Gene structure, Transcription, & Translation Flashcards
Gene expression in prokaryotes
- coordinated gene expression: clustered genes (operon) controlled by one promoter and transcribed by polycistronic mRNA (mRNA that encodes for multiple different polypeptides), encoding multiple gene products
Gene expression in eukaryotes:
- interrupted genes (exons/introns) - monocistronic mRNAs - post-transcriptional modifications (1. a 5’ CAP is added; 2. a polyA tail is added; 3. introns are spliced out)
What is the 5’ cap?
The 5′ cap is found on the 5′ end of an mRNA molecule and consists of a guanine nucleotide connected to the mRNA via an unusual 5′ to 5′ triphosphate linkage. This guanosine is methylated on the 7 position directly after capping in vivo by a methyl transferase. It is referred to as a 7-methylguanylate cap, abbreviated m7G.
What is the Open Reading Frame (ORF)
- the DNA sequence between a start (AUG codon) and a stop site which is to be transcribed.
Phases of DNA Transcription:
- Initiation
- Elongation
- Termination
Describe the Initiation phase of DNA transcription
- RNA polymerase binds to promoter sequence in duplex DNA (“Closed complex”)
- Polymerase melts duplex DNA near transcription start site, forming a transcription bubble (“Open complex”)
- Polymeraze catalyzes phosphodiester linkage of two initial rNTPs (ribonucleoside triphosphates)
Describe the Elongation phase of DNA transcription
- Polymerase advances down the template strand in the 3’ to 5’ direction, melting duplex DNA and adding rNTPs to growing RNA
Describe the Termination phase of DNA transcription
- at the transcription stol site, polymerase releases completed RNA and dissociates from DNA
A chemical view of RNA synthesis:
What are the requirements for RNA synthesis?
- Enzyme: RNA polymerase
- DNA Template (3’ to 5’ strand)
- No primer required
- Nucleoside triposphates - ATP, GTP, CTP, UTP
- Synthesis is 5’ to 3’
Describe the roles of RNA in protein synthesis:
Three types of RNA are involved in translation:
- mRNA carries information copied from DNA
- tRNa deciphers the code and delivers the specified amino acid
- rRNA associates with a set of proteins to form ribosomes
How does tRNA acquire and carry appropriate amino acids?
An enzyme called amynoacyl tRNA synthetase attaches amoni acids to their corresponding tRNA molecules using energy from ATP. Each amino acid has its own tRNA molecule with the anticodon for that amino acid.
What determines the fidelity of protein synthesis?
- correct aminoacylation of tRNA (tRNA with the correct anticodon is conjucated with the correct amino acid)
- correct codon-anticodon pairing in the ribosome
Describe Amynoacyl tRNA synthetase and its mechanism for error correction:
Amynoacyl tRNA synthetase:
- at least one for every amino acid
- for different codons there are different synthetases
- error correction is in the specificity of synthetase and tRNA. No mechanism exists for error correction once tRNA is mischarged and separated from the synthetase.
Double sieve mechanism for error correction:
- synthetases have 2 sites: active site and hydrolytic site.
Amio acids larger than the correct one are not activated because they are too large to fit into the active site. Smaller amino acids fit into the hydrolytic sites (which excludes the correct amino acid) and are hydrolyzed.
When is nonstandard base pairing most likely to occur?
When the base is in the third (or “wobble”) position of an mRNA codon; in these cases the codon may be recognized by tRNA that has these bases in first position of the anticodon
