Module 6 - Structure and function of RNA Flashcards
RNA: what is it, how much is in cells, and what differences between RNA and DNA?
Ribonucleic acid
10x more RNA than DNA in cells
- Ribose contains an OH in the 2’ carbon whereas DNA only has a H (deOXY) so less stable
- Uracil used instead of thymine (uracil has one less CH₃, saving energy over stability)
- Single stranded and can pair with itself using palindromic sequences to do complementary base pairing to form complex structures
Biological role of RNA
Transfer of information (as mRNA) from DNA to ribosomes to synthesize proteins
Synthesis of proteins (rRNA and tRNA)
Post-transcriptional modifications (snRNA)
Processing and modification of rRNA (small nucleolar RNA - snoRNA)
Catalyctic RNA (self-splicing introns, naturally and artificially synthesized ribozymes)
RNA content in a cell
A single cell contains 10 pg (10⁻¹¹g) RNA
- 80-85% ribosomal RNA
- 10-15% low m/w species (tRNAs, snRNAs etc)
- 1-5% mRNAs
Regulation of RNA
Controlled by:
- Regulation of synthesis (ie lac operon)
- Regulated degradation (ie transferrin receptor mRNA will remove iron from RBCs but be degraded if enough iron is present)
- Translation efficiency (ie proteins using RNA in many different ways)
RD and TE are both caused by UTR of RNAs
Regulation of transferrin receptor mRNA
In the absence of Fe²⁺, Iron response element binding element (IRE-BP) bound to 3’ of the untranslated region (UTR) so the RNA is stable so the receptor is made
In presence of Fe²⁺, IRE-BP binds with Fe²⁺ so dissociates from 3’UTR so the RNA is degraded and the receptor is not made
Ferritin gene: what does it do, how does it react with low iron levels, and how does it react with high iron levels?
Produces ferritin which binds iron in the cell
Low Iron - IRE-BP binds to IRE and prevents translation initiation
High Iron - IRE-BP is removed from IRE and degraded and translation is initiated
Three types of mRNA editing
Capping
Polyaldenation
Splicing
Capping: what is it, what does it do, why it is used?
The first modification made to RNA polymerase II-transcribed RNA and takes place co-transcriptionally in the nucleus
It involves binding the 5’ to 5’ of ribose molecules via the phosphates - not normal
Linked to transcription initiation, increases stability of mRNA, required for efficient splicing, and nuclear export and Translation initiation
Polyadenylation: what is it, what is the signal for it, what does it do, why it is used?
The addition of a poly tail to an RNA transcript
AAUAAA, G/U, and U-rich region
Cleavage and Polyadenylation specificity Factor (CPSF) binds to AAUAA, Cleavage stimulatory factor (CstF) binds to G/U, - recruits cleaving factors and Poly- A polymerase (PAP), and finally cleavage and addition of polyA occurs
PolyA tailing functionally linked to transcription and a major role in stability and translation efficiency
Splicing: what is it, what does it involve, why it is used?
The removal of introns to form mature RNA
Signalled by conserved sequences at the 5’ splice site and the 3’ splice site which are recognised by factors which remove introns
Allows for variation in the protein manufactured after the expression of the gene
Splicing process
1) Cleavage at 5’ splice site and lariat formation at the branchpoint sequence
2) Cleavage at 3’ splice site, removal of intron region, and exon ligation
What does the splicing?
Small nuclear ribonucleoprotein particles (snRNPs made of snRNA and proteins) and protein splicing factors
The main snRNP’s involved in splicing that form the spliceosome are U1, U2, U4 U5 and U6
U2, U5 and U6 = the active site of the spliceosome
