Watching Hopeful Monsters, Horizon's film (BBC2) about recent advances in genetic science, offered the intellectual equivalent of skating on thin ice. The sense of rapid forward motion was exhilarating, but it was difficult to shake the apprehension that it was only forward motion that was keeping you out of trouble. Every now and then I remembered that I might have to offer a paraphrase of what I thought I had just understood, and there was an ominous glacial creaking. I don't mean by this that it was a bad programme - it was actually very elegant - but the complexity of molecular biology is such that any real solidity of explanation would take a series rather than a single film. Because this was a single programme, and because it concerned a development that more than justified the mythical hyperbole (the scientists involved had "cracked open nature's great mysteries") it was almost obliged to shape its material as a graspable story - and the most accessible story for science programmes is still that of personal triumph. The person in this case was Mike Levine, Professor of Molecular Biology at the University of California in Berkeley, and one can hardly blame the film-makers for their selection, because he was captivating, recounting the progress from eureka moment (when he saw a picture of a fruit-fly with perfectly formed legs where its antenna should be) to the sinking realisation that his finest work was probably now behind him. I warmed to him immediately when he grumbled about the atmosphere of "social oppression" in Switzerland and described an anonymous colleague as a "schmuck" but, given that he was just one of several signatories to the paper which had described the breakthrough, it seemed likely that he wasn't the only person who could have explained it. I doubt if anyone else would have dramatised the matter with such unselfconscious zeal, though. His eyes shone as he sketched in the magnitude of his shared discovery.

And what I thought I understood was this: virtually every living thing on the planet shares a set of supervisory genes, which act as regulators for hierarchical cascades of other genes. This genetic gearing system means that relatively small changes can have substantial effects in a body plan, allowing evolution to move much faster than had previously been assumed. It also means that a chemical messenger that operates in the construction of fruit-fly bodies can have astounding medical applications in humans. Administered in one spot it will promote bone growth, in another it will transform ordinary brain cells into dopamine-producers, offering the prospect of a cure for Parkinson's disease. The ice groaned very loudly here - it wasn't at all clear to me why you wouldn't inadvertently turn the Parkinson's sufferer into a bone-head - but the sense of being close to the edge of bafflement probably boosted the general thrill.

Silent Witness, (BBC1) back for a new series, opened with a molecular model of DNA, a kind of logo for scientific braininess which then neatly tied in to a lecture our heroine was giving about the fallibility of DNA evidence. Someone should write a thesis about the lecture scene in television drama - they are rarely very credible, but writers keep returning to them because they're such an easy way to underline the theme of a story or to deliver information.And, naturally, the story that followed was about fallibility - a two-part tale in which Sam Ryan finds that the murder victim she is called to examine is a close friend. Cue much agonising about professional detachment and posthumous fidelity and cue also a dangerous number of dramatic coincidences. Cambridge is a small place and collegiate Cambridge is even smaller, but even so there is something a little too convenient in the way that Ryan keeps bumping into people involved in the case she's working on. Shortly after she had left her last meeting with the murder victim, for example, a man staggered across her field of vision drenched in blood.

Had he been wearing a T-shirt saying "I'm an obvious suspect but don't jump to any hasty conclusions" the instructions to the viewer could not have been clearer.