Discussion
In what way does the ras oncogene contribute to cancers ?
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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