RNA is transcribed with only four bases (adenine, cytosine, guanine and uracil), but there are numerous modified bases and sugars in mature RNA's. Pseudouridine (Ψ) in which the linkage between uracil and ribose is changed from a C–N bond to a C–C bond and ribothymidine (T) are found in various places (most notably in the TΨC loop of tRNA). Another notable modified base is hypoxanthine, a deaminated adenine base whose nucleoside is called inosine (I). Inosine plays a key role in the wobble hypothesis of the genetic code. There are nearly 100 other naturally occurring modified nucleosides, of which pseudouridine and nucleosides with 2'-O-methylribose are the most common. The specific roles of many of these modifications in RNA are not fully understood. However it is notable that in ribosomal RNA many of the post-transcriptional modifications occur in highly functional regions, such as the peptidyl transferase center and the subunit interface implying that they are important for normal function
The functional form of single stranded RNA molecules, just like proteins, frequently requires a specific tertiary structure. The scaffold for this structure is provided by secondary structural elements which are hydrogen bonds within the molecule. This leads to several recognizable "domains" of secondary structure like hairpin loops, bulges and internal loops. Since RNA is charged, metal ions such as Mg2+ are needed to stabilize many secondary and tertiary structures.
The functional form of single stranded RNA molecules, just like proteins, frequently requires a specific tertiary structure. The scaffold for this structure is provided by secondary structural elements which are hydrogen bonds within the molecule. This leads to several recognizable "domains" of secondary structure like hairpin loops, bulges and internal loops. Since RNA is charged, metal ions such as Mg2+ are needed to stabilize many secondary and tertiary structures.
