【多选题】How does the cell determine which parts of the RNA transcript to remove during RNA splicing?
A.
Unlike the coding sequence of an exon, most of the nucleotide sequence of an intron is unimportant. Although there is little overall resemblance between the nucleotide sequences of different introns, each intron contains a few short nucleotide sequences that act as cues for its removal from the pre-mRN
B.
These special sequences are found at or near each end of the intron and are the same or very similar in all introns. The special sequences are recognized primarily by small nuclear ribonucleoproteins (snRNPs), which direct the cleavage of the RNA at the intron–exon borders and catalyze the covalent linkage of the exon sequences.
C.
Guided by these sequences, an elaborate splicing machine cuts out the intron in the form of a “lariat” structure, formed by the reaction of an adenine nucleotide. The removed intron is eventually degraded in the nucleus.
D.
Unlike the other steps of mRNA production, RNA splicing is carried out largely by RNA molecules rather than proteins. These RNA molecules, called small nuclear RNAs (snRNAs), are packaged with additional proteins to form snRNPs. RNA molecules that catalyze reactions in this way are known as ribozymes. Together, these snRNPs form the core of the spliceosome, the large assembly of RNA and protein molecules that carries out RNA splicing in the nucleus.