RNA Synthesis Flashcards
What is the main function of chromosomes ?
To carry genes
What is a gene?
- Unit of heredity: contains instructions for an organisms phenotype
- DNA segment containing instructions for making a particular product
- includes regulatory genes and structural genes
What is transcription?
The synthesis of mRNA from DNA
What is translation?
Protein production from the mRNA transcript
How many RNA types are found in the cell and what are they?
- 3
- mRNA, tRNA and rRNA
What are the main differences between RNA and DNA?
- RNA HAS ribose not deoxyribose
- RNA has uracil instead of thymine
- RNA is single stranded
- RNA shorter than DNA
Where is RNA made?
In the nucleus
How are sa attached to tRNA?
By aminoacyl tRNA synthases
How does RNA Pol know where to start transcribing?
- In prok/euk DNA, there are regions called promoter regions
- They are specific sequence of nucleotides that bind with RNA Pol
Where is the TATA box found?
25 nucleotides to the start of where the gene is found
What are the requirements for transcription?
- RNA Pol II
- rNTPs
- Promoter regions
- Transcription factors (forms pre-initiation complex)
- Enhancers and silencers
Describe initiation
- Transcription Factor II D binds to the promoter region (Tata- box Binding Protein is a subunit of TFIID tata binding protein)
- TFIIB binds to the TBP and it links RNA polymerase to the TFIID
- WHEN RNA polymerase binds to the TFIIB it is already associated with TFIIF
- TFIIE and TFIIH associate with RNA polymerase
- TFIIH unwinds the DNA around the initiation site (cistron) and phosphorylated the RNA polymerase II
ATP is needed
Describe elongation
- RNA Polymerase moving along DNA synthesizing the RNA which is released at the other end
- Transcription Bubble formed which is 12 - 14 bps.
Describe termination
- Near the stop codon there is a sequence called AAUAAA which is the polyadenylation signal or the termination signal which is after the stop codon.
- A specific endonuclease recognizes the termination signal. The CSPF is the enzyme that does the cleavage after the termination signal.
- Polyadenylate polymerase adds 250 adenines to the end of the 3’ end of the pre-mRNA and this is called the poly A tail.
When does capping occur and what is it’s importance?
- When mRNA is still being transcribed
- Acts as a marker of RNA Pol to transcribed RNA
- Stimulates splicing
- Protects from any nucleus attack
- Enables multiple ribosomes to translate simultaneously
Describe the capping mechanism
- Forms a capping enzyme complex of methyl transferase, triphosphatase and gunnel transferase
- Triphosphatase removes a phosphate group from mRNA
- Guanyl transferase transfers G to 5’ end
- Methyl transferase transfers methyl group to the 7th position on G
Where does the the polyadenylation occur?
- At the beginning of the 3’ end
- Within the mRNA transcript there is a cleavage signal that is recognised by a specific endonuclease
- poly A pol. adds around 200 ‘A’s
What are the requirements of polyadenylation?
- CPSF: Cleavage and polyadenylation specificity factor
- CSTF: Cleavage stimulation factor
- Poly A Pol.
- PABP: Poly A binding protein stims Poly A Pol
What is the purpose of capping and polyadenylation?
1) Stabilty: prevents degradation in the nucleus and cytoplasm
2) Transport: ensures mRNA is intact
3) Integrity prior translation: so cell does not waste energy translating incomplete/ incorrect mRNA
What is splicing?
The removal of introns and the joining of axons
Describe splicing
- GU recognised by an RNA within the splicer zone where protein binds
- Protein undergoes conformation and cleaves 5’ splice site
- Ends join to form lariat like structure
- Cleavage at 3’ splice site and exons join together
What is alternative splicing?
- Ligations of exons can differ so the sequence of mature mRNA changes
- The proteins made may have similar functions as there are common exons, but different function as the sequence of exons differs.
How is RNA exported out of the nucleus?
- tRNA and microRNAs bind directly to export receptors.
- Large RNAs such as ribosomal RNAs and mRNA recruit specific adaptor proteins.
- They are exported through the nuclear pores
What proteins bind to mRNA for export from nucelus?
- Cap binding complex: recognises cap
- TREX: transcriptional-coupled export complex
- EJC: exon-junction complex recognises successful splicing events
