L8 - RNA Transcription Flashcards
(29 cards)
What is transcription?
The process of RNA synthesis from a DNA template
mRNA
Messenger RNA
Codes for proteins
3-5% of total RNA
tRNA
Transfer RNA
Participates in translation carrying amino acids
Each has a specific anticodon loop
rRNA
Ribosomal RNA
Major constituent of ribosomes
4 main types
Primary structure of RNA
Ribose instead of deoxyribose
Uracil instead of thymine
Synthesised as a single strand
Very unstable
Secondary structure of RNA
A binds to U G binds to C Can also have non Watson Crick airing - G binds to U Give rise to stem loop secondary structures that can base pair on themselves through H bonds
Tertiary structure of RNA
Folding of a molecule in 3D
RNA polymerase I
Ribosomal RNA
RNA polymerase II
Protein encoding genes
RNA polymerase III
tRNA, snRNA, 5s rRNA
Differences between RNA and DNA transcription
Multiple RNA polymerases bind on the same gene
No primer needed
Only one strand of DNA is used as the template
Transcript does not remain bound to the template
Higher error rate
What direction does RNA transcription occur?
5’ to 3’ direction
RNA polymerase moving from left to right - RNA made using bottom strand as template
RNA polymerase moving from right to left - RNA made using top strand as template
Template strands described as antisense
Transcription creates supercoiling
DNA with fixed end
- Unwind 10 DNA base pairs - DNA helix forms one supercoil
What do topoisomerases do?
Release supercoils to allow progression
Where does transcription start?
Promoter Sequences in the DNA tell RNA polymerase where to start - INR - BRE - TATA - DPE
Many proteins in the polymerase complex bind directly to DNA including
TATA binding protein binds to TATA box found upstream
Protein complex is required for transcription to start
General and specific transcription factors
RNA processing techniques
Splicing of introns
Capping of 5’ end
Polyadenylation of 3’ end
Splicing of introns
Eliminates non-coding intron regions to generate mature mRNA for protein synthesis
Specific to eukaryote mRNA
Some mRNA can be spliced multiple ways - alternative splicing
Capping of 5’ end
Necessary for
- Stability
- Binding of mRNA to ribosomes
- Initiation of translation
Polyadenylation of 3’ end
Part of termination process
Addition of PolyA tail
Determine mRNA stability
Helps mRNA nuclear export and translation
Cleavage is 1-30 nucleotides downstream of PolyA signal
Splicing mechanism
- OH of branch site A attacks bond on donor site G
- Cleavage at donor site leads to formation of a lariat ]
- 3’ OH donor site G attacks bond on acceptor site G
- Lariat is freed then degraded
What is mRNA splicing performed by?
Splicesome - nuclear complex made of 150 proteins + 5RNAs
Functions of the splicesome
Recognise 5’ donor and branch sites
Bring sites together
Catalyse RNA cleavage
