Cell Signalling Pathways Questions and Answers

Q: In what way does the ras oncogene contribute to cancers ?

The ras oncogene acts because ras codes for a GTPase switch protein, which in its mutated form cannot be switched off.Ras is a small G protein that relays signals from growth factor receptors to the MAP kinase pathway. It is on when bound to GTP and turns itself off by hydrolysing GTP to GDP. Point mutations, commonly at codons 12, 13 or 61, block this hydrolysis, so Ras stays in the GTP-bound state and keeps telling the cell to divide even without a growth signal. Mutant ras is found in many cancers, including about 90 per cent of pancreatic cancers.An overproduced anti-apoptotic protein describes bcl-2, an overproduced transcription factor describes myc, and a truncated, always-active growth factor receptor describes erbB. None is how ras works.

Q: Rous sarcoma virus, which induces the formation of sarcomas in chickens. Which of the following is the product of oncogene V-src ?

The product of v-src is a tyrosine kinase.The src gene of Rous sarcoma virus was the first oncogene discovered. Its product, pp60v-src, is a non-receptor protein kinase that adds phosphate groups to tyrosine residues of target proteins. In the viral form it is permanently active, so it keeps growth signals switched on and transforms the cell.It is not a growth factor such as EGF (the sis oncogene encodes a growth factor, PDGF). Tyrosinase is the enzyme of melanin synthesis, which has nothing to do with oncogenes, and serine kinases such as Raf phosphorylate serine and threonine, not tyrosine.

Q: As part of the circadian clock, transfer of phosphate groups from adenosine triphosphate (ATP) to PER is required. This is achieved by one of the following :

PER is phosphorylated by Caesin kinase 1 (casein kinase 1).In the circadian clock, PERIOD proteins accumulate and are progressively phosphorylated by casein kinase 1: Doubletime (DBT) in Drosophila, CK1ε and CK1δ in mammals. The phosphorylation times PER's degradation and its entry into the nucleus, and so sets the length of the cycle. The hamster tau mutation, which shortens the period, is a CK1ε mutation.Casein kinase 2 also phosphorylates clock proteins, but CK1 is the kinase textbooks name for PER. PER is phosphorylated on serines and threonines, so tyrosine kinases are not involved.

Q: Which one of the following pairs execute their functions by binding with cytoplasmic receptors ?

The pair that acts through cytoplasmic receptors is estrogen and progesterone.Both are lipid-soluble steroid hormones. They cross the plasma membrane and bind intracellular receptors of the nuclear receptor family, found in the cytoplasm or nucleus. The hormone-receptor complex binds hormone response elements in DNA and changes gene transcription.Each wrong pair contains a hormone that cannot cross the membrane and so uses a cell-surface receptor. Noradrenaline acts on G-protein coupled adrenergic receptors, growth hormone on a cytokine-type receptor linked to JAK2, and GABA on GABA-A ion channels and GABA-B receptors.

Q: One of the signalling molecules used for bacterial cell-cell communication are :

The bacterial signalling molecule is N-acyl homoserine lactone.Gram-negative bacteria such as Vibrio fischeri and Pseudomonas aeruginosa use acyl homoserine lactones (AHLs) for quorum sensing. A LuxI-type synthase makes the AHL, which diffuses across membranes; when the population is dense enough, the AHL reaches a threshold, binds a LuxR-type regulator, and switches on genes for bioluminescence, virulence or biofilm formation.The other names are distortions of the real one. A laccase is a copper-containing oxidase, not a signal, so both 'laccase' options are wrong; and the acyl chain, not an acetyl group, is the variable part that gives each species its own signal, which rules out 'N-acetyl homoserine lactone'.

Q: Which of the following molecules plays a pivotal role in presenting the paracrine factors to their target cells ?

The molecule is heparan sulphate (printed 'Heparin sulphate').Heparan sulphate proteoglycans on cell surfaces and in the extracellular matrix bind paracrine factors such as FGFs, Wnts and Hedgehog proteins. They concentrate these factors, control how far they spread, and present them to their receptors; FGF signalling, for example, needs heparan sulphate to form an active receptor complex.Fibronectin, laminin and collagen are extracellular matrix proteins concerned with cell adhesion, migration and structural support. They do not have the central role in presenting paracrine factors.

61.

In what way does the ras oncogene contribute to cancers ?

A.ras codes for anti-apoptotic protein, which is produced in abnormally large amounts
B.ras codes for a GTPase switch protein, which in its mutated form cannot be switched off.
C.ras codes for a transcription factor, which is produced in abnormally large amounts.
D.ras codes for a truncated form of a growth factor receptor, which is continually active.

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

The ras oncogene acts because ras codes for a GTPase switch protein, which in its mutated form cannot be switched off.

Ras is a small G protein that relays signals from growth factor receptors to the MAP kinase pathway. It is on when bound to GTP and turns itself off by hydrolysing GTP to GDP. Point mutations, commonly at codons 12, 13 or 61, block this hydrolysis, so Ras stays in the GTP-bound state and keeps telling the cell to divide even without a growth signal. Mutant ras is found in many cancers, including about 90 per cent of pancreatic cancers.

An overproduced anti-apoptotic protein describes bcl-2, an overproduced transcription factor describes myc, and a truncated, always-active growth factor receptor describes erbB. None is how ras works.

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62.

Which one of the following statements is incorrect ? Calcium ions are necessary for :

A.muscle contraction.
B.release of neurotransmitter from the presynaptic terminal.
C.propagation of action potential in a neuron.
D.mediating the action of several second messengers.

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The incorrect statement is that calcium is necessary for the propagation of action potential in a neuron.

A nerve impulse travels along the axon by the opening of voltage-gated sodium channels, which depolarise the membrane, followed by voltage-gated potassium channels, which repolarise it. Calcium ions are not needed for this conduction along the axon.

The other roles are genuine. Calcium binds troponin to allow muscle contraction. Its entry into the presynaptic terminal triggers vesicle fusion and the release of neurotransmitter. And it is itself a second messenger, released for example by IP₃, mediating the action of other signals through proteins such as calmodulin.

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63.

Rous sarcoma virus, which induces the formation of sarcomas in chickens. Which of the following is the product of oncogene V-src ?

A.Epidermal Growth Factor (EGF)
B.Tyrosine Kinase
C.Tyrosinase
D.Serine Kinase

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The product of v-src is a tyrosine kinase.

The src gene of Rous sarcoma virus was the first oncogene discovered. Its product, pp60^v-src, is a non-receptor protein kinase that adds phosphate groups to tyrosine residues of target proteins. In the viral form it is permanently active, so it keeps growth signals switched on and transforms the cell.

It is not a growth factor such as EGF (the sis oncogene encodes a growth factor, PDGF). Tyrosinase is the enzyme of melanin synthesis, which has nothing to do with oncogenes, and serine kinases such as Raf phosphorylate serine and threonine, not tyrosine.

Discuss in Forum

64.

As part of the circadian clock, transfer of phosphate groups from adenosine triphosphate (ATP) to PER is required. This is achieved by one of the following :

A.Caesin kinase 1
B.Caesin kinase 2
C.Tyrosine kinase 1
D.Tyrosine kinase 2

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: A

PER is phosphorylated by Caesin kinase 1 (casein kinase 1).

In the circadian clock, PERIOD proteins accumulate and are progressively phosphorylated by casein kinase 1: Doubletime (DBT) in Drosophila, CK1ε and CK1δ in mammals. The phosphorylation times PER's degradation and its entry into the nucleus, and so sets the length of the cycle. The hamster tau mutation, which shortens the period, is a CK1ε mutation.

Casein kinase 2 also phosphorylates clock proteins, but CK1 is the kinase textbooks name for PER. PER is phosphorylated on serines and threonines, so tyrosine kinases are not involved.

Discuss in Forum

65.

Which one of the following pairs execute their functions by binding with cytoplasmic receptors ?

A.Estrogen and noradrenaline
B.Growth hormone and progesterone
C.Growth hormone and GABA
D.Estrogen and progesterone

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

The pair that acts through cytoplasmic receptors is estrogen and progesterone.

Both are lipid-soluble steroid hormones. They cross the plasma membrane and bind intracellular receptors of the nuclear receptor family, found in the cytoplasm or nucleus. The hormone-receptor complex binds hormone response elements in DNA and changes gene transcription.

Each wrong pair contains a hormone that cannot cross the membrane and so uses a cell-surface receptor. Noradrenaline acts on G-protein coupled adrenergic receptors, growth hormone on a cytokine-type receptor linked to JAK2, and GABA on GABA-A ion channels and GABA-B receptors.

Discuss in Forum

66.

One of the signalling molecules used for bacterial cell-cell communication are :

A.N-acetyl hydroxy laccase
B.N-acyl homoserine laccase
C.N-acetyl homoserine lactone
D.N-acyl homoserine lactone

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The bacterial signalling molecule is N-acyl homoserine lactone.

Gram-negative bacteria such as Vibrio fischeri and Pseudomonas aeruginosa use acyl homoserine lactones (AHLs) for quorum sensing. A LuxI-type synthase makes the AHL, which diffuses across membranes; when the population is dense enough, the AHL reaches a threshold, binds a LuxR-type regulator, and switches on genes for bioluminescence, virulence or biofilm formation.

The other names are distortions of the real one. A laccase is a copper-containing oxidase, not a signal, so both 'laccase' options are wrong; and the acyl chain, not an acetyl group, is the variable part that gives each species its own signal, which rules out 'N-acetyl homoserine lactone'.

Discuss in Forum

67.

Which of the following molecules plays a pivotal role in presenting the paracrine factors to their target cells ?

A.Fibronectin
B.Heparin sulphate
C.Laminin
D.Collagen

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: B

The molecule is heparan sulphate (printed 'Heparin sulphate').

Heparan sulphate proteoglycans on cell surfaces and in the extracellular matrix bind paracrine factors such as FGFs, Wnts and Hedgehog proteins. They concentrate these factors, control how far they spread, and present them to their receptors; FGF signalling, for example, needs heparan sulphate to form an active receptor complex.

Fibronectin, laminin and collagen are extracellular matrix proteins concerned with cell adhesion, migration and structural support. They do not have the central role in presenting paracrine factors.

Discuss in Forum

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