What is siRNA?

siRNA is short for small interfering RNA and acts to 'silence' a gene. It's short as its name suggests, they're usually only around 21 nucleotides long. They're also double stranded. This means that they are made up of a series of two complementary base pairs.


siRNA is called "interfering" because it disrupts translation (a part of protein synthesis which takes place within the cytoplasm). The siRNA and accosiated proteins bind to target mRNA
proteins cut mRNA into sections so it cant be translated.


siRNA prevents expression of the specific gene as its protein cant be made during translation so the protein isn't made.

What is Speciation? | A Level Biology Revision

Speciation is pretty much the evolution of new species from existing species. A species being a group of organisms with similar genes which are able to interbreed to produce fertile offspring. If populations become seperated (geographical isolation) then they are no longer able to interbreed. This might be for a number of reasons  - lets take a huge mountain for example. Selection pressures will be different in both locations, for example ease of getting food or temperature may differ. Within these slightly different environments some are better able to compete than others - this  causes the allele frequencies to change in each population. The genes then, over time become so different they now can't interbreed.

Other keywords:

Stabilizing selection - this is selection which tends to favor the average (gets rid of the extremes)
Directional selection - this favors one extreme

This is specifically for AQA A Level Biology Unit 4