RNA Processing

Ribosomal RNAs are synthesized as long primary transcripts that contain several different rRNAs separated by spacer regions (see figure). The individual rRNAs are cut apart by endoribonucleases that cleave within the spacer regions. Other enzymes then trim the ends to their final length. Ribosomal RNAs are also modified at many specific sites within the RNA. Ribosomal RNA synthesis and processing occurs in a special structure within the nucleus called the nucleolus. The mature rRNAs bind to ribosomal proteins within the nucleolus and the assembled ribosomes are then transported to the cytoplasm to carry out protein synthesis.

Transfer RNAs are transcribed individually from tRNA genes. The primary transcripts are trimmed at both the 5′ and 3′ ("five prime," or "upstream" and "three prime," or "downstream") ends, and several modifications are made to internal bases. Many eukaryotic tRNAs also contain an intron, which must be removed by RNA splicing. The finished tRNAs are then transported from the nucleus to the cytoplasm.

Messenger RNAs are transcribed individually from their genes as very long primary transcripts. This is because most eukaryotic genes are divided into many exons separated by introns. Genes may contain from zero to more than sixty introns, with a typical gene having around ten. Introns are spliced out of primary RNA transcripts by a large structure called the spliceosome. The spliceosome does not move along the RNA but is assembled around each intron where it cuts and joins the RNA to remove the intron and connect the exons. This must be done many times on a typical primary transcript to produce the mature mRNA.

In addition to removal of the introns, the mRNA is modified at the 5′ end by the addition of a special "cap" structure that is later recognized by the translation machinery. The mRNA is also trimmed at the 3′ end and several hundred adenosine nucleotides are added. This modification, which is called either polyadenylation or poly (A) addition, helps stabilize the 3′ end against degradation and is also recognized by the translation machinery. Finally, the processed mature mRNA is transported from the nucleus to the cytoplasm.

Some RNAs, called small nuclear RNAs (snRNAs) and small nucleolar RNAs (snoRNAs), are processed in the nucleus and are themselves part of the RNA processing systems in the nucleus. Most snRNAs are involved in mRNA splicing, while most snoRNAs are involved in rRNA cleavage and modification.