Asked by: Tom England, Liverpool
Bananas produce ethylene gas (C2H4), which acts as a plant hormone. Plants have genes called ETR1 and CTR1 that regulate lots of other genes involved with growth, ageing and cell death. When ethylene gas is present, ETR1 and CTR1 are shut off, which allows the other genes to swing into action.
Some fruit plants use this mechanism to control the sequence of cellular changes in their ripening process. Bananas actually only produce moderate levels of ethylene but apples, pears and melons are so sensitive to the hormone that it has a powerful effect on their ripening.