22. Short RNAs

Last updated on January 11, 2020 at 13:07


  • There are many types of short RNA molecules that do not code for proteins, but have roles in regulation or processing
  • miRNA downregulate RNA translation. They are single-stranded.
  • saRNAs have a long-acting activating effect on genes. They are double-stranded.
  • snRNAs are found in the nucleus. They are a component of the spliceosome.
  • snoRNA are involved in the base modification of other RNAs. They can perform methylation and pseudouridylylation.
  • piRNA are the largest class of small RNAs. They create complexes with Piwi proteins and silence retrotransposons.
  • rasiRNA are a subclass of piRNA. They help establishing the heterochromatin structure.
  • tmRNA have properties of both tRNA and mRNA. They are involved in recycling ribosomes that have stalled because they were translating an mRNA that had lost its stop-codon
  • qiRNAs are expressed in response to DNA damage. They have a down-regulatory effect on protein translation
  • RISC is a protein which inhibits mRNA by cleaving it

More details

miRNA (microRNA) are approx. 22 nucleotides long. They bind to a sequence on the 3’-end of mRNAs to downregulate their translation into proteins.

saRNAs (small activating RNAs) induce a long-acting activation of genes.

siRNAs (small interfering RNAs) binds to the mRNA and silences it.

snRNAs (small nuclear RNAs) are a component of small nuclear ribonucleoprotein particles (snRNPs), which are involved in splicing introns out of mRNAs.

snoRNAs (small nucleolar RNAs) modify bases on tRNA, rRNA and snRNAs. They can perform methylation (transfer of a methyl group to a base) or pseudouridylylation (conversion of uridine to pseudouridine).

piRNAs (Piwi-interacting RNA) are the largest class of small RNAs. They are 25-30 nucleotides long, and create complexes with Piwi protein. Their function is to silence retrotransposons in germ line cells.

rasiRNA (repeat associated small interfering RNA) are a subclass of piRNAs. They perform the same functions as piRNAs, but also help establishing and maintaining the heterochromatin structure.

tmRNA (transfer-messenger RNA) help ribosomes that are stuck. If a ribosome tries to transcribe an mRNA that doesn’t contain a stop codon, the ribosome never gets the signal to release the polypeptide (because that signal is the stop codon). The ribosome will do nothing, with the polypeptide still attached to it. tmRNAs recycles the ribosome, makes sure the mRNA is degraded, and adds a tag to the polypeptide that signals that it should be broken down.

qiRNAs are 20-21 nucleotides long. Its expression is induced by DNA damage. It then inhibits protein translation.

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21. RNA metabolism

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23. Retroviruses

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