Summary RNA Processing II: Capping and Polyadenylation
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Molecular Biology
RNA Processing II: Capping and Polyadenylation:
Describe the addition of a 5' cap and a poly(A) tail to the pre-mRNA molecule.
Discuss the roles of these modifications in mRNA stability, nuclear export, and translation initiation.
Emphasize the importance of proper capping and polyadenylation ...
RNA Processing I: Splicing ● Splicing, by attracting the exon junction complex to mRNAs, Splic
Evidence for split gene enhances gene expression, primarily by making translation ●
● Hexon - encode structural proteins. Exon - coding regions, more efficient.
or expressed regions. ● advantage of introns comes from at least two sources: ●
● The first introns were found in major late locus of adenovirus ● They stimulate efficient mRNA 3’-end formation, and they Exon
● R – looping - DNA template strands are separated to allow make translation more efficient. ●
a double-stranded hybrid to form between one of these ● exon junction complex (EJC) – necessary and sufficient
strands and the RNA product. for stimulation of gene expression by introns, probably by
RNA SPLICING facilitating the association of mRNAswith ribosomes. Splic
● The process of cutting introns out of immature RNAs and ● EJC - also makes possible the destruction of faulty mRNAs ●
stitching together the exons to form the final product is called that have premature stop codon
RNA splicing. Branched Intermediate ●
● Messenger RNA synthesis in eukaryotes occurs in stages. ● first step is the formation of the lariat-shaped intermediate
The first stage is synthesis of the primary transcription ○ 2’-hydroxyl group of an adenosine nucleotide in the snRN
product, an mRNA precursor that still contains introns copied middle of the intronattacks the phosphodiester bond ●
from the gene, if any were present. This precursor is part of a between the first exon and the G at thebeginning of
pool of large nuclear RNAs called hnRNAs (heterogeneous the intron (the 5’-splice site), forming the loop of the
nuclear RNA) lariat and simultaneously separating the first exon
● The second stage of mRNA maturation. Part of the from the intron.
maturation of an mRNA precursor is the removal of its introns ● second step completes the splicing process:
3’-Sp
in a process called splicing. ○ The 3’- hydroxyl group left at the end of the first exon
●
Splicing Signals attacks thephosphodiester bond linking the intron to
● Signals must occur in the mRNA precursor that tell the the second exon (the 3’-splice site).
splicing machinery exactly where to “cut and paste.” ● Thus, forming the final product exon-exon phosphodiester
● The first two bases of the intron are almost always GU, and bond and releases intron, in lariat form, at the same time
Contr
the last two are almost always AG Signal at the Branch
●
● The 5’- and 3’-splice sites have consensus sequences that ● first hint of a special region within the intron came from
extend beyond the GU and AG motifs, and there is also a experiments with the yeast actin gene performed by
branchpoint consensus sequence. Christopher Langford and Dieter Gallwitz in 1983.
● The most striking observation, first made by Chambon, is ○ cloned the actin gene, made numerous mutations in it,
●
that almost all introns in nuclear mRNA precursors begin and and reintroduced these mutant genes into normal
end the same way: yeast cells. Then they assayed for splicing by S1
○ exon/GU–intron–AG/exon mapping.
○ first two bases in the intron of a transcript are GU and ● In addition to the consensus sequences at the 5’- and 3’-ends
the last two are AG of nuclear introns, branchpoint consensus sequences also
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