Biology · MU Sofia

6. RNA processing.

Covers how primary RNA transcripts become mature RNA in prokaryotes and eukaryotes, including rRNA and tRNA maturation, then the three steps of eukaryotic mRNA processing: 5'-cap formation, 3'-polyA tail addition and splicing by the spliceosome, with alternative splicing as a closing example. It also includes seven mnemonics, such as "CAP & TAIL the message, then EDIT", that summarise the sequence of steps.

Inside: 3 sections · 4 labelled pictures · 24 flashcards · 5 min

Topic 7 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 781 words (5 min of reading), with 24 flashcards.

In this note

  1. Day 2
  2. RNA processing.
  3. Mnemonics

Day 2

6-10 (Cheat Sheets at the end)

RNA processing.

RNA processing is the conversion of primary RNA transcript into mature RNA.

In prokaryotes, the primary RNA transcript must be converted into mature RNA.

  • Polycistronic mRNA is generally ready for translation.
  • The three rRNAs (16S, 23S, and 5S) are copied as a single primary transcript. This transcript is cut into 3 chains, which then require additional shortening of their ends and methylation.
  • Some prokaryotic tRNAs are also produced as a common primary transcript and undergo similar processing.
  • Redundant parts are removed by endo- or exoribonucleases (RNases). These enzymes recognize specific RNA sequences or specific conformations.

Maturation of eukaryotic rRNAs involves several steps:

  • The eukaryotic rRNAs are 28S, 18S, 5.8S, and 5S.
  • The 28S, 18S, and 5.8S rRNAs are synthesized together as a 45S chain.
  • During processing, some nucleotides are methylated, and some ribosomal proteins are bound to the 45S chain.
  • Non-coding parts are removed.
  • Other proteins and the 5S rRNA join the complex.
  • Finally, ribosomal subunits are assembled.

Eukaryotic tRNA maturation has similarities to prokaryotic tRNA maturation.

  • The CCA sequence is missing in tRNA genes and is added after transcription by an enzyme called tRNA nucleotidyl transferase.
  • Some eukaryotic tRNAs contain an intron, which is a part not needed for functioning and is excised during processing. This intron is located in the anticodon arm.
  • Mature tRNAs undergo complex post-transcriptional modification, with up to 10% of nucleotides being chemically modified; the function of this modification is unclear.

Maturation of eukaryotic mRNA is a complex process involving capping, polyadenylation, and splicing.

The primary transcripts of RNA polymerase II are called heterogeneous nuclear RNA (hnRNA).

5'-cap formation:

  • A G (guanine) is added to the terminal phosphates by a guanylyl transferase, forming an unusual 5′–5′ triphosphate linkage.
  • This added G is then methylated by a methyl transferase. The next two riboses can also be methylated.
  • This structure is called the 5’-cap.
  • Capping ensures the mRNA’s stability during translation and serves as a signal structure.

3'-polyA tail formation:

  • In mRNA genes, ААТААА is the signal for the end of transcription.
  • An enzyme recognizes the sequence AAUAAA in the RNA transcript and cuts the chain 10 – 30 bases after it.
  • Then, the enzyme adds 100-200 adenine (A) nucleotides to the 3'-end of the mRNA.
  • The resulting structure is called the 3’-polyA tail.
  • In eukaryotes, all mRNAs are polyadenylated at the 3' end, with histone mRNA being an exception.

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