PCR makes copies of DNA, gene sequencing reads the order of its bases, and CRISPR can be used to edit DNA. They answer different questions: how can we get enough DNA to study, what does its sequence say, and how might we change it?

Studying DNA

PCR (polymerase chain reaction) amplifies a chosen DNA fragment, producing many copies from a small starting sample. The DNA strands are separated by heating. Short DNA primers bind on either side of the target region as it cools, and a DNA polymerase makes new strands. Repeating this cycle increases the number of copies. For example, PCR can provide enough copies of a fragment for analysis by gel electrophoresis.

Gene sequencing determines the order of bases in DNA. Researchers can compare sequences to identify differences between alleles or study an organism’s genome. Sequencing tells you what the DNA says; it does not, by itself, change it.

Altering DNA

CRISPR is a gene-editing approach. A guide RNA directs a Cas enzyme to a chosen DNA sequence, where the enzyme cuts the DNA. When the cell repairs the cut, the sequence may be disrupted; under suitable conditions, a specified change can be introduced. A cut alone does not guarantee the exact edit intended.

For AQA A-level Biology, focus your revision of recombinant DNA technology on the named methods: making DNA fragments, amplifying them by PCR or in host cells, and using vectors to transfer them into cells. DNA probes and genome sequencing are also in the topic. CRISPR is useful context for the question, but it is not named as a required technique in the supplied AQA topic content.

Exam mistake to avoid: PCR copies DNA; it does not read the base sequence or deliberately edit it.