Mitosis

Question 1

What are the stages of mitosis?

Answer 1

• Interphase (pre)
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis (post)

Question 2

Interphase

Answer 2

• Before mitosis
• Nuclear envelope intact
• No visible chromosomes

Question 3

Prophase

Answer 3

• Chromosomes condense and become visible

- Bipolar spindle develops

Question 4

Prometaphase

Answer 4

• Nuclear envelope dissolves
• Chromosomes begin to migrate to the metaphase plate
• 2 chromatids visible

Question 5

Metaphase

Answer 5

• Chromosomes fully condensed and at metaphase plate

Question 6

Anaphase

Answer 6

• Each centromere splits

- 2 chromatids of each chromosome are pulled to opposite poles

Question 7

Telophase

Answer 7

• Chromosomes reach poles and start to decondense
• Nuclear membrane reforms
• Cytoplasm begins to divide

Question 8

Cytokinesis

Answer 8

• Cytoplasm division completed resulting in 2 daughter cells

Question 9

How is the genome organized during interphase?

Answer 9

• DNA double helices are wrapped around histones

- Nucleosomes are coiled into a chromatin fiber

Question 10

What are the two structural maintenance complexes (SMC)?

Answer 10

• Related complexes that organize chromosomes during mitosis

- Elongated coil-coil ATPases

Question 11

Cohesin

Answer 11

• An SMC
• Keeps sister chromosomes together during segregation of metaphase and ensures ones sister chromatid ends up in each daughter cell
• Load before replication (G1) and maintained through metaphase
• Dissolution of cohesion during anaphase

Question 12

Condensin

Answer 12

• An SMC
• Compacts chromosomes: uses ATP to pump a loop of DNA through itself creating a more compact chromosome structure
• Organizes for efficient segregation

Question 13

What is the role of topoisomerase in mitosis?

Answer 13

• Topoisomerase decatenates and relaxes supercoils, helping with chromosome compaction

Question 14

What is the role of histone phosphorylation in mitosis?

Answer 14

Chromosome compaction

Question 15

How are the mitotic cyclins activated?

Answer 15

1. M-cyclin (cyclinB) binds Cdk-1 forming an inactive M-Cdk complex
2. Cdk-activating kinase (CAK) and Cdk-inhibitory kinase (Wee1) phosphorylate M-Cdk, which remains inactive
3. Phosphatase (Cdc25) removes the inhibitory phosphate and M-Cdk is activated

Question 16

How is mitotic cyclin activation regulated?

Answer 16

Positive feedback:

• active M-Cdk inhibits Wee1 inhibitory kinase
• active M-Cdk activates Cdc25 phosphatase

G2/M DNA damage checkpoint:

• Via a signaling cascade, DNA damage causes Cdc25 export from the nucleus
• M-Cdk will not be activated

Question 17

How does the nuclear envelope disassemble during prophase/prometaphase and reassemble during telophase?

Answer 17

• Phosphorylation of nuclear pore proteins and lamins => membrane disassembly. Nuclear envelope forms vesicles.
• Dephosphorylation allows for reassembly around the daughter chromosomes.

Question 18

What elements make up the structure of the nuclear envelope?

Answer 18

• Inner and outer nuclear membranes contiguous with ER
• Nuclear pores
• Lamins = intermediate filaments inside the inner membrane for structure and organization

Question 19

What are the structure and function of the kinetochore?

Answer 19

• Interface between chromosome and MT spindle that controls all chromosome movement

Different structural elements:

• Centromeric chromatin: histones
• Inner kinetochore: associated with centromere DNA, present throughout the cell cycle
• Outer kinetochore: associates with MTs (via various proteins), dynamic and only present during mitosis.

Question 20

How do microtubule spindles form?

Answer 20

• Grow outward from centrosome attachments in a - => + direction via dynamic instability

Question 21

What are the functions of molecular motors in mitosis?

Answer 21

• Spindle formation
• MT dynamics
• Chromosome alignment

Question 22

What are kinesin and dyenin?

Answer 22

• Kinesin = + end directed molecular motor (away from centromere)
• Dyenin = - end directed molecular motor (towards centromere)

Question 23

What microtubule structures are present in the dividing cell?

Answer 23

• Aster microtubules: branch out from spindle pole
• Kinetochore microtubules: bind to kinetochores
• Interpolar microtubules: associate with microtubules from opposite pole

Question 24

What does the mitotic checkpoint check for?

Answer 24

All chromosomes must be attached to mitotic spindle

Question 25

What happens if spindle assembly is not complete at the mitotic checkpoint?

Answer 25

- Unattached kinetochore induces the mitotic checkpoint complex (MCC) - MCC inhibits APC, which is required for anaphase progression

Question 26

What does APC activation after metaphase cause?

Answer 26

- proteolysis of mitotic cyclins (cyclinB-Cdk1) - loss of chromosome cohesion (via proteolysis of securin, which inhibits separase, which degrades cohesin) - separation of sister chromatids

Question 27

What is APC/C and what are its targets?

Answer 27

An E3 ubiquitin ligase targeting cyclinB-Cdk-1 and securin

Question 28

What are microtubule poisons used to treat?

Answer 28

Cancer, via MT breakdown or inhibition of MT dynamics

Question 29

Compare anaphase A and anaphase B.

Answer 29

A: - forces generated mainly at kinetochores - kinetochore MTs shorten via depolymerization with kinetochores holding on to the shortening ends - daughter chromosomes move towards the poles B: - forces involve molecular motors - interpolar MTs elongate pushing poles apart - kinesin-14 and -5 walk along + ends of spindle MTs and generate a sliding force pushing poles apart - dyneins bind plasma membrane and pull poles apart

Question 30

Cytokinesis

Answer 30

- Actin-myosin ring forms at equator (as per signals from the spindle-cortex interactions and spindle midzone) - Contractile ring constricts

Question 31

What are some types of errors in chromosome kinetochore attachment?

Answer 31

- Amphitelic = success - Syntelic = both kinetochores attached to one pole - Monotelic = only one kinetochore attached to one pole - Merotelic = one extra MT attaches to one kinetochore (3/4 or 4/4 sisters may end up in one cell)

Question 32

What is aneuploidy?

Answer 32

Having too many or too few chromosomes

Question 33

What are some types of errors in chromosome segregation?

Answer 33

- SAC mutations - premature loss of chromatid cohesion - aberrant kinetochore attachments - supernumary centrosomes: extra centrosome means cell is split with > 2 poles => tumorigenesis

Question 34

What are trisomies and how are they tolerated?

Answer 34

- Missegregation of chromosomes during meiosis I or II leading to an extra copy of a chromosome. - Tolerated: 13, 18, 21, X, and Y - Miscarriage: 15, 16, 22 - Not tolerated: 1-12, 14, 17, 19, 20

Question 35

What is anaphase lag?

Answer 35

- caused by merotelic attachment of kinetochores => aneuploidy and micronuclei (lone chromatid gets a mini nucleus with incomplete machinery)

Question 36

What are chromosome bridges?

Answer 36

- a chromosome segregation error caused by fused sister chromatids entering mitosis => aneuploidy