Biology 1002B Chapter Notes - Chapter 14.3: Small Interfering Rna, Dicer, Microrna
3 May 2018
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14.3a Posttranscriptional Regulation Controls mRNA Availability
• Variations in pre-mRNA processing
o Variations in pre-mRNA processing regulates which proteins are made in cells
o Alternative splicing produces different mRNAs from same pre-mRNA by removing
different combinations of exons and introns
o Resulting mRNAs are translated to produce a family of related proteins
o Regulatory proteins control which exons are removed by binding to regulatory
sequences
• Masking proteins
o Masking proteins bind to mRNA and make them unavailable for protein synthesis
o Keep mRNA inactive in egg cells until egg is fertilized
o When mRNA is to become active, other proteins remove masking proteins to allow
translation
• Variations in the rate of mRNA breakdown
o Regulatory hormone can increase or decrease rate of breakdown
• If hormone prolactin is present, casein (milk protein) mRNA half life increases
- rate of breakdown decreases
o 5' UTR has a controlling sequence recognized by regulatory proteins
• If 5' UTR is transferred experimentally from one mRNA to another, the half-life
of the receiving mRNA becomes the same as that of the donor mRNA
• Regulation of gene expression by small RNAs
o RNAi: silencing a gene posttranscriptionally by RNA
o miRNA: RNA that is encoded by nuclear genome and regulates gene expression by
binding to their mRNA and reducing translation
• RNA polymerase transcribes miRNA gene and pre-miRNA forms loop structure
after base pairing with itself
• Pre-miRNA is exported to cytoplasm
• Dicer enzyme removes loop from pre-miRNA
• Protein complex binds to double stranded RNA
• Enzyme in protein complex degrades one of the RNA strands and complex is
now called miRISC (miRNA-induced silencing complex)
• miRNA in miRISC binds to 3' UTR of target mRNA
• Imperfect pairing blocks translation (common in animals); perfect pairing
causes mRNA degradation (common in plants)
o siRNA: RNA that is NOT encoded by nuclear genes (ie. Virus genes) and regulates
gene expression by binding to their mRNA and reducing translation
• Viruses with RNA genomes involves a double-stranded RNA stage
• Cells attacked by such a virus can defend themselves using siRNA that they
produce from the virus's RNA
• RNAi process for siRNA is similar to miRNA (dicer cuts double stranded viral
RNA, protein complex formed is siRISC)
• Target RNA is viral mRNA for proteins, single-stranded RNA viral genome, or
single-stranded RNA produced from viral genome during replication
o Expression of any gene can be knocked out COMPLETELY using siRNA
• Introduce a double-stranded RNA complementary to mRNA transcribed from
that gene
• This is equivalent to creating mutated version of the gene, but without
changing the gene's DNA sequence
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