Oooc oc

Ribosomal RNA

Transfer RNA

Primary Transcript

Endoribonuclease Cleavage

Exoribonuclease Cleavage x x

Internal Modification

Mature rRNA

Mature tRNA

Messenger RNA

Primary Transcript

5' Capping and 3' Cleavage

3' Poly (A) Addition

Splicing

Mature mRNA

The principal steps in processing various types of RNAs in eukaryotic nuclei. RNA sequences that are in the mature RNAs are indicated in blue while RNA sequences that are removed during processing are in black. Small "x's" indicate sites of internal modifications of RNAs. The various steps do not necessarily occur in the order shown.

nucleolus portion of the nucleus in which ribo-somes are made cytoplasm the material in a cell, excluding the nucleus

RNA splicing is similar to trimming in that it removes extra RNA sequences, but it is different because the sequence is removed from the middle of an RNA and the two flanking pieces are joined together again (see figure). The part of the RNA that is removed is called an intron, whereas the two pieces that are joined together, or spliced, are called exons. Just as with the cleavage enzymes, the splicing machinery recognizes particular sites within the RNA, in this case the junctions between exons and introns, and cleaves and rejoins the RNA at those positions.

Modification of RNA nucleotides can occur at the ends of an RNA molecule or at internal positions. Modification of the ends can protect the RNA from degradation by exoribonucleases and can also act as a signal to guide the transport of the molecule to a particular subcellular compartment. Some internal modifications, particularly of tRNAs and rRNAs, are necessary for these RNAs to carry out their functions in protein synthesis. Some internal modifications of mRNAs change the sequence of the message and so change the amino acid sequence of the protein coded for by the mRNA. This process is called RNA editing. As with the other types of RNA processing, the enzymes that modify RNAs are directed to specific sites on the RNA.

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