RNA

This tour of different biological processes showcases the role of RNA. In many of these roles, RNA interacts with DNA and proteins to support life. Early on in the evolution of life, according to the "RNA world" hypothesis, DNA and proteins did not yet exist and RNA had DNA-like roles (genetic storage) and protein-like roles (metabolism, biosynthesis, etc), some of which it retains to the present day.

Genetic information

RNA plays key roles in gene expression. The classic three types of RNA are mRNA (messenger RNA), tRNA (transfer RNA) and rRNA (ribosomal RNA), all of which have a crucial role in gene expression. Beyond these roles, RNA also serves to store genetic information, to regulate gene expression and to modulate the sequences of protein through alternate splicing.

Transcription

RNA is synthesized by RNA polymerases using DNA (or sometimes RNA) as a template. During synthesis, the template and the newly synthesized RNA form a base-paired, anti-parallel structure. Shown here is the initiation of transcription by phage T7 RNA polymerase (⚞RNA⚟) ^[1]

Post-transcriptional modification

Introns are segments of RNA that have catalytic activity. These elements can splice RNA (remove the intron) and sometimes act as retrotransposons (i.e. they can insert themselves into pieces of RNA, which then - via reverse transcription - changes the sequence of DNA). The class IIc intron shown here in complex with a small piece of its substrate (the exon) catalyzes phophodiester transesterification using bound magnesium ions.^[2]

Translation

Ribosomes synthesize proteins with sequences determined by mRNA. The ribosome is a complex of rRNA and protein. Shown here is a small part of the ribosome illustrating protein-RNA interactions: ribosomal RNA (⚞RNA⚟). When synthesizing proteins, ribosomes follow the genetic code to translate an RNA sequence (grouped into codons of 3 nucleotides) into a protein sequence.tRNAs are the adapter molecules that embody the genetic code. Each tRNA is covalently bound to a specific amino acid at one end and display an anti-codon on the other end (for more details, see transfer RNA)

Storage of genetic information

All known organisms store genetic information on DNA. Some viruses, however, use RNA instead. Depending on how they use RNA to store genetic information, they are classified as double-stranded RNA viruses and plus or minus single-stranded RNA viruses . For example, human immunodeficiency virus carries genetic information in the form of single stranded RNA. In the figure, the RNA is located in the dark rectangular blob inside the circular virus particles. The single-stranded RNA folds onto itself, as illustrated for the part of the RNA called packaging signal.

Catalysis

In addition to the examples of catalytic RNA already mentioned in the previous section (ribosome, introns), other ribozymes exist.

Hammerhead RNA

Hammerhead ribozyme (animate)

RNAse P

RNAse P modifies folded tRNA.

Regulation

Only a small fraction of the human genome codes for proteins, tRNA or rRNA. The remainder has been called "junk DNA" in the past, but some of it codes for RNA that plays a role in gene regulation.

Riboswitches

Riboswitches work by binding to segments of RNA, thereby changing the structure of it (for example, by competing with an interaction supporting one structure, they allow it to fold into a different structure. The structure of the riboswitches is sometimes dependent on binding to small molecules, like the riboswitch bound to S-adenosylmethionine shown here.

RNA silencing

siRNA (⚞RNA⚟)