The "cloverleaf" tertiary structure of tRNAs is a common textbook example of the link between RNA structure and function.
===microRNA===
''Micro RNAs'' ([[microRNA]])are short RNA molecules involved in post-transcriptional [[gene regulation]]. microRNAs work by binding to complementary sequences on target mRNAs; thus targeting the mRNA for degradation via the [[RISC|RNA-induced silencing complex]] or, less commonly, blocking translation of the mRNA by [[steric hindrance|steric]] (mechanical) hindrance.
In their mature, functional form, microRNA molecules are 20 to 24 nucleotides long. This length allows for microRNAs to have a very high degree of target specificity (some microRNAs may only have a single target mRNA).
Because the mechanics of [[RNA polymerase]] require a minimum transcript length of about 50-100 nucleotides, precursor microRNAs are at least 80 nucleotides long (most are longer). As such, several different microRNAs are usually derived from a single precursor transcript.
Together with siRNAs, microRNAs are a major component of ''gene regulatory networks'', which allow the cell to rapidly and efficiently respond to environmental stimuli.
===siRNA===
''Short interfering RNAs'' ([[siRNA]]) are short (~21 base pairs), double-stranded RNA molecules which function in a manner similar to microRNAs (described above).
siRNAs are commonly used in research for targeted gene knockdown (blocking the expression of a specific gene), and they are also used in several gene therapies.
==References==