DNA replication, mutation and gene editing

Learning objectives

Explain semi-conservative replication as the separation of the two strands by helicase and the assembly of a complementary new strand on each template by DNA polymerase, and describe how the Meselson–Stahl experiment distinguished this from conservative replication

Describe the polymerase chain reaction as the repeated cycling of denaturation, primer annealing and extension by a heat-stable polymerase to amplify a chosen sequence of DNA

Explain how gel electrophoresis separates fragments of DNA by length, and describe the use of DNA profiling in paternity testing and forensic investigation

(HL) Describe replication at the fork in terms of the antiparallel strands, explaining continuous synthesis of the leading strand and the production of Okazaki fragments on the lagging strand, and state the roles of primase, DNA polymerase III, DNA polymerase I and DNA ligase

Classify gene mutations as base substitution, insertion or deletion, explain why a substitution may be silent, missense or nonsense while an insertion or deletion causes a frameshift, and use sickle cell anaemia as an example of the consequences of a single substitution

Explain that mutation is random and is increased in frequency by mutagens such as ionising radiation and some chemicals, and (HL) outline gene knockout and CRISPR–Cas9 gene editing as techniques that make a targeted change to a genome