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)
