins:

• APC (Anaphase-Promoting Complex) –> ubiquitinates (attaches a ubiquitin chain to) the active cyclin-Cdk complex (followed by destruction of cycline in proteosome) –> prevents exit of mitosis
• Wee1 –> can phosphorylate + inactivate mitotic Cdk (Cdc25 = phosphotase necessary to re-activate the complex)
• p27 + p21 = Cdk inhibitors that bind to cyclin-Cdk before the S phase to lock the cell in the G1 phase

Question 9

What do mammalian cells require for proliferation ?

Answer 9

Extracellular signals = mitogens

Question 10

How do mitogen receptors (MRs) work ?

Answer 10

When activated, this MRs can stimulate the cell cycle by activating cyclin-Cdk complexes that can phosphrylate (inactivate) retinoblastoma (Rb) –> Rb releases a cellular transcription factor and allows the cell cycle to proceed

Question 11

How is Rb relevant to cancer research ?

Answer 11

If Rb can be kept in an active state in cells, this could prevent cell proliferation –> prevent tumor proliferation/cellular degeneration

Question 12

What can arrest the cell cycle in G1 ?

Answer 12

DNA damage by any ionizing radiation (e.g. by UV light).

Question 13

To what cascade can DNA damage lead to ?

Answer 13

1. DNA damage
2. Activation of protein kinases that phosphorylate p53, thus stabilizing/activating it (in absence of DNA, p53 = degraded by lysosomes).
3. Active p53 binds to regulatory regions of p21 gene
4. p21 transcribed and translated into p21 (Cdk inhibitor protein 1A)
5. p21 binds and inactivates G1/S-Cdk and S-Cdk complexes

Question 14

What are mutations in p53 taken very seriously ?

Answer 14

Because p53 mutations are found in about 50% of all human cancers.

Question 15

What is Cdc6 ?

What effects does it have a DNA replication ?

Answer 15

• Cdc6 protein that increases during G1
• Cdc6 binds to ORC (Origin Recognition Complex sitting on origin) formed at the end of G1
• binding to Cdc6 to ORC attracts a helicase dimer + leads to unbinding of Cdc6
• S-Cdk complex phosphorylates/activates the helicase dimer –> dimers dissociate and unwind DNA –> completion of replication
• S-Cdk also targets Cdc6 for degradation, thereby preventing re-replication

Question 16

What is the role of cohesins and condensins ?

Answer 16

They help configure duplicated chromosomes for segregation.

Question 17

What are cohesin rings ?

Answer 17

Ring-shaped complexes that hold 2 sister chromatids together –> condense DNA to form/stabilize chromosome structures during S phase

Question 18

What are condensin rings ?

Answer 18

Ring-shaped complexes that hold different DNA molecules together and thus folds the chromosome into a series of loops at the start of the M phase

Question 19

What seperates the two daughter cells during cytokinesis ?

Answer 19

A contractile ring made of actin and myosin filaments.

Question 20

What does the centrosome of the cell do during interphase ?

Answer 20

It duplicates to form the two poles of a mitotic spindle.

Question 21

What pulls the two poles of the cell apart during mitosis ?

Answer 21

The mitotic spindle.

Question 22

When does the centrosome start duplicating ?

Answer 22

At the same time as DNA replication and is triggered by the same G1/S and S-CDKs.

Question 23

What does the mitotic spindle “wait” for to become operational ?

Answer 23

The breakdown of the nuclear envelope (to pull chromosomes apart).

Question 24

What are the 5 stages of mitosis ?

Answer 24

Prophase, prometaphase, metaphase, anaphase and telophase.

Question 25

What happens during prophase ?

Answer 25

• duplicated chromosomes, each consisting of 2 closely associated sister chromatids, condense
• outside the nucleus, the mitotic spindle assembles between the 2 centrosomes, which have begun to move apart

Question 26

What happens during prometaphase ?

Answer 26

• breakdown of nuclear envelope

- chromosomes can now attach to spindle via their kinetochores and undergo active mvnt

Question 27

What happens during metaphase ?

Answer 27

• the chromosomes are aligned at the equator of the spindle, midway between the spindle poles
• the kinetochore MT on each sister chromatid attach to opposite poles of the spindle

Question 28

What happens during anaphase ?

Answer 28

• the sister chromatids synchronously seperate and are pulled slowly toward the spindle pole to which they are attached
• the kinetochore MTs get shorter + the spindle poles move apart, both contributing to chromosome segregation

Question 29

What happens during telophase ?

Answer 29

• the 2 sets of chromosomes arrive at the pole of the spindle
• a nuclear envelope reassembles around each set –> completing the formation of 2 nuclei + marking the end of mitosis
• division of cytoplasm –> begins w/ assembly of contractile ring

Question 30

What happens during cytokinesis in an animal cell ?

Answer 30

• cytoplasm divided in 2 by a contractile ring of actin + myosin filaments
• contractile ring –> pinches the cell into 2 daughters, each w/ one nucleus

Question 31

What are the 3 classes of MTs that make up the mitotic spindle ?

Answer 31

Aster (don’t attach anything), kinetochore (attach kinetochores of chromatids) and interpolar (attach each other on opposite poles).

Question 32

How does APC influence the activity of the proteolytic enzyme seperase ?

Answer 32

Securin is continuously bound to separase, keeping it in an inactive state. APC leads to unbiquitylation and degredation of securin. The active seperase can then break the cohesin rings between sister chromatids + allow them to be pulled apart
N.b. APC activation is blocked by signals from unattached chromosomes !

Question 33

What are the 2 processes that segregate daughter chromosomes at anaphase ?

Answer 33

A. Chromosomes are pulled poleward: though a shortening of kinetochore MTs, forces (= loss of tubulin subunits of both sides of kMTs) are generated at kinetochores to move chromosomes towards their spindle pole
B. 1. A sliding force is generated between iMTs from opposite poles (MT growth at +ve ends of iMTs) to push the poles apart (done w/ kinesins)
2. A pulling force acts to pull the poles toward the cell cortex, thereby moving the 2 poles apart (done w/ dyneins)

Question 34

What is the role of aMTs ?

Answer 34

They use dynein motor proteins which slide and move MTs apart.

Question 35

When and how does the nuclear envelope breakdown ?

When is it reformed ?

Answer 35

The nuclear envelope breaks down in prometaphase by phosphorylation of nuclear pore proteins and lamins.
It reforms during telophase by dephosphorylation of these structures.

Question 36

How is cytokinesis different in plants ?

Answer 36

Cytokinesis in a plant cell is guided by a specialized MT-based structure called the phragmoplast.