Fragments of DNA can be amplified by in vitro and in vivo techniques.

The idea

A single copy of a gene is no use for screening, sequencing or manufacture, so the fragment must be amplified. In vitro means outside a living organism, and the standard method is the polymerase chain reaction, carried out in a thermal cycler with primers, free DNA nucleotides and a heat-stable DNA polymerase. Each cycle doubles the number of DNA molecules, so n cycles give two to the power n copies. PCR is very fast, needs only a trace of starting material and produces the fragment on its own, but it copies any errors and makes only DNA. In vivo means inside a living organism. The fragment, with promoter and terminator regions added, is spliced into a plasmid, transferred into a host cell, and the transformed cells are cultured; every time a bacterium divides by binary fission the plasmid is copied too.

What earns the marks

• Using PCR to amplify the DNA or sample, or a correct description of amplification.
• Identifying PCR as in vitro, taking place outside living cells.
• The in vivo route being insertion of the fragment into a vector and culture of transformed host cells.
• The plasmid being copied each time the host cell divides, so copy number rises with the population.
• A valid comparison, such as PCR being faster and needing a smaller sample, while in vivo culture also yields the protein and larger quantities.

Common mistakes and how to put them right

• Using in vitro and in vivo the wrong way round.

• Describing PCR as taking place inside a bacterium.

• Saying PCR produces protein, when it produces only DNA.

• Claiming PCR 'makes a gene', when it copies a fragment that must already be present.

• Giving a comparison with no criterion, such as 'PCR is better', with nothing measured.

Exam tips

• Fix the terms by their literal meaning: in vitro is in glass, in vivo is in the living.
• Comparison questions want a named criterion - speed, quantity, accuracy or whether protein is produced.
• If the question says 'amplify', the first mark is almost always the word PCR or a description of doubling.