Mitosis & Apoptosis

Question 1

Briefly describe what happens in each of the four phases of the cell cycle.

Answer 1

G1: Cell grows and carries out normal metabolism; organelles duplicate.
S: DNA replication and chromosome duplication.
G2: Cell grows and prepares for mitosis.
M: Mitosis

Question 2

What happens in prophase?

Answer 2

The nuclear envelope and cytoskeleton disassemble. The Golgi complex and ER fragment and disappear. The mitotic spindle assembles. Chromosomal material condenses into sister chromatids.

Question 3

What happens in prometaphase?

Answer 3

Chromosomal microtubules attach to kinetochores of chromosomes.
Chromosomes are moved to the spindle equator.

Question 4

What happens in metaphase?

Answer 4

Chromosomes are aligned along the metaphase plate, attached by chromosomal microtubules to both poles.

Question 5

What happens in anaphase?

Answer 5

The sister chromatids come apart at their centromere. The chromosomes are pulled toward either pull with the mitotic spindle.

Question 6

What happens in telophase?

Answer 6

Chromosomes cluster at opposite poles and become dispersed. The nuclear envelope assembles around chromosome clusters. The Golgi and ER reform. Daughter cells are formed by cytokinesis.

Question 7

What is the timeline for the centrosomal cycle? (ie where does it fit into the overall cell cycle?)

Answer 7

Since centriole duplication is required for mitosis, it occurs before M phase. The point at which the daughter centriole is growing at a right angle from the mother centriole occurs during S phase. By the end of G2 there are two complete centrosomes.

Question 8

What are the three important types of microtubules used in mitosis?

Answer 8

Astral, chromosomal, and polar.

Question 9

What is the role of chromosomal spindle fibers?

Answer 9

These microtubule fibers attach to the kinetochore and are the entities that actually pull the chromosome apart.

Question 10

What is the role of polar spindles?

Answer 10

Polar spindles are responsible for movement and shape changes. Polar spindle complexes can also form in the absence of centrosomes through the action of minus end directed microtubule motors. They can also act as microtubule organizing centers.

Question 11

What is the kinetochore?

Answer 11

The kinetochore is the highly active junction and specialized region of the centromere between mitotic microtubules and the chromosomes. It is highly enriched in heterochromatin.

Question 12

What are the functions of the outer kinetochore?

Answer 12

Microtubule binding, motor activity, and signal transduction.

Question 13

What are the functions of the inner kinetochore?

Answer 13

Centromere replication, chromatin interface, and kinetochore formation.

Question 14

What can labeling studies on mitotic microtubules reveal about them?

Answer 14

They demonstrate that mitotic microtubules are not stable but rather constantly cycling tubulin subunits in a process known as tubulin flux at a rate of about 1 µm/min.

Question 15

During prometaphase, what are kinesins (plus end motors) doing?

Answer 15

During prometaphase plus end motors (kinesin) cause polar MT from opposite poles to slide relative to each other and move apart both halves of the spindle.

Question 16

When chromosomes move toward the pole, which type of motor protein helps them accomplish this? How about for movement away from the pole?

Answer 16

Poleward movement of the chromosomes occurs through minus end motors (dyneins) and movement away from the pole occurs via plus end motors (kinesisn).

Question 17

At metaphase, describe the motor protein activity.

Answer 17

At metaphase the two halves of the spindle maintain their position by plus end motors associated with polar MTs and the balanced activity of plus/end motors at the kinetochore.

Question 18

What is the activity of motor proteins like during anaphase?

Answer 18

At anaphase kinesin depolymerase catalyses the depolymerization of kinetochore MTs while polar MTs move poleward via the activity of the plus end.

Question 19

What controls the entry into anaphase? What else does this complex do?

Answer 19

The entry into anaphase is controlled by the anaphase promoting complex activity, which also controls degradation of cyclin B and ends mitosis.

Question 20

How does APC interact with MPF?

Answer 20

APC targets the cyclin component of MPF for proteolytic degradation. MPF is responsible for the phosphorylation of several APC subunits, which is necessary to activate the APC. APC subunits are low-affinity targets for MPF, so they require peak levels of MPF.

Question 21

What is Cdh1, and what activates and inactivates it?

Answer 21

Cdh1 is an APC adaptor protein that directs APC activity to M-cyclins. Cdh1 is activated by Cdc14 phosphatase and inactivated by G1 CDK kinase.

Question 22

What is APC’s role in promoting anaphase?

Answer 22

APC sets up the process by which sister chromatids are released from each other by degrading a specific inhibitor which would otherwise block the activity of cohesins (cohesins keep the sister chromatids aligned).

Question 23

What is the target of APC, and how does it degrade this target?

Answer 23

The securin protein is the target of APC. It is degraded (like APC does with MPF) through the addition of ubiquitin.

Question 24

What happens after the degradation of securin?

Answer 24

The degradation of securin leads to the release of separase, which is a cysteine protease whose target is cohesin.

Question 25

How do levels of MPF affect kinase and phosphatase activity?

Answer 25

When MPF is very high, the activity of kinases is favored. When MPF is degraded, the background activity of phosphatases is favored.

Question 26

What happens when the activity of phosphatases is favored over the activity of kinases?

Answer 26

This means MPF levels are low, and since kinase activity is no longer favored, and kinases phosphorylate, many of the molecules that were phosphorylated become dephosphorylated. The nuclear envelope is reestablished and the organelles and structures that were previously degraded are remade.

Question 27

What is the function of the actin/myosin contractile ring apparatus?

Answer 27

This ring cleaves the daughter cells in cytokinesis.

Question 28

How is phosphatase activity necessary for cytokinesis?

Answer 28

Normally MPF activity inhibits the myosin light chain. When MPF is turned off and phosphatase is allowed to take over, the light chain becomes active, leading to cytokinesis.

Question 29

What is the death domain of the extrinsic apoptotic cascade?

Answer 29

The death domain is an 80 amino acid C-terminal domain shared among a large family of transmembrane proteins.

Question 30

What is the function of the death domain?

Answer 30

The death domains act as dimerization domains to facilitate signaling pathways that depend on cytosolic death domain adapters TRADD and FADD. Death domain proteins activate a signaling cascade that is the default state in the absence of growth factors.

Question 31

What is the function of the death domain adapters TRADD and FADD?

Answer 31

TRADD and FADD enable linkage between the death domain receptors and the beginning of the caspase pathway of proteases.

Question 32

Describe the caspase family of proteases.

Answer 32

Proteases degrade proteins, and caspases work by cleaving proteins after their aspartic acid residues. They have an N-terminal pro domain that must be cleaved off to form an active enzyme complex.

Question 33

What are the two types of caspases?

Answer 33

The initiators and the effectors. The activity of initiator caspases is not directed toward the general protein structure of the cell but to specific proteolytic cleavage of effectors. The effectors are the caspases actually involved in “cutting up” proteins.

Question 34

List the three steps in the process of cell death.

Answer 34

1. Formation of apoptosome starts the cascade of caspases.
2. This leads to a loss of electron transport (and release of more cytochrome C) and the build-up of toxic free radicals.
3. General loss of all cellular enzymes through their digestion by general caspase targets.

Question 35

What are four of the important targets of caspase?

Answer 35

1. Kinases
2. Lamins
3. Cytoskeleton
4. CAD DNase

Question 36

Where are the intrinsic apoptotic pathways initiated?

Answer 36

These pathways are initiated on the mitochondrial surface, in the apoptosome.

Question 37

What process promotes the release of cytochrome C in the intrinsic apoptotic pathway?

Answer 37

Oligomerization of Bcl-2 family members (Bad/Bax/Bak) on mitochondria promotes the release of cytochrome C. Cytochrome C is an important factor in the electron transport and cellular metabolism processes. The release of cytochrome C is like acid leaking from a battery—not a good thing.

Question 38

What does cytochrome C complex with to form the apoptosome, and what does this accomplish?

Answer 38

Apaf-1 and procaspase-9. This allows for the activation of procaspase-9 into its active form: caspase 9. Caspase 9 in turn activates the executioner caspase which will initiate apoptosis.

Question 39

What blocks the release of procaspase 9 by Apaf-1?

Answer 39

The overexpression of antiapoptotic proteins Bcl-2 and Bcl-xl can block this release.

Question 40

What promotes the release of procaspase 9 by Apaf-1?

Answer 40

The overexpression of Bax/Bad (proapoptotic proteins) promotes the release.

Question 41

What is the function of the condensin protein?

Answer 41

Condensin works to compact chromosomes. In the presence of a topoisomerase and ATP, condensing is able to bind to DNA in vitro and curl the DNA into positively supercoiled loops.

Question 42

What activates condensin?

Answer 42

Condensin is activated at the onset of mitosis by phosphorylation of several of its subunits by the cyclin-Cdk responsible for driving cells from G2 into mitosis. Condensin is one of the targets through which Cdks are able to trigger cell cycle activities.

Question 43

What is the function of the multiprotein complex cohesin?

Answer 43

Following replication, cohesin holds the two sister chromatids together continuously through G2 and into mitosis when they are ultimately separated.

Question 44

What are the three functions of the kinetochore?

Answer 44

1. The site of attachment of the chromosome to the dynamic microtubules of the mitotic spindle
2. The residence of several motor proteins involved in chromosome motility.
3. A key component in the signaling pathway of an important mitotic checkpoint.