Lesson 16: Post-Transcriptional Control Flashcards
How are hairpins used to reduce gene expression?
- Hairpins prevent protein formation.
1) Hairpins are often bound to translation repressor proteins which physically block the start codon, therefore no protein can be made
2) If the start codon is in the hairpin, the ribosome won’t be able to translate it. Therefore, no protein will be made.
- Thus, the hairpin needs to be melted (by increasing the temperature). This will allow the start codon to be accessible and the protein to be made
What are regulatory RNAs?
Noncoding RNAs whose function is to silence gene expression
ex: microRNAs (miRNAs) and small interfering RNAs (siRNAs)
- Cell will use complementary base pairing to “seek” out a desired mRNA
What is a microRNA (miRNA)?
- transcribed from the genome, estimated that about 1/3 of human protein coding genes are regulated by miRNAs.
- And new ones are being discovered
all the time
What is the function of miRNA?
Repress translation and target mRNA for destruction
Explain how miRNAs work.
1) Precursor miRNA folds up onto itself by using complementary base pairing
2) Precursor miRNA gets processed in the nucleus into a small, mature ds miRNA molecule by dicer and exported to the cytoplasm
- the miRNA gets cleaved by the dicer (dices dsRNA
into pieces) so now we are left with a ds miRNA
intermediate.
3) The ds miRNA intermediate binds to a protein complex called he RISC
4) RISC proteins help unzip ds miRNA using ATP, leaving a ss miRNA molecule in the complex.
5) This RISC-ss miRNA complex then searches for mRNAs with complementarity.
6) mRNAs are targeted for destruction or translation is repressed.
- Extensive match: mRNAS to be rapidly degraded by
nuclease within RISC
- Less Extensive match: translation reduced; mRNA
sequestered and eventually degraded by nucleases
in the cytosol
7) RISC- ss miRNA releases and targets another mRNA
What is RNA interference (RNAi)?
It is a mechanism by which siRNAs are used as a template to search for complementary mRNAs molecules to target them for destruction.
- NOTE: This mechanism may have evolved as an early adaptive “immune system” to kill invading viruses that use dsRNA as their genetic information or in their cycle.
What is a small interfering RNA (siRNA)?
NOT found in the genome but may come from a double-stranded virus (or a scientist)
Explain the process of RNAi (with the help of siRNAs).
1) siRNA gets processed by the dicer (enzyme that dices up dsRNA into pieces) into small, mature ds siRNA molecule
2) The ds siRNA intermediate binds to the protein complex called RISC.
3) The RISC proteins help unzip the ds siRNA using ATP, leaving a ss siRNA molecule in the complex.
4) RISC-ss siRNA complex searches for RNAs with complementarity.
5) RNAS are targeted for destrcution.
6) RISC-ssRNA releases and can target another RNA.
- NOTE: The process is very similar to he machinery that is used with miRNAs.
What is a difference between the machinery used with miRNAs and RNAi?
In RNAi, the RISC-ss siRNA complex only searches for a PERFECT match. There is NO less extensive match that can be used (like with miRNAs). This is because the goal is to degrade the foreign RNA. Therefore, the best way to do that is with the perfect match.
What are the key themes in post-transcriptional control?
1) Hairpins & translation repressor proteins can limit accessibility. (both in prokaryotes and eukaryotes)
2) Regulatory RNAS (miRNAs and siRNAs) can use complementary base pairing to reduce mRNA leading to a reduction in protein production. (JUST eukaryotes)
Are there many ways to regulate gene expression?
YES!
While transcription is the main level of control. Control at several points in the protein making process allows thecell to fine tune the concentration of mRNA and protein, ultimately controlling cell function and the ability of a cell to respond to its environment.
