BIOB11H3 Lecture Notes - Lecture 6: Covalent Bond, Ribozyme, Precursor Mrna

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Molecular Cell Biology Lecture 6
1. Conserved (consensus) sequences exist at the junctions of introns/exons
2. These sequences are recognized by spliceosome components and interactions occur by RNA::RNA
base pairing.
3. Self-splicing RNAs exist in nature and can be categorized as group I or group II introns. This
involves elaborate intrastrand basepairing, forming complex 3D structures. Self-splicing implies that
RNA has catalytic activity, like an enzyme-thus the term „ribozyme‟ was coined.
4. Northern blotting (RNA gel blotting) can be used to demonstrate the processivity of RNA splicing as
well as alternative splicing (we will get to this as a means of gene regulation in chapter 12.
5. The consequences of incorrect splicing: frameshifts and the generation of missense and/or nonsense
mutations, abolishing protein function.
6. Small interfering RNAs (siRNAs) and microRNAs (miRNAs) play roles in development and defense
by mediating destruction of dsRNA or inhibiting translation.
o How does the same basic splicing machinery recognize the exon-intron boundaries in thousands of
different pre-mRNAs?
The sequence most commonly found at the exon-intron borders within mammalian pre-mRNA
molecules is shown in Figure 11.30
The G/GU at the 5’ end of the intro (5’ splice site), the AG/G at the 3’ end of the intron (3’
splice site), and the polypyrimidine tract near the 3’ splice site are present in the vast
majority of eukaryotic pre-mRNAs
In addition the adjacent regions of the intro contain preferred nucleotides, which play an
important role in splice site recognition
Additional clues that allow the splicing machinery to distinguish between exons and
introns are provided by specific sequences, most notbly the exonic splicing
enhancers (ESEs) situated within exons
Estimated that approximately 15 percent of inherited human disease results directly
from mutations that alter pre-mRNA splicing. In addition, much of the “nor- mal” genetic
variation in susceptibility to common diseases that is present in the human population
may result from the effects of this variation on RNA splicing efficiency.
o First evidence that RNA molecules were capable of catalyzing chemical reactions was obtained in
1982 = revealing the existence of RNA enzymes, or ribozymes.
A self-splicing intron, called a group II intro, was subsequently discovered in fungal
mitochondria, plant chloroplasts, and a variety of bacteria and archaea.
Group II introns fold into a complex structure shown in 2D in Figure 11.31
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