Sequencing methods are continuously updated and have become automated.

The idea

Early sequencing was slow and done largely by hand: the chain-termination method read a few hundred bases at a time, and the first human genome took more than a decade and enormous expense. Modern high-throughput methods are automated. The DNA is broken into very large numbers of small fragments, these are sequenced in parallel by machines that detect fluorescently labelled bases as each is added, and computer software then finds the overlaps between reads and assembles them back into the complete sequence. The consequences matter more than the chemistry: whole genomes can now be sequenced in hours rather than years, at a tiny fraction of the original cost, and with fewer human errors because the reading is done by machine.

What earns the marks

• Sequencing now being automated, carried out by machines rather than by hand.
• The DNA being cut into many fragments that are sequenced at the same time and reassembled by computer.
• The consequence being that sequencing is faster and cheaper, with fewer errors.
• An application made possible, such as DNA or base sequencing evidence leading to species being reclassified or renamed.
• Whole genomes of individuals being sequenced, allowing screening and personalised medicine.

Common mistakes and how to put them right

• Writing that modern methods sequence the protein, when sequencing reads the order of DNA bases.

• Answering only 'it is automated' when the question asks for an advantage, so never stating faster, cheaper or more accurate.

• Claiming a machine reads a whole chromosome in one pass, with no fragmentation or reassembly step.

• Confusing DNA sequencing with genetic fingerprinting, which compares fragment lengths and never reads bases.

• Saying improved technology changed the genetic code rather than our ability to read it.

Exam tips

• Advantages need a comparison - faster than what, cheaper than what - so name the old method or the old timescale.
• Link automation to a biological consequence; the mark is rarely for the technology on its own.
• If a question asks why a classification has been revised, name the evidence precisely - base sequencing scores, and so does electron microscopy of greater resolution or improved staining, so do not cross a microscopy answer out.