U1 attaches to 5' end of intron; commits intron to splicing; no direct role in splicing

Positions 5' end of intron near branch point for lariat formation Holds 5' end of intron near branch point

Positions 5' end of intron near branch point Anchors first exon to spliceosome subsequent to cleavage; juxtaposes two ends of exon for splicing

Juxtaposes two ends of exon for splicing

Delivers U6 to intron; no direct role in splicing between different snRNAs (summarized in Table 14.2). These interactions depend on complementary base pairing between the different RNA molecules and bring the essential components of the pre-mRNA transcript and the spliceosome close together, which makes splicing possible.

The spliceosome is assembled on the pre-mRNA transcript in a step-by-step fashion (I Figure 14.12). First, snRNP U1 attaches to the 5' splice site, and then U2 attaches to the branch point. A complex consisting of U5 and U4-U6 (which form a single snRNP) joins the spliceo-some. At this point, the intron loops over and the 5' splice site is brought close to the branch point. U1 and U4 disassociate from the spliceosome. The 5' splice site, 3' splice site, and branch point are in close proximity, held together by the spliceosome. The two transesterification reactions take place, joining the two exons together and releasing the intron as a lariat.


An animation of the splicing

Nuclear organization RNA splicing takes place in the nucleus and must occur before the RNA can move into the cytoplasm. For many years, the nucleus was viewed as a biochemical soup, in which components such as the spliceosome diffused and reacted randomly. Now, the nucleus is believed to have a highly ordered internal structure, with transcription and RNA processing taking place

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