Unit 10

Question 1

Cell Cycle

Answer 1

• The mechanism by which a cell duplicates its contents & divides in 2
  Requires:
  1.) Replication of DNA & other cellular contents
  2.) Partitioning of the duplicated material into 2 “daughter” cells

Question 2

4 Phases of the eukaryotic cell cycle

Answer 2

1.) M-phase: Includes mitosis (division of the genetic material) and cytokinesis (physical separation of the cells into 2 cells)
2.) S-phase: DNA Replication
3.) G1 (Gap 1) Phase: Interval between M phase & S phase
4.) G2 (Gap 2) phase: Interval between S phase & M Phase

Question 3

Timing of Cell cycle

Answer 3

M Phase ~ 1 hour
G1 Phase takes ~ 11 hours
S Phase takes ~ 8 hours
G2 Phase takes ~ 4 hours

Question 4

Prophase

Answer 4

• First phase of mitosis
  1.) 2 daughter centrosomes separate & move to opposite poles of the cell
  2.) Some of the microtubules growing from opposite centrosomes overlap. Microtubule-associated proteins cross-link those microtubules together in a way that stabilises their plus (+) ends by decreasing the probability of depolymerisation
  3.) The interacting sets of microtubules form the mitosis spindle, & the two centrosomes that give rise to these microtubules are called spindle poles
  4.) The chromatin starts to condense

Question 5

Prometaphase

Answer 5

• 2nd phase of mitosis
  1.) The nuclear envelope breaks down, facilitated by the phophorylation of the lamin proteins underlying the nuclear envelope. The membrane fragments into small vesicles
  2.) Microtubules now have access to the chromosomes and attach to the chromosomes via the kinetochore. These microtubules are called kinetochore microtubules
  3.) Chromosomes are now fully condensed

Question 6

Metaphase

Answer 6

• 3rd phase of mitosis
  1.) Chromosomes align at the equator of the spindle, 1/2way through spindle poles, forming metaphase plate
  2.) Kinetochore microtubules from 1 spindle pole attach to 1 sister chromatid. 2nd sister chromatid of the chromosome attaches to a kinetochore microtubule from the 2nd spindle

Question 7

Anaphase

Answer 7

• 4th phase of mitosis
  1.) Paired chromatids synchroneously separate to form 2 daughter chromosomes
  2.) Each daughter chromosome is gradually pulled toward the spindle pole to which it is attached. This movement distributes / segregates the 2 identical sets of chromosomes to opposite ends of the spindle

Question 8

3 types of microtubules during mitosis

Answer 8

1.) Kinetochore microtubules - Attached to mitotic chromosomes at their centromeres via kinetochores, play critical role in separation of mitotic chromosomes
2.) Interpolar microtubules - are not attached to chromosomes & overlap each other; are stabilised by proteins called spindle kinesins
3.) Aster microtubules: extend outward from the centrosomes to tell the cell periphery & have freely exposed plus (+) ends

Question 9

Dynamics of kinetochore microtubules

Answer 9

• Dissassemble but remain attached to chromosomes, pulling chromatids to opposite poles

Question 10

Dynamics of Interpolar microtubules

Answer 10

• Spindle kinesins of interpolar microtubules generate a sliding force to push microtubules apart. This forces spindle poles further apart

Question 11

Dynamics of Aster microtubules

Answer 11

• Pulled towards cell periphery by dynein that is anchored to plasma membrane. This forces spindle poles further apart

Question 12

Telophase

Answer 12

• 5th phase of mitosis
  1.) Separated chromosomes arrive at spindle poles
  2.) Nuclear envelope begins to reassemble around chromosomes. Assisted by dephophorylation of lamin proteins. Chromosomes themselves start to decondense
  3.) A contractyle ring, composed largely of actin filaments starts to form at equator between 2 spindle poles

Question 13

Cytokinesis

Answer 13

1.) Cytoplasm divided in 2 by the contractile ring. Closure of cleavage furrow is aided by non-muscle myosin II
2.) Other cytosolic components such as cytoskeleton, organelles, & other cytosolic proteins equally distributed between newly-formed daughter cells
3.) Chromosomes are fully decondensed & nuclear envelope is reformed
4.) Microtubules emerge from single centrosome in each daughter cell
5.) The remaining microtubules from the mitotic spindle that are no longer attached to a centrosome are referred to as the midbody

Question 14

Health relevance of proper cell division

Answer 14

• Improper segregation of genome leads to aneuploidy, where there is an abnormal # or chromosomes in cell. This can be caused by defects in chromosome segregation & can result in disease
• Can be caused by improper detachment of microtubules to kinetochores, improper sindle pole formation
• Down syndrome & cancer can be caused by these