The Eukaryotic Cell Cycle - Interphase

Question 1

What is Cell Division?

Answer 1

• This cycle of duplication and division, known as the cell cycle, is the essential mechanism by which all living things reproduce/grow
• In 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

Also important as a repair/renew process (e.g skin, blood cells)

Question 2

What is the cell cycle?

Answer 2

-is an ordered sequence of events that leads to the DNA replication (but also some macromolecules and organelles) and cell division

Question 3

Do all cells divide?

Answer 3

-No, where division rates are also different from cells to cells

• Highly specialised cells (muscle and nerve cells) do not or rarely divide (a subset)
• Epithelial cells in the gut divide twice a day, whereas liver cells only once a year or two

Question 4

What happens when a cell enters a G0?

Answer 4

-This means that the cell is entering a resting state, called G0 where the cell reaches a certain size, growth either stops its cycle or the cell must divide

Question 5

Eukaryotic Cell Cycle consists of 4 phases, what are they?

Answer 5

1) M Phase- comprising mitosis (nucleus division) & cytokinesis (cytoplasm division)

2) G1 Phase

3) S Phase- (DNA replication)

4) G2 Phase

Question 6

What is a quiescent (usually reversible state)?

Answer 6

-This is the G0 phase, resting phase

Question 7

Why does cell division need to be controlled?

Answer 7

Uncontrolled cell division can lead to hallmarks of tumour) can result in cancer

Question 8

What is the Cell Cycle Control System?

Answer 8

-is an internal control to ensure the proper progression of the cell cycle and that key steps occur in the right sequence

• Using a set of checkpoints (biochemical switches) to pause the cycle at 3 main transition points (G1/S, G2/M & metaphase/anaphase transition phases)
• It checks that all the required events for progression have occurred and it does not trigger the next step in the cycle unless the cell is properly prepared.

Question 9

Cell Cycle Control System. Progression of every checkpoint of the cell-cycle control system depends on …?

Answer 9

• some cyclically activated Cyclin-dependent protein kinases (Cdks)
• Cdks must bind to a specific regulatory protein called cyclin to become active (cyclin-Cdk complex)
• Cdks must also be in a particular phosphorylation state
• Cyclins do not have enzymatic activity

-Activated Cdks phosphorylate some proteins crucial for the checkpoint transitions, which allows the cell to proceed along the cell cycle. E.g proteins to initiate DNA replication at G1/S transition (e.g. helicases)

Question 10

What is the G1 Phase ?

Answer 10

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

• G1-decision-making for the cell (to pass the G1/S checkpoint), according to: Cell size, Presence of nutrients, grow factors,DNA integrity
• Cells can proceed to S phase; extracellular signals (mitogens) includes progression

-Delay the entrance in S phase (to further grow or if DNA is damaged)

• Exit the cell cycle to G0 (temporarily or permanently)
• Induce a programmed cell death (apoptosis), if there is a severe DNA damage

Question 11

G1 phase diagram??

Question 12

What is DNA replication?

Answer 12

• it is the process by which DNA makes a copy of itself
• DNA must rapidly and accurately copy (replicate) its nucleotide sequence (to avoid mistakes*mutations)

Question 13

What happens during DNA replication?

Answer 13

-each of the 46 chromosomes (at that point, in the form of chromatin) forms two identical duplicated DNA molecules (two sister chromatids) joined at the centromere

-NOT duplications of Chromosomes

Question 14

What phase is DNA replicated?

Answer 14

-The S-phase

Question 15

What happens during DNA replication in the S phase?

Answer 15

– The phase in which DNA is replicated, therefore this phase is highly regulated

• Once entered into the S phase, S-Cdk activates helicases and other proteins to form the replication forks and initiate the DNA replication
• Sister chromatids are then connected by cohesins (for chromosome segregation)
• In S phase, centrosome is also duplicated

Question 16

What is the Centrosome ?

Answer 16

-Centrosome is a cellular structure that controls location, number, and orientation of the microtubules

• Composed of 2 centrioles (9 triplets of microtubules)

Question 17

What are the functions of the Centrosome?

Answer 17

-It serves to organise microtubules to make cytoskeleton

• It organises microtubules during cell division and forms the mitotic spindle (microtubules machinery that controls chromosome movement during mitosis)

Question 18

What is the G2 Phase?

Answer 18

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

Question 19

G2 Phase diagram?

Question 20

Checkpoint end of G2/entry into mitosis, what is this?

Answer 20

-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 21

What is the M Phase?

Answer 21

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

Question 22

M Phase diagram ?

Question 23

M phase to Anaphase transition?

The 6 phases

Answer 23

-Prophase: condensation of chromosomes; starting formation of two poles

-Prometaphase: Nuclear envelope breakdown; chromosomes attachment to spindle microtubules

-Metaphase: Chromosomes aligned at the equator (midway) of the spindle

-Anaphase: Sister chromatids separated; moving to the two poles

-Telophase: Reformation of nuclear envelopes (nuclear separation), formation of contractile ring

• Cytokinesis: separation of two daughter cell

Question 24

Stage 1 - Prophase ?

Answer 24

-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 25

Stage 2 - Prometaphase ?

Answer 25

-Nuclear envelopE and nuclear lamina break down

• 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 26

Stage 3- Metaphase?

Answer 26

• 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 27

Stage 4 - Anaphase ?

Answer 27

• Cohesins, which kept sister chromatids together, break down

-Sister chromatids synchronously separate (now individual chromosomes) and are pulled slowly towards the opposite spindle poles to which they are attached

• Kinetochore microtubules shorten and the spindle poles also move apart

Question 28

Stage 5- Telophase ?

Answer 28

-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 29

Stage 6- Cytokinesis ?

Answer 29

• 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 30

What is Apoptosis?

Answer 30

• Apoptosis is a form of programmed cell death induced by external and internal stimuli
• Removing cells during embryonic development (fingers and toes formation)
• Cells no longer needed
• Cells with severe DNA damage
• Cells infected by viruses (preventing damage to neighbouring cells)
• Different from cells dying for accidental death (necrosis) due to acute injury
• non-controlled event, causing rupturing of cells and leakage into surrounding tissues (inflammation)

Question 31

The eukaryotic cell cycle consists of what?

Answer 31

-several distinct phases. In interphase, the cell grows and prepares for the cell division, in M phase, the nucleus divides (mitosis) followed by the cytoplasm (cytokinesis)

Question 32

What does Interphase consist of?

Answer 32

-Interphase consists of S phase when DNA duplicated and two gap phases: G1 and G2

Question 33

What does the The cell cycle control system do?

Answer 33

-co ordinates events of the cell cycle by sequentially and cyclically switching on and off the appropriate parts of the cell-cycle machinery

Question 34

Why to cells require survival signals from other cells?

Answer 34

-to avoid apoptosis-a form of cell suicide mediated by a proteolytic caspase cascade; ensuring the survival of only needed cells.