Lecture 17- The cell cycle

Question 1

Cell cycle (3)

Answer 1

• Cell cycle- The fundamental mechanism of all living organisms to reproduce themselves and reproduce themselves to pass down their genetic material to the next generation.
o Duplication of cell contents DNA, organelles, cytoplasm
o Division into new progeny cells.

Question 2

Cell cycle for ___cellular organisms (3)

Answer 2

• Unicellular organisms such as bacteria or yeast: - each cell cycle gives rise to 2 new organisms.
• Multicellular organisms, such as humans:
o Single fertilised egg (zygote) must undergo many rounds of the cell cycle to make a new fully-grown organism.
o Must also constantly replace any cells that die during the lifetime of the organism.

Question 3

Phases of the cell cycle (6)

Answer 3

Interphase = G1 + S + G2 
G1 = Gap 1 phase: growth and preparation for S phase. 
S = Synthesis phase: chromosome duplication, DNA synthesis.
G2 = Gap 2 phase: growth and preparation for M phase.

M = Mitotic phase: mitosis + cytokinesis (cell division).

Between S and M two phases G1 and G2 cells prepare for next stage, G0 non-dividing stage.

Question 4

Why are cells in G0 not able to re-enter the cell cycle? (3)

Answer 4

o Cell cycle re-entry not possible (e.g. nerve cells).
o Maintained in G0 unless stimulated to divide (e.g. hepatocytes-liver cells).
o Constantly in the cell cycle (e.g. epithelial cells of the gut, haematopoietic cells [blood forming cells- stem cells that continue to replicate as fresh blood is needed continuously] in the bone marrow).

Question 5

How can change in DNA content be detected in cell cycle? Change? (2)

Answer 5

• Fluorescence activated cell sorter (FACS) analysis.

* In G1 phase 2N- diploid state, in M phase 4N state as chromosome has been duplicated.

Question 6

M phase (nuclear division) Name and describe phases (7)

Answer 6

Prophase
• Chromosomes condense, centrosomes move to opposite poles, mitotic spindle forms.
Prometaphase
• Breakdown of nuclear envelope, chromosomes attach to mitotic spindle.
Metaphase
• Centrosome are at opposite poles, chromosomes are at their most condensed and line up at the equator of the mitotic spindle.
Anaphase
• Sister chromatids separate synchronously, each new daughter chromosome moving to the opposite spindle pole.
Telophase
• Chromosome arrives at the spindle poles, chromosomes decondense, nuclear envelope reforms.

Cytokinesis: cytoplasmic division
• At the position of the metaphase plane.
• Contractile ring of actin and myosin II constrict the cell into two daughter cells.

Question 7

M phase - Prophase (3)

Answer 7

Chromosomes condense.
Centrosome moving to opposite poles.
Mitotic spindle forms.

Question 8

M phase - Prometaphase (2)

Answer 8

Chromosomes attach to mitotic spindle.

Nuclear envelope degrades.

Question 9

M phase - Metaphase (3)

Answer 9

Centrosome at opposite poles.
Chromosome in its most condensed form.
Line up at equator of mitotic spindle.

Question 10

M phase - Anaphase (2)

Answer 10

Sister chromatids separate synchronously.

Each new daughter chromosome moving to opposite spindle pole.

Question 11

M phase - Telophase (3)

Answer 11

Nuclear envelope reforms.
Chromosomes decondense.
Chromosomes arrive at spindle poles.

Question 12

M phase - Cytokinesis (2)

Answer 12

At position of metaphase plane.

Contractile ring of actin and myosin II constrict the cell in to two new daughter cells.

Question 13

Mitotic spindle (5)

Answer 13

“Bipolar” array of microtubules – they have a plus (growing) end and a minus end (shrinking).
Start to assemble during prophase from the centrosomes at each pole.
Attach to the chromosomes via the kinetochore (a large protein structure assembled on the centromere).
Pull apart the sister chromatids.
3 types of spindle microtubules.

Question 14

Types of spindle microtubules (3)

Answer 14

o Astral microtubules
o Kinetochore microtubules
o Interpolar microtubules

Question 15

Kinetochore (1)

Answer 15

A protein structure formed on a chromatid, where the spindle fibers attach to pull the chromatids apart during cell division.

Question 16

Centromere (1)

Answer 16

A part of the chromosome connected to the spindle fibre.

Question 17

Chromatids (1)

Answer 17

The two chromosomes that have been replicated and linked through the centromere.

Question 18

Centrosome cycle (4)

Answer 18

Microtubule-organising centre in somatic animal cells.
Centrosome consists of a pair of centrioles surrounded by pericentriolar matrix (a cloud of amorphous material (doesn’t have a fixed form)).
Duplicated during interphase.
Migrate to opposite poles in preparation for M phase.

Question 19

Cytokinesis (5)

Answer 19

• Final step in the cell cycle.
• Divides the cytoplasm into two daughter cells.
• Contractile ring
o Cytoskeletal structure composed of actin and myosin bundles.
o Accumulates between the poles of the mitotic spindle beneath the plasma membrane.
o Ring contracts and forms an indentation or cleavage furrow, dividing the cell in two.

Question 20

Cell organelle distribution in new daughter cells (2)

Answer 20

• All cell organelles must be redistributed between the 2 new daughter cells.
• Cell organelles cannot spontaneously regenerate so must be already present in the new daughter cells.

Question 21

Meiosis - Overview (6)

Answer 21

• Four cells
• Haploid (1n) DNA
• Cell divides twice
• Homologous recombination occurs
• No exchange of DNA sequences between tightly linked chromosomes
• “chiasm” structure allows exchanges of DNA between father and mother.

Question 22

Mitosis - Overview (4)

Answer 22

• Two cells
• Diploid (2n) DNA
• Cell divides once
• No recombination between homologous chromosomes.

Question 23

Regulation of the cell cycle (4)

Answer 23

Strictly controlled.
Phase must only occur once.
Phases in correct order: G1-S-G2-M.
Phases must be non-overlapping.

Question 24

Checkpoints in the cell cycle (names and questions) (4,3,2 = 9)

Answer 24

G1
Check for DNA damage?
Check extracellular environment?
Growth factors? Mitogenic signals (encourages cell to undergo division)?

G2
Check for DNA damage?
Has DNA been replicated properly?

Metaphase
Are all chromosomes aligned on the mitotic spindle?

Question 25

Cell cycle regulators (3)

Answer 25

Cdks (Cyclin dependent kinases) - enzymes that phosphorylate the target proteins.
Activated when bound to cyclin (regulator of Cdk).
Different cyclins produced at each phase of the cell cycle.

Question 26

Cell cycle control - Yeast (3)

Answer 26

A single Cdk binds successively to different cyclins.
During S phase – S-cyclin binds and triggers S phase after it is degraded at the end. Free Cdk waits for next cyclin (M-cyclin) triggering M phase.

Question 27

Cell cycle control - Humans (5)

Answer 27

Different classes of Cdk and cyclins act at each stage of the cell cycle.
Cdk2/Cyclin A - Works through G1, S and G2.
Cdk1/Cyclin B – Ensures the transition of G2 and M phase.
Cdk4/Cyclin D – Acts through G1 to S phase, initiate S phase.
Cdk2/Cyclin E - Start emerging at the end of G1 phase, work in S phase, work to ensure the smooth transition of G1 to S phase.

Question 28

Cell cycle control basic principles - Cdk (5)

Answer 28

• Cdk levels fairly stable throughout the cell cycle.
• Cyclin levels vary as part of the cell cycle.
• Cdk bound to cyclin is active and can phosphorylate “target protein”.
• Cdk activation triggers the next step in the cell cycle such as entry into S phase or M phase.
• Cyclin degradation terminates Cdk activity.

Question 29

G1 checkpoint - Entry into the cell cycle (9)

Answer 29

Check for DNA damage?
Check extracellular environment?
Growth factors/ Mitogenic signals (encourages cell to undergo division) –> Induction/Expression of Cyclin D binds to Cdk 4 –> phosphorylation of pRB –> release and activates E2F –> promotes transcription of genes required for G1/S phase progression –> Cyclin A and Cyclin E.

Question 30

G1 checkpoint - DNA damage repair (6)

Answer 30

o Normally p53 degraded quickly, unstable and maintained at very low levels.
o Phosphorylated (active) p53 is not degraded.
o Active p53 promotes transcription of genes that induce cell cycle arrest, it binds to promoter region of p21 gene and stimulates p21 expression.
o p21 binds and inhibits G1/S-Cdk and S-Cdk complexes.
o Cell arrests in G1 (allowing time to repair the damaged DNA).
o If DNA repair is not possible→ cells undergo apoptosis.

Question 31

Cyclin dependent kinase inhibitors (CKIs) (2)

Answer 31

• Two families of CKIs
o Inhibitor of Kinase 4 family (INK4): specifically inhibit G1 CDKs (e.g. CDK4)
o CDK Inhibitory Protein/Kinase Inhibitory Protein (CIP/KIP) family: inhibit all other CDK-cyclin complexes (late G1, G2 & M) gradually sequestered by G1 CDKs thus allowing activation of later CDKs.

Question 32

Cell cycle and cancer (4)

Answer 32

• Neoplasia - The presence or formation of new, abnormal growth of tissue.
• Mis-regulation of cell cycle causes cancer.
• Cells escape normal cell cycle checkpoint -> uncontrolled progression through the cell cycle.
• Many genes that regulate cell cycle (e.g. p53 and pRB) are often mutated in human cancers.

Question 33

Symptoms predisposing to cancer - DNA repair defects (6)

Answer 33

Ataxia telangiectasia.
Bloom's syndrome.
Fanconi's anaemia.
Li-Fraumeni syndrome.
Lunch type II (HNPCC).
Xeroderma pigmentosum.

Question 34

Symptoms predisposing to cancer - Chromosomal instability (2)

Answer 34

Down’s syndrome.

Klinefelter’s syndrome.

Question 35

DNA damage (2)

Answer 35

• DNA damage may be recognized and repaired.

* Failure to repair the damage results in a mutation, which cannot be repaired.

Question 36

DNA damage - Mismatched bases (3) = Source (1) Repair System (1) Inherited defects (1).

Answer 36

Replication errors, particulary in repeat regions.
Strand-drected mismatch repair (MMR, degrades the new strand with nicks).
Hereditary Nonpolyposis Colon Cancer (HNPCC).

Question 37

DNA damage - Chemical modification of bases (3) = Source (1) Repair System (1) Inherited defects (1).

Answer 37

Chemicals (alkylating agents, oxidative DNA damage, free radicals- may be exogenous or endogenous).
Base Excision Repair (BER).
Colorectal and gastric cancers.

Question 38

DNA damage - Bulky DNA adducts (3) = Source (1) Repair System (1) Inherited defects (1).

Answer 38

Intercalating chemicals (benzo[a]pyrene), pyrimidine dimers (UVB light).
Nucleotide excision repair.
Xeroderma Pigmentosum.

Question 39

DNA damage - Double-strand breaks (3) = Source (1) Repair System (1) Inherited defects (1).

Answer 39

Ionising radiation (radioactive decay, cosmic rays, X-Rays).
Homologous recombination (HR) and non-homologous end joining (NHEJ).
Bloom's syndrome, BRCA1 and BRCA2 defects, translocations.

Question 40

How many kinetochores are there in a human cell at mitosis? (4)

Answer 40

Human diploid genome contains 23 pairs of homologous chromosomes (2N, N=23, thus 46 in total).
During mitosis, DNA duplication occurs, and this becomes 4N.
There is one kinetochore per each chromatid.
Therefore, total number of kinetochores in a mitotic cell is 4 x 23 = 92.

Question 41

TRUE/FALSE and why ==> The regulation of cyclic-cdk complexed depends entirely on phosphorylation and dephosphorylation. (3)

Answer 41

FALSE – not ENTIRELY dependent on these factors, below also affect this:
Cdk activation triggers the next step in the cell cycle such as entry into S phase or M phase.
Cyclin degradation terminates Cdk activity.