RNA & Transcription Flashcards
What is the central dogma of the cell?
DNA > protein.
What type of RNA is used in transcription?
mRNA.
What is the transcriptome?
Set of all RNA molecules, varies depending on cell/cell type, sequences determined by RNAseq.
What is the purpose of transcription?
Produces an RNA molecule that is complementary to one strand of DNA (coding strand) and therefore a copy of the template strand.
This is used to translate the genetic code into functional proteins for cellular processes without risking damage to the DNA.
Differences between RNA and DNA.
RNA has a ribose sugar, DNA has a deoxyribose sugar.
Ribose has a hydroxyl group on 2nd C, in deoxyribose an H replaces this.
RNA is > reactive than DNA because of the 2 OHs.
Uracil in RNA vs Thymine in DNA.
If uracil were used in DNA, cytosine deanimation (losing an anime group to become uracil) would be difficult to detect & repair.
RNA used for short term information transfer, DNA used for long term information storage.
RNA has multiple functions, DNA has 1.
RNA is single stranded, DNA is double stranded.
Give the general structure of RNA.
Single stranded polymer of repeating RNA nucleotides.
Can base pair & form variety of 3D shapes with other RNA or itself.
Give the key enzyme that synthesises an RNA strand from DNA.
RNA polymerase.
Give the process of transcription.
DNA locally unwound by DNA helicase.
Left to Right direction of RNA polymerase along DNA template strand (3’ to 5’).
RNA strand synthesised 5’ to 3’.
Ribonucleotides enter through groove, base pair with exposed template strand of DNA.
RNA polymerase catalyses the formation of phosphodiester bonds between ribonucleotides to form mRNA transcript.
How is transcription driven?
Loss of pyrophosphate when phosphodiester bond is formed between ribonucleotides.
Pyrophosphate broken down & the energy is used to drive the reaction in the forward direction.
Where does transcription take place in prokaryotes?
In the cytoplasm, simultaneously with translation.
In eukaryotes, how is mRNA modified before translation?
5’ end is capped with atypical nucleotide (usually G) with an atypical bond to protect from exonuclease degradation.
3’ end is attached to string of poly-A nucleotides (polyadenylation).
Introns are spliced out by the spliceosome.
When were introns discovered, how and by who?
Sharp & Roberts in 1993.
They looked at the betaglobin gene.
What is alternative splicing?
More than one protein can be expressed from a single gene.
How does the cell know when an mRNA has been modified & is ready for export?
RNA binding proteins mark a mature mRNA for selective export from the nucleus.
What does the stability of RNA depend on?
Sequence, which affects how many times the RNA can be translated before it is degraded.
Give the structure & function of tRNA.
Double stranded via base paired stems & loops.
Becomes charged by coupling with a specific amino acid via a high energy ester bond.
Anticodon loop binds to appropriate codon in mRNA to deliver the correct amino acid.
What is rRNA & it’s structure?
Ribosome. Large & small subunit. E site = exit site. P site = peptidyl-tRNA site. A site = aminoacyl-tRNA site.
Describe the first stage of translation.
Initiation (eukaryotes).
P-site filled with the initiator tRNA but A site is empty.
Small subunit binds at 5’ cap & scans until it finds start codon (AUG).
Initiator tRNA charged with methionine binds using complementary base pairing via it’s anticodon (UAC).
Describe the second stage of translation.
Elongation.
tRNA with an anticodon that corresponds to the mRNA codon in the A site binds.
The 2 amino acids begin to react, ester bond of charged tRNA in P site breaks & peptide bond is formed using that energy.
(1st) tRNA that has transferred its amino acid moves to E site & is released.
tRNA & mRNA move along to P site so new charged tRNA can fill A site.
Describe the third stage of translation.
Termination.
At the end of the coding region the ribosome reads a stop codon, termination factor binds in place of tRNA.
mRNA & ribosomal units disassemble & releases protein.
Give the directionality of protein synthesis.
N terminus to C terminus.
