Transcription and Translation Flashcards
Exons
Introns
- RNA sequences that remain in mature mRNA after processing of primary RNA transcript (hnRNA)
- sequences that are removed from primary transcript during processing
4 important coding regions of DNA
- 5’ promotor
- Info to initiate transcription and specifies direction
- Not transcribed into RNA
- Exons
- Specify the AA sequence of a protein
- 1st exon contains 5’ UTR
- Last exon contains 3’ UTR
- Introns
- Generally do not encode protein sequence
- May encode regulatory RNAs w/ important cellular functions
- Enhancer Sequences/Silencer Sequences
- Enhancer sequences are binding sites for additional transcription factors that inc the transcription of one or more genes
- Whereas silencer sequences facilitate binding of repressors that reduce transcription
What happens in nucleus v cytoplasm?
Nucleus- DNA –> hnRNA or pre-mRNA –> mRNA
Mature mRNA then transported –> cytoplasm
8 Functional Classes of RNA
- Ribosomal RNA
- Transfer RNA
- Messenger RNA
- Small Nuclear RNA
- Micro RNA
- Small Interfering RNA
- PIWI-associated RNA
- long noncoding RNA
What direction does RNA synthesis proceed in?
5’ –> 3’ (just like DNA replication)
Which RNA polymerase is involved in making mRNA?
RNA Polymerase II
Promotor
a base pair sequence on every gene that sets direction and location of transcription on DNA template
TATA box
Determines direction of transcription
about 30 bps upstream of transcription initiation site of many genes
**These boxes are usually found in tissue-specific genes but not house-keeping genes (house keeping genes normally have CG-rich sequences usually in genomic regions containing CpG islands that can be modified by methylation of C residues to repress gene expression)
CAAT box
another promotor proximal element
**These boxes are usually found in tissue-specific genes but not house-keeping genes (house keeping genes normally have CG-rich sequences usually in genomic regions containing CpG islands that can be modified by methylation of C residues to repress gene expression)
Basic Steps of Transcription
1- binding of transcription factors (TFIIA, TFIIB, etc) and RNA polymerase II to promoter
2- RNA synthesis - elongation (product is long primary transcript w/ introns and exons)
3- RNA Processing
How is transcription initiated?
- At many promoters, transcription starts at TATA box; TATA box is bound by TFIID
- This then enables adjacent binding of TFIIB
- Next, rest of general transcription factors and RNA polymerase itself assemble at promotor
- TFIIH uses ATP to unwind DNA dbl helix at transcription starting point which allows transcription to begin
- TFIIH also phosphorylates RNA polymerase II —> changes confirmation that that it is released from general factors and can begin elongation phase
- **Activators also attract ATP-dep chromatin remodeling complexes and histone-modifying enzymes
- **Mediator coordinates assembly of all these proteins at promoter
RNA Processing
- Capping rxn- addition of 7-methyl G residue to 5’ end
- The 3’ Polyadenylation rxn- (poly A tail)
- RNA Splicing
Capping Rxn
- By guanylyl transferase through 5’-5’ phosphodiester bond
- Cap is important for mRNA protection, splicing, stability and translation
- Steps
- Lose a phosphate
- Addition of GMP
- Methylation of nitrogen atom 7 of the added G nucleotide and methylation of carbon 2’ of ribose of adjacent nucleotide (+ its neighbor in vertebrates)
The 3’ Polyadenylation rxn- (poly A tail)
- The AAUAAA located near the 3’ end of hnRNA acts as polyA+ addition signal
- A nuclease cleaves the RNA approx 11-30 nucleotides downstream of the polyA signal and poly A polymerase subsequently adds 50-200 adenine residues to the 3’ end of RNA
- **Important for mRNA stability, nuclear export and translation
RNA Splicing In General
- Remove introns and join exons
- Catalyzed by small nuclear ribonucleoproteins (snRNPs) and occurs in large complex (spliceosome)
- Introns always start w/ GU sequences (splice donor site) and end w/ AG (splice acceptor site)
Where does translation occur?
cytoplasm
In what direction are proteins synthesized?
Amino terminal –> carboxyl terminal
Genetic Code
- 64 codons (3 bases ea)
- 61 encode AA
- 3 are termination codons (UAA UAG UGA)
- Initiates w/ AUG (which codes for methionine)
- Redundant (A given AA can be encoded by multi codons BUT a single codon only corresponds to 1 AA)
tRNA
- tRNAs contain anticodons that are complementary and anti-parallel to mRNA
- Fewer tRNAs (~50) than codons, many tRNAs can recognize more than one codon using wobble base pairing b/n 3rd position in mRNA and 1st position in tRNA
Ribosomes
- Site of protein synthesis
- Large 60S subunit- contains 50 proteins + 28S 5.8S and 5S rRNAs
- Small 40S subunit- contains 30 proteins + 18S RNA
-Act as ribozymes (RNA component of ribosome catalyzes peptide transferase rxn)
aminoacyl-tRNA synthetase
couples a particular AA to its corresponding tRNA (CHARGING)
Protein Synthesis
1- Attachment of AA to tRNAs
2- Initiation
3- Elongation
4- Termination
Attachment of AA to tRNA
- AA carboxyl group attached to CCA sequence of 3’ end of tRNA (via aminoacyl-tRNA synthetases)
- This rxn requires ATP and energy released by ATP hydrolysis is captured in ester bond b/n AA and tRNA
- This energy is subsequently used to form peptide bond
Where is protein synthesis initiated?
Small ribosome subunit (40S)
-Assembly scans mRNA until AUG is found then larger 60S ribosome subunit attaches –> prod of initiation complex
Translocation
- When ribozyme moves to next codon
- requires GTP hydrolysis and translocase
peptidyl transferase
catalyzes formation of peptide bond b/n carboxyl group of 1 AA and amino group of next AA
Termination of Protein Synthesis
- UAA UAG UGA reached - binding of release factor to A-site bearing a stop codon terminates translation
- Complete polypeptide is released
- After action of ribosome recycling factor, the ribosome dissociates into 2 sep subunits
Splice Donor Site
Splice Acceptor SIte
GU sequence/more upstream (encountered by spliceosome first)
AG sequence/ further downstream (encountered by spliceosome second)
