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11. Angive at de primære transkripter fra tRNA, rRNA og mRNA bliver processeret
Devlin, s.220-3, fig.5.10; fig. 5.11
Stryer, s. 797-8, fig. 28.23

All RNA copies of DNA sequences must be modified to mature, functional molecules in eukaryotes. In prokaryotes, tRNA and rRNA are modified, but not mRNA.

 The reactions of RNA processing can include:

• Removal of extra nucleotides

• Addition of nucleotides

• Base modification

• Separation of different RNA sequences by the action of specific nucleases

tRNA precursors are converted into mature tRNAs by a serious of alterations:

1. cleavage of a 5`-leader sequence (by RNase P)

2. splicing to remove an intron (exonuclease and ligase)

3. replacement of the 3´-terminal UU by CCA (with a free 3`-OH end)

4. modification of several bases

rRNA primary product of transcription is a long rRNA, termed 45S RNA, which contains the sequences of 38S, 5.8S and 18S rRNAs. Processing of 45S rRNAs goes on in the nucleolus. The processing is carried out by large ribonucleotide protein assemblies and follows a sequential order.

mRNA

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