🧬 11. cell cycle

Question 1

What is the cell cycle?

Answer 1

The ordered series of events by which a cell grows, replicates its DNA, and divides into two daughter cells.

Question 2

Why is the cell cycle biologically important?

Answer 2

It enables growth, development, tissue repair, immune responses, and reproduction.

Question 3

What happens when cell cycle regulation fails?

Answer 3

Diseases such as cancer, autoimmune disorders, and premature ageing can occur.

Question 4

What are the two major functional halves of the cell cycle?

Answer 4

Interphase and M phase.

Question 5

What phases make up interphase?

Answer 5

G1, S, and G2.

Question 6

What occurs during G1 phase?

Answer 6

Cell growth, organelle synthesis, protein production, and assessment of environmental conditions.

Question 7

What is the restriction point (START)?

Answer 7

A checkpoint in late G1 where the cell commits to division.

Question 8

What controls passage through the restriction point?

Answer 8

Growth factors, nutrients, and absence of DNA damage.

Question 9

What happens if a cell does not pass the restriction point?

Answer 9

It enters G0.

Question 10

What is G0 phase?

Answer 10

A quiescent state where cells are metabolically active but not dividing.

Question 11

Which cells commonly remain in G0 permanently?

Answer 11

Neurons.

Question 12

What occurs during S phase?

Answer 12

DNA replication.

Question 13

How does DNA content change during S phase?

Answer 13

From 2n to 4n.

Question 14

What forms after DNA replication?

Answer 14

Sister chromatids joined at the centromere.

Question 15

Why must DNA replicate only once per cycle?

Answer 15

To maintain genome stability.

Question 16

How is re-replication prevented?

Answer 16

MCM helicase is displaced after initiation and cannot reload until mitosis is complete.

Question 17

What occurs during G2 phase?

Answer 17

Further growth, protein synthesis, and DNA integrity checks.

Question 18

What is unique about G2 arrest in oocytes?

Answer 18

It can last for decades until hormonal stimulation.

Question 19

What is M phase?

Answer 19

The phase where nuclear division (mitosis) and cytoplasmic division (cytokinesis) occur.

Question 20

How long does M phase typically last?

Answer 20

Approximately 1 hour.

Question 21

What is the purpose of mitosis?

Answer 21

Equal segregation of duplicated chromosomes.

Question 22

What are the stages of mitosis?

Answer 22

Prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis.

Question 23

What are cell cycle checkpoints?

Answer 23

Surveillance mechanisms that ensure correct order and fidelity of cell cycle events.

Question 24

What does the G1 checkpoint assess?

Answer 24

Growth conditions, growth factor availability, and DNA integrity.

Question 25

What does the spindle assembly checkpoint ensure?

Answer 25

All chromosomes are correctly attached to the spindle before anaphase.

Question 26

What experimental method measures DNA content in cells?

Answer 26

Flow cytometry.

Question 27

How does flow cytometry distinguish cell cycle phases?

Answer 27

By measuring fluorescence proportional to DNA content.

Question 28

What DNA content indicates G1 cells?

Answer 28

2n.

Question 29

What DNA content indicates G2/M cells?

Answer 29

4n.

Question 30

What are cyclin-dependent kinases (Cdks)?

Answer 30

Protein kinases that drive cell cycle progression by phosphorylating target proteins.

Question 31

Are Cdks active on their own?

Answer 31

No, they require cyclin binding.

Question 32

What do cyclins do?

Answer 32

Regulate the timing and specificity of Cdk activity.

Question 33

What is MPF?

Answer 33

Maturation-promoting factor composed of Cdk1 and cyclin B.

Question 34

What is the role of MPF?

Answer 34

Triggers entry into mitosis.

Question 35

How was MPF discovered?

Answer 35

Through cytoplasm transfer experiments in frog oocytes.

Question 36

What organism helped identify Cdk1 genetically?

Answer 36

Yeast.

Question 37

What organism helped identify cyclins?

Answer 37

Sea urchin embryos.

Question 38

How are cyclin levels regulated?

Answer 38

By synthesis during interphase and degradation during mitosis.

Question 39

What enzyme complex degrades cyclins?

Answer 39

The ubiquitin–proteasome system.

Question 40

What happens when cyclin B is degraded?

Answer 40

Cdk1 is inactivated and mitosis ends.

Question 41

How many mechanisms regulate Cdk activity?

Answer 41

Four.

Question 42

What is the first mechanism regulating Cdks?

Answer 42

Cyclin binding.

Question 43

What is the second mechanism regulating Cdks?

Answer 43

Activating phosphorylation by CAK.

Question 44

What does CAK do?

Answer 44

Phosphorylates Cdk threonine 160 to activate it.

Question 45

What is the third mechanism regulating Cdks?

Answer 45

Inhibitory phosphorylation by Wee1 kinase.

Question 46

What does Wee1 kinase do?

Answer 46

Adds inhibitory phosphate groups to Cdks.

Question 47

What removes inhibitory phosphates from Cdks?

Answer 47

Cdc25 phosphatase.

Question 48

What is the fourth mechanism regulating Cdks?

Answer 48

Cdk inhibitors (CKIs).

Question 49

What are the two CKI families?

Answer 49

Ink4a and Cip/Kip.

Question 50

Give an example of a Cip/Kip inhibitor

Answer 50

p21.

Question 51

Which cyclin–Cdk pair drives early G1?

Answer 51

Cyclin D–Cdk4/6.

Question 52

Which cyclin–Cdk pair drives late G1 and the restriction point?

Answer 52

Cyclin E–Cdk2.

Question 53

Which cyclin–Cdk pair controls S phase?

Answer 53

Cyclin A–Cdk2.

Question 54

Which cyclin–Cdk pair controls mitosis?

Answer 54

Cyclin B–Cdk1.

Question 55

How do growth factors promote cell cycle entry?

Answer 55

By activating signalling pathways that increase cyclin D synthesis.

Question 56

Which signalling pathway links growth factors to cyclin D?

Answer 56

RTK → Ras → Raf → MEK → ERK.

Question 57

What is pRb?

Answer 57

Retinoblastoma protein, a tumour suppressor.

Question 58

What does pRb bind?

Answer 58

E2F transcription factors.

Question 59

What is the function of E2F?

Answer 59

Activates transcription of S-phase genes.

Question 60

How does pRb inhibit the cell cycle?

Answer 60

By binding E2F and preventing transcription.

Question 61

How is pRb inactivated?

Answer 61

Phosphorylation by cyclin D–Cdk4/6.

Question 62

What happens when pRb is phosphorylated?

Answer 62

E2F is released and S-phase genes are expressed.

Question 63

What senses double-stranded DNA breaks?

Answer 63

ATM kinase.

Question 64

What senses single-stranded or unreplicated DNA?

Answer 64

ATR kinase.

Question 65

What kinases transmit DNA damage signals?

Answer 65

Chk1 and Chk2.

Question 66

How do DNA damage checkpoints block the cell cycle?

Answer 66

By inhibiting Cdc25 and preventing Cdk activation.

Question 67

What is p53?

Answer 67

A tumour suppressor transcription factor known as the guardian of the genome.

Question 68

How is p53 activated?

Answer 68

By ATM/Chk2 signalling after DNA damage.

Question 69

What gene does p53 induce to stop the cell cycle?

Answer 69

p21.

Question 70

What does p21 do?

Answer 70

Inhibits cyclin–Cdk complexes causing G1 arrest.

Question 71

What happens when p53 is lost or mutated?

Answer 71

DNA damage is not repaired and mutations accumulate, promoting cancer.