Section A: Step by step flashcards

Question 1

What are the main types of signal sequences used in protein targeting?

Answer 1

NLS (nucleus), MTS (mitochondria), PTS1/PTS2 (peroxisomes), Signal peptide (ER)

Signal sequences are essential for directing proteins to their correct cellular locations.

Question 2

What is the step-by-step process of SRP-mediated ER translocation?

Answer 2

1. Signal peptide emerges during translation
2. SRP binds and halts translation
3. SRP–ribosome complex docks at SRP receptor on ER
4. Ribosome binds Sec61 translocon
5. Translation resumes into ER lumen
6. Signal peptide is cleaved, protein folds in ER

SRP stands for Signal Recognition Particle, crucial for targeting proteins to the ER.

Question 3

What are the key steps of N-linked glycosylation in the ER?

Answer 3

1. Oligosaccharide assembled on dolichol phosphate
2. Transferred en bloc to Asn residue (Asn-X-Ser/Thr)
3. Glucose trimmed → calnexin binding and quality control
4. Proper folding → exits to Golgi
5. Golgi modifies glycans (complex/hybrid types)

N-linked glycosylation is vital for protein folding and stability.

Question 4

Describe COPII vesicle formation.

Answer 4

1. Cargo proteins gather at ER exit sites
2. Sar1-GTP inserts into membrane
3. Sec23/24 complex recruited
4. Sec13/31 forms outer coat
5. Vesicle buds off and uncoats before reaching Golgi

COPII vesicles are essential for transporting proteins from the ER to the Golgi apparatus.

Question 5

What is the step-by-step role of SNAREs in vesicle fusion?

Answer 5

1. Rab GTPase on vesicle engages tethering protein
2. v-SNARE (vesicle) pairs with t-SNARE (target membrane)
3. SNARE complex forms and draws membranes together
4. Membranes fuse, releasing cargo
5. SNAREs disassembled by NSF and α-SNAP

SNARE proteins are critical for the fusion of vesicles with target membranes.

Question 6

What are the steps of MAPK signalling from RTK to the nucleus?

Answer 6

1. Growth factor binds RTK → dimerisation and autophosphorylation
2. Grb2 binds RTK, recruits SOS
3. SOS activates Ras (GDP → GTP)
4. Ras activates Raf → MEK → ERK
5. ERK enters nucleus → activates gene expression (e.g., Cyclin D, Myc)

MAPK signalling is important for cell growth and differentiation.

Question 7

How does PI3K–Akt signalling promote survival?

Answer 7

1. PI3K converts PIP2 → PIP3
2. PIP3 recruits PDK1 and Akt
3. PDK1 phosphorylates Akt → activation
4. Akt inhibits pro-apoptotic factors (BAD, FOXO)
5. Akt activates mTOR → promotes growth and protein synthesis

PI3K-Akt pathway is a key player in cell survival and metabolism.

Question 8

Explain the sequence leading to G1/S transition.

Answer 8

1. Mitogenic signals induce Cyclin D
2. Cyclin D binds CDK4/6 → phosphorylates Rb
3. Rb releases E2F → S-phase gene transcription
4. Cyclin E–CDK2 reinforces S-phase entry
5. Positive feedback increases CDK activity

The G1/S transition is a critical checkpoint in the cell cycle.

Question 9

How do CDK inhibitors regulate the cell cycle?

Answer 9

1. CKIs (e.g., p21, p27) bind to Cyclin–CDK complexes
2. Block Rb phosphorylation → halts G1/S progression
3. Akt signalling can inhibit CKIs
4. Cyclin E–CDK2 suppresses CKIs further (positive feedback)

CDK inhibitors are essential for regulating cell cycle progression and preventing uncontrolled cell division.

Question 10

Name 3 model organisms and their unique advantages.

Answer 10

1. Yeast: simple genetics, secretion pathways
2. C. elegans: apoptosis, transparent body, RNAi by feeding
3. Drosophila: gene expression control, developmental studies

Model organisms are widely used for genetic and developmental studies due to their unique characteristics.

Question 11

What are the steps of RNA interference (RNAi)?

Answer 11

1. Double-stranded RNA (dsRNA) introduced or expressed
2. Dicer cleaves dsRNA into siRNAs
3. siRNA loaded into RISC complex
4. RISC guides siRNA to complementary mRNA
5. mRNA is cleaved and degraded

RNA interference is a powerful tool for gene silencing.

Question 12

Describe the mTOR pathway’s role in ageing.

Answer 12

1. Nutrient signals activate mTOR
2. mTOR promotes protein synthesis, inhibits autophagy
3. Caloric restriction or rapamycin inhibits mTOR
4. mTOR inhibition → increased autophagy, extended lifespan

The mTOR pathway is a central regulator of growth and metabolism, influencing ageing and longevity.

Question 13

What are the main causes of cellular ageing?

Answer 13

1. Telomere shortening → senescence
2. DNA damage accumulation
3. Mitochondrial dysfunction and ROS
4. Loss of proteostasis → protein aggregation
5. Stem cell exhaustion

Cellular ageing is a complex process influenced by multiple factors.

Question 14

What are the steps of caloric restriction’s impact on longevity?

Answer 14

1. Reduced nutrient intake triggers AMPK activation
2. Inhibits mTOR → increases autophagy
3. Enhances FOXO and SIRT1 activity
4. Reduces inflammation, oxidative stress
5. Extends lifespan in model organisms

Caloric restriction is linked to increased lifespan and healthspan in various species.

Question 15

List the steps of the proteasome degradation process.

Answer 15

1. Protein marked by polyubiquitin (E1, E2, E3)
2. Recognised by 26S proteasome
3. Protein is unfolded and translocated into core
4. Degraded into peptides using ATP
5. Peptides recycled to amino acids

The proteasome is crucial for protein turnover and cellular homeostasis.

Question 16

What are the sequential steps in autophagy?

Answer 16

1. Stress or starvation activates ULK1 complex
2. Phagophore formation begins
3. Expansion driven by ATG proteins
4. Cargo enclosed → autophagosome forms
5. Fuses with lysosome → autolysosome
6. Contents degraded and recycled

Autophagy is a key cellular process for recycling and degrading damaged organelles and proteins.