Mod 6 - structure and function of RNA Flashcards
what is RNA?
- ribonucleic acid
- the product of transcription from a DNA template
- several types of RNA that play a role in gene expression
how does the chemical structure of RNA differ from DNA?
- Ribose instead of deoxyribose
- ribose contains 2’ - OH whereas deoxyribose lacks the oxygen - RNA = less stable than DNA
- uses uracil instead of thymine
what is the role of mRNA?
- transfer of information from DNA to protei
- codes for proteins
what is the role of rRNA and tRNA ?
protein synthesis
what is the role of snRNA? (small nuclear RNA)
- processing of pre-mRNA
-involved in splicing introns from pre-messenger RNA
what is the role of snoRNA? (small nucleolar RNA)
- processing and modification of rRNA
what are the RNA contents within a cell?
- single cell contains 10pg of RNA (10^-11g)
- 80-85% ribosomal RNA
- 10-15% low m/w species (tRNA, snRNA, etc)
- 1-5% mRNA
what are some physical properties of RNA?
- generally single-stranded
- palindromic sequences allows for self-complementary base pairing, which allows it to fold into complex structures
how does the lac operon control the amount of certain RNA’s?
- transcriptional-level control
- transcription can be turned on or off rapidly to control synthesis of RNA
how is RNA abundance maintained apart from at the synthesis/transcriptional level?
- regulated degredation - taking advantage of RNA’s unstable structure to make it break down much faster
- can be achieved via transferrin receptor for mRNA
- translation efficiency - proteins controlling usage of certain RNA’s
all under the control of the UTR (untranslated region)
How does the Transferrin Receptor work to control mRNA levels?
- TfR mRNA (transferrin receptor mRNA) codes for the transferrin receptors that transports Fe2+ (toxic) ions into the cell
- when intracellular Fe2+ content gets too high, TfR mRNA becomes degraded, meaning no more transferrin receptors can be coded for, so no more Fe2+ enters the cell.
- the protein IRE-BP (iron response element binding protein) protects the 3’ - UTR stem loop structure in the mRNA - keeps it stable
- Fe2+ binds to IRE-BP which forces it to dissociate from 3’ - UTR stem loop, leaving it exposed and unstable so it breaks down much more easily
what is the function of the ferritin mRNA?
- regulates translation using RNA secondary structure
- ferritin binds to Fe2+ within the cell
- the ferritin mRNA has stem-loops in the 5’ UTR
- these IRE’s act as binding sites for IRE-BP
- when Fe2+ levels are high, they stop IRE-BP binding to IRE’s
- when Fe2+ levels are low, the IRE-BP binds to the IRE’s, which causes translation to be inhibited
what are the 4 steps in RNA processing (pre-mRNA -> mature mRNA)
- transcription
- capping
- cleavage + polyadenylation
- splicing
describe the process of mRNA capping
- adding GTP to the 5’ end of RNA
- linkage is from 5’ of the RNA to the 5’ phosphates of the GTP
- methylation occurs on the first 2 nucleotides and on the added G
why is mRNA capping important?
- linked to transcription initiation
- increase stability of mRNA
- required for efficient splicing
- nuclear export + translation initation
describe the process of 3’ cleavage and polyadenylation
precursor - signal sequence of AAUAA, G/U or a U rich region
- CPSF (cleavage and polyadenylation specificity factor) binds to AAUAA
- CstF (cleavage stimulatory factor) binds to G/U region
- these two factors recruit other factors such as cleaving factors and Poly-A polymerase (PAP)
- cleavage takes place at CA sequence
- polA tailing then occurs
what are the 3 coserved sequence signals for pre-mRNA splicing?
5’ splice site - AGGU RAGU
3’ splice site - Y AGGU
branchpoint region - YNYURAY
(Y = pyrimidine)
(R = purine)
(N = any base)
describe the 2-step process of splicing
- cleavage at 5’ splice site + lariat formation at branchpoint sequence
- 5’ phosphate of intron binds with free -OH of A residue of branchpoint
(this is why the A of the branchpoint sequnce is the most important) - cleavage at 3’ splice site - removal of intron region entirely
what is a snRNP (snurp)?
small nuclear ribonuclear particles
- consists of snRNA and numerous proteins
- snurps involved in splicing are U1, U2, U4, U5, and U6
what do the different snurps do?
U1 - attatches to exon 1 at the boundary between exon and intron
U2 - attatches at branchpoint
U4, U5, U6 - bind to exon 1, displaces U1
U2 then brings branchpoint into close proximity to exon 1 - U4 is displaced
U2, U5, U6 forms active site of spliceosome
what is the function of RNA processing?
- increase stability + functional efficiency
- increase number of possible products that can arise from one gene
what is a function of alternative splicing?
create isoforms - different proteins that arise from a single gene
