[C] before [D] = CB4d?
IDK how education works in the UK, but given COVID-19, thought it could be relevant. CB4d is a category found under the Biology regimen.
CB4d = Genetic Engineering / Breeds and varieties
The process of genetic engineering - Higher
The main steps of genetic engineering:
Restriction enzymes are used to isolate the required gene leaving it with sticky ends. Sticky ends are a short section of unpaired bases
A vector, which is usually a bacterial plasmid or a virus, is cut by the same restriction enzyme leaving it with corresponding sticky ends.
The vector and the isolated gene are joined together by ligase enzyme.
The vector inserts the gene into required cells.
The genes are transferred to animal, plant or microorganism cells, during early development, which allows them to develop with the desired characteristics.
This diagram shows how the genetic engineering of insulin works.
Sauce: https://www.bbc.co.uk/bitesize/guides/z9rqcj6/revision/4