The lac operon is a segment of DNA in the bacterium E. coli that controls the metabolism of lactose; it consists of a promoter, an operator and three structural genes (z, y and a). It is an inducible operon. In the absence of lactose, a repressor protein binds to the operator and blocks transcription, so the genes are switched off. When lactose is present, it acts as an inducer: it binds to the repressor and inactivates it, so the repressor leaves the operator and transcription of the genes takes place, allowing the bacterium to use lactose.

DNA fingerprinting is a technique used to identify an individual on the basis of the unique patterns in certain highly variable regions of their DNA (called repetitive DNA or VNTRs), which differ from person to person (except in identical twins). Applications: (1) in forensic science, to identify criminals or victims from biological samples such as blood, hair or semen; and (2) in settling disputes of parentage (paternity or maternity testing). It is also used to study genetic diversity and evolution.

Darwin's theory of natural selection (survival of the fittest) states that: (1) organisms produce far more offspring than can survive, so there is a struggle for existence for limited resources; (2) individuals within a species show variations, some of which are useful (favourable); (3) individuals with favourable variations are better adapted, so they survive, reproduce more and pass on these variations to their offspring (natural selection); and (4) over many generations these favourable variations accumulate, leading to the origin of new species.