The eukaryotic cell cycle & interphase

Question 1

Cell division

Answer 1

continuity of life

Question 2

Why is duplication and cell division so important?

Answer 2

Cell cycle
- essential mechanism by which all living things reproduce/grow
e.g unicellular organisms, each cell division produces a new organism
rounds of cell divisions are required from the fertilised egg cell to develop into multicellular organisms

Question 3

How often do cells divide?

Answer 3

-not all cells divide
-rates are different in different cells
-highly specialised cells (muscle and nerve cells) do not/rarely divide
-epithelial cells in gut divide x2 a day
-liver cells only one in a year/two

Question 4

Resting state

Answer 4

when cells reach certain size, growth either stops its cycle or cell MUST divide

Question 5

Eukaryotic cell cycle - phases?

Answer 5

M phase
G1 phase
S phase (DNA replication)
G2 phase

(non-dividing cells are usually reversible) state - G0 phase or RESTING PHASE (of the replicative cell cycle)

Question 6

Why does cell division have to be controlled?

Answer 6

To avoid uncontrollable cells division - CANCER

Question 7

checkpoints in cycle…

Answer 7

biochemical switches - to pause the cycle at 3 main transition points

Question 8

Cdks

Answer 8

Cyclically activated Cyclin-dependent protein kinases

Question 9

How does Cdks come active?

Answer 9

must bind to a specific regulatory protein called cyclin
(cdks must be in a particular phosphorylation state)

Question 10

do cyclins have enzymatic activity?

Answer 10

No

Question 11

G1 phase

Answer 11

Period of metabolic activity, cell growth, and
general repair. The cell grows in mass to
prepare the cell for division

Question 12

To pass the checkpoint, according to

Answer 12

1. Cell size
2. Presence of nutrients, grow factors
3. DNA integrity

Question 13

Cells can …… in G1 phase

Answer 13

1. Proceed to S phase; extracellular signals(mitogens) induces progression
2. Delay the entrance in S phase (to further grow or if DNA is damaged)
3. Exit the cell cycle to G0 (temporarily or permanently)
4. Induce a programmed cell death (apoptosis), if there is a severe DNA damage
   checkpoint
   G1-to-S

Question 14

DNA replication

Answer 14

Process by which DNA makes a copy of itself

DNA must rapidly and accurately copy (replicate) its nucleotide sequence (to avoid
mistakes/mutations)

Question 15

During DNA replication

Answer 15

46 chromosomes (in form of chromatin) forms two identical duplicated DNA molecules (x2 sister chromatids - joined at centreomere)

Question 16

S phase

Answer 16

DNA is replicated, therefore this phase is highly regulated

Question 17

What happens immediately in S phase?

Answer 17

Once entered into the S phase, S-Cdk activates
helicases and other proteins to form the replication
forks and initiate the DNA replication

Question 18

Duplicated chromosome

Answer 18

Sister chromatids are connected by cohesins (for chromosome segregation
In s phase - centrosome is also duplicated

Question 19

Centrosome

Answer 19

Cellular structure that controls location, number and orientation of microtubles

Question 20

Centrosome structure?

Answer 20

Composed of 2 centrioles (9 triplets of microtubules)

Question 21

G2 phase

Answer 21

• Rapid cell growth and protein synthesis
  (accumulation of enzymes) to prepare for mitosis
• To check for unreplicated or damaged DNA

Question 22

Checkpoint end of G2/ entry into mitosis

Answer 22

• If DNA is fully replicated and not damaged, proteins involved in early mitosis are
  activated and the cell enters in mitosis (onset of prophase)
• Incomplete replication can arrest the cell cycle

Question 23

M phase

Answer 23

Cell division is a continuous sequence of events (5 stages), usually symmetric

Question 24

M phase key points:

Answer 24

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
6. Cytokinesis

Question 25

Stage 1 - prophase

Answer 25

• Chromatin is condensed into visible chromosomes (by condensins)
• Transcription activities ceased

Outside the nucleus, the mitotic spindle assembles between the two centrosomes,
which have begun to move apart

Question 26

Stage 2 - Prometaphase

Answer 26

• Nuclear envelope and nuclear lamina breakdown
• The 2 centrosomes are now at the 2 spindle poles (opposite ends) of the cell
• Chromosomes attach to spindle microtubules of one pole via their kinetochores (protein complexes of the chromosomes’ centromere on both chromatids)

Question 27

Stage 3 - Metaphase

Answer 27

• Mitotic spindle is fully developed
• The chromosomes are aligned at the spindle equator (midway), between poles
• Kinetochores on each sister chromatid attach to opposite poles of the spindle
• M checkpoint controls the proper chromosomes alignment and attachment

Question 28

Stage 4 - Anaphase

Answer 28

• Cohesins, which kept sister chromatids together, break down
• Sister chromatids synchronously separate (now individual chromosomes) and are pulled slowly toward the opposite spindle poles to which they are attached
• Kinetochore microtubules shorten and the spindle poles also move apart

Question 29

Sage 5 - Telophase

Answer 29

• The two sets of chromosomes arrive at the spindle poles and decondense
• A new nuclear envelope and nuclear lamina reassembles around each set of chromosomes, completing the formation of two nuclei (end event of mitosis)
• It starts the division of the cytoplasm with the formation and contraction of the contractile ring (actin and myosin), at midway between the spindle poles

Question 30

Stage 6 - Cytocynesis

Answer 30

• The cytoplasm is divided in two by a contractile ring, which pinches the cell into two daughters, each with one nucleus (same DNA).
• The mitotic spindle determines the plane of cytoplasmic cleavage

Question 31

Programmed cell death (Apoptosis)

Answer 31

Apoptosis is a form of programmed cell death induced by external and internal stimuli

Question 32

Removing cells during embryonic development (fingers/toes formation)

Answer 32

 Cells no longer needed
 Cells with severe DNA damage
 Cells infected by viruses (preventing damage to neighbouring cells)

Question 33

Necrosis - cells dying for accidental cell death due to acute injury

Answer 33

Non controlled event, causing rupturing of cells and leakage into surrounding tissues (inflammation)

Question 34

Apoptosis & caspases

Answer 34

• Both pathways activate initiator caspases

Question 35

How are cells killed quickly and neatly?

Answer 35

Initiator caspases cleave, and activate,
downstream executioner caspases, which
dismember numerous key proteins (e.g.
enzymes, structural proteins)

Question 36

What is the process of cells dying?

Answer 36

 cytoskeleton collapses
 the nuclear envelope disassembles
 the nuclear DNA breaks up into fragments
 cell shrinkage

Question 37

Survival factors

Answer 37

Promoting cell survival and suppress apoptosis

Question 38

Apoptosis and health

Answer 38

1. The rate of cells dying should balance the rate of cells produced by mitosis
2. Cell signalling plays a role in maintaining the correct balance

Question 39

Either excessive or insufficient apoptosis can contribute to disease

Answer 39

1. Not enough apoptosis leads to the formation of tumours
2. Too much apoptosis leads to cell loss and degeneration

Question 40

Not enough Apoptosis - Contributes to tumours

Answer 40

• Cancerous cells have mutations allow them to block/escape apoptosis (cancer hallmark). As a result, they can survive longer and gives more time for the accumulation of mutations and be more malignant

Question 41

Too much apoptosis

Answer 41

• Cell loss and degeneration