Cell Signalling Pathways Questions and Answers

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.

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.

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.

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.

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.