Cell Cycle, Cell Division Apoptosis

Question 1

Karyotype

Answer 1

Observed appearance of a set of homologous chromosomes during metaphase, based on banding pattern after staining (such as G banding after Giemsa staining).
Each chromosome has a distinct and recognizable banding pattern, with a different position of the centromere (“waist” of chromosome).
Humans have 22 pairs autosomal chromosomes plus sex chromosomes (XX or XY).
Diploid genome in humans is 46 chromosomes (2n)
Haploid genome (germline: egg or sperm) has 23 chromosomes (1n).

Question 2

Mitosis

Answer 2

Duplication of DNA genome, and generated of two identical daughter cells from somatic cell (2x2n).

Question 3

Go

Answer 3

Resting phase; non-proliferating, fully differentiated cell.

Question 4

Cyclin:cdk (cyclin dependent kinase)

Answer 4

Stimulates progression through the cell cycle at different transitions (G1/S, S/G2, G2/M). Cyclins are active for a transient time periods, protein levels are regulated by proteolytic degradation.
Cyclin:Cdk complex will phosphorylate proteins required for progression through the cell cycle.

Question 5

G1 (Gap 1)

Answer 5

• Variable in time; cell goes through active metabolism.
• Entrance into G1 is first step of cell cycle in response to growth factors and signals for cell proliferation.
• DNA repair may take place in response to elevation of p53 levels (DNA damage, cytotoxic agents). Requirement to pass the R (restriction point), prior to commitment to S-phase. When cell commits to division, cyclin D accumulates and interacts with Cdk4 and Cdk6.

Question 6

G1-S

Answer 6

• Initiation of centrosome duplication in response to cyclin E/cdk2.
• Stimulation of cyclin E (trigger for DNA synthesis)
• Checkpoint proteins are activated (p53, ATM), cell cycle will pause until DNA is repaired prior to entry into S-phase.

Question 7

S

Answer 7

Synthesis phase, DNA replicaiton
G1/S checkpoint restricts progession (p53 levels are elevated)
Phosphorylation of Rb protein resets the rheostat to allow entry into S-phase

Question 8

GS

Answer 8

• Gap phase prior to mitosis.
• Synthesis of cyclin B, regulatory subunit of CDK1 complex -> induces entry into mitosis.
• Breakdown of nuclear envelope and disassembly of nuclear lamina (degraded) precede formation of mitotic spindles and chromosome segregation. Process involves hyperphosphorylation of nuclear proteins medicated by lamin kinases such as CDK1 and protein kinase C (PKC).
• Chromosome condensation and nucleosome reorganization is associated with phosphorylation at specific sites in histone proteins.
• Double strand breaks are repaired by homologous recombination, using sister chromatid as template. Repair precedes mitosis.
• Centrioles will acquire appendages used for anchoring microtubules. Two mature centrioles are formed at end of G2.

Question 9

Tumor Suppressor Genes (gate keepers)

Answer 9

p53 and Rb
Proteins encoded by these genes inhibit cell proliferation and cell cycle progression.
Loss of function requires inactivating mutations in both alleles (recessive mutations).

Question 10

Retinoblastoma (Rb) Tumor Suppressor

Answer 10

Regulates the transition from G1 to S phase in mammalian cells. Unphosphorylated form of Rb is active

In differentiated cell at Go, RB binds to E2F (transcription factor family) resulting in growth arrest and transcription repression.
In response to growth factor stimulation (cyclin:cdk), Rb is phosphorylated causing the release of E2F.

Question 11

E2F

Answer 11

E2F (family of transcription factors) allows transcription activation of genes involved with DNA synthesis (e.g., DNA polymerase, thymidine kinase) or other S-phase genes.
-> Allows progression through the cell cycle.

Question 12

Mutation of Rb gene

Answer 12

Loss of function results in pediatric eye cancer.
Loss of phosphorylation control or interaction with viral oncoproteins can result in other types of cancer, due to loss of cell cycle control.
Any defect in the ability of RB to bind to E2F will result in constitutive activation of DNA synthesis and uncontrolled proliferation.
Loss of function was discovered by loss of Rb gene known as “loss of heterozygosity.”
Observed by comparing normal with tumor tissue, or by karyotyping in which the q arm of chromosome 13 will be shorter (due to deletion).

Question 13

p53 tumor suppressor protein

Answer 13

Accumulation in response to DNA damage -> activates transcription of kip (kinase inhibitory proteins) such as p21 to restrain cyclin E/cdk2 complex formation. G1 will be longer.

Question 14

P21

Answer 14

Will inhibit phosophorylation of Rb

Question 15

Apoptosis

Answer 15

Will be triggered if DNA repair is inadequate

Question 16

Centrosome duplication cycle

Answer 16

Late G1- physical separation of paired centrosome is coupled to initiation of duplication of DNA in response to cyclin E-cdk2.
G2, mother centriole acquires appendages.
Each daughter cell will receive one centrosome after cytokinesis.

Question 17

Trisomy 13

Answer 17

Patau syndrome

Question 18

Trisomy 18

Answer 18

Edwards syndrome

Question 19

Other trisomies

Answer 19

usually result in miscarriages

Question 20

Turner’s Syndrome

Answer 20

XO, only one X chromosome is present

Question 21

Caspases

Answer 21

Characterized by shrunken cytoplasm, chromatin condensation and DNA fragmentation. Phosphatidylserine is exposed on the outer surface of the cell membrane.

Proteases that cleave next to aspartate residues; activated by proteolytic cleavage.
Initiator caspases are activated by either death receptors pathway or mitochondrial pathway.

Question 22

Extrinsic pathway

Answer 22

Binding of ligands which activates a set of receptors to initiate activation of pro-caspases.

Question 23

Intrinsic pathway (mitochondrial)

Answer 23

Induced by intracellular signals such as low oxygen, low intracellular calcium. Cytochrome C release initiates the mitochondrial pathway. Cytochrome C binding to a pro-apoptotic protease activating factor (Apaf) will cause formation of apoptosome complex and activation of initiator caspases.

When more pro-apoptotic than anti, and balance is disturbed, that is when the pathway is triggered

Question 24

Necrosis

Answer 24

Pathological type of premature death, which results in inflammation.
Does not follow a programmed biochemical pathway.

Question 25

Cyclins + Cyclin Dependent Kinases (Cyclin+Cdk)= Active complex

Answer 25

• Responds to growth factors
• Phosphorylate regulatory protiens
• Stimulate cell cycle progression at transition points (G1/S, S/G2, G2/M)

Question 26

Cyclin Dependent Kinase Inhibitors (CKI)

Answer 26

• Bind to cylcin-CDK complex
• Slow cell cycle progression
• INK family (inhibitors of CDK4 and CDK6)
• KIP family (kinase inhibitory proteins)

Question 27

p16

Answer 27

Blocks the phosphorylation of Rb

Question 28

CDK-2

Answer 28

Centromeres are duplicated in response to CDK2

Question 29

Binding of cyclin B to Cdk1 complex

Answer 29

Mitosis promoting factor (MPF). MPF is regulated by phosporylation-dephosphorylation

Allows entrance into mitosis after breakdown of nuclear envelope

Question 30

Degradation of cyclin B

Answer 30

Inactivation of Cdk-1 kinase and exit from mitosis

Question 31

High levels of p53

Answer 31

Cell will arrest and apoptosis is induced

Question 32

Synapsis

Answer 32

Pairing of homologous chromosomes

Cross-over exchange occurs randomly along a chromosome

Question 33

Purpose of Mitosis and Meiosis

Answer 33

Preserve the integrity of the genome
Preserve a constant chromosome number
Preserve the progeny from one generation to another

Question 34

Extrinsic pathway

Answer 34

Causes conformational change. Casphase 8 stimulates Casphase 3 which causes destruction of cellular proteins

Question 35

Death receptor

Answer 35

TNF binds to death receptor which triggers conformational change. Caspase 8 is activated and stimulates Caspase 3

Question 36

Apoptosis

Answer 36

Extrinsic: triggered by binding of ligand to death receptor

Intrinsic: triggered by damage to mitochondria
Imbalance of Bcl-2 family of proteins
Both pathways are stimulated by proteases called caspases