Download Transcription and Genetic Code

Protein
Synthesis
DNA
molecule
Gene 2
Gene 1
Gene 3
DNA
template
strand
TRANSCRIPTION
mRNA
Codon
TRANSLATION
Protein
Amino acid
Promoter, RNA-coding sequence and terminator
The transcription process
RNA polymerase synthesis of RNA on a DNA template
Chemical reaction
The Bacterial Promoter
Two DNA sequences are critical for
specifying the initiation of transcription
-35 5’-TTGACA- 3’ (consensus)
-10 5’-TATAAT- 3’ (Pribnow box)
E. coli – Initiation and elongation of transcription
(four polypeptides + sigma)
•Only one RNA polymerase in prokaryotes. It transcribes protein
coding genes, tRNA genes, and rRNA genes.
Termination of transcription in bacteria
• Rho dependent – type II terminators
• Rho independent – type I terminators
Termination of transcription – Rho independent terminat
Hairpin loop stalls
RNA polymerase
String of Us destabilizes the
RNA- DNA coupling
Eukaryotes – RNA polymerases
• RNA polymerase I – rRNAs
• RNA polymerase II – protein coding
genes
• RNA polymerase III – tRNAs, and other
small RNA molecules
The Eukaryotic Promoter
Includes:
1. +1 site – short sequence which
includes the transcription initiation
site
2. TATA box – at -30, consensus
sequence 5’ TATAAAA 3’
3. Promoter proximal elements
-50 to -500, interact with activators
Eukaryotes – assembly
transcription initiation
complex
Co-transcriptionally –
after the RNA chain is
around 20 to 30
nucleotides long
Eukaryotes
production of mature
mRNA – 5’ capping
Sequence of events – processing of primary transcript
Splicing mechanism
Intron removal by spliceosome
Simultaneous transcription and translation in E. coli
DNA
molecule
Gene 2
Gene 1
Gene 3
DNA
template
strand
TRANSCRIPTION
mRNA
Codon
TRANSLATION
Protein
Amino acid
General features of the Genetic Code
• Written in linear form using bases in mRNA as
“letters”.
• Each “word” within the mRNA has three “letters”.
With three exceptions, each group of three
ribonucleotides, a codon, specifies an amino acid;
the code is thus a triplet codon.
• The code is unambiguous.
• The code is degenerate.
• The code contains one “start” and three “stop”
signals, triplets that initiate and terminate
translation.
• The code has no internal punctuation. It is
commaless.
• The code is nonoverlapping.
• The code is nearly universal.
Evidence for triplet code
Frameshift Mutations
Reversion of
frameshift
mutation
Restoring the reading frame – three addition mutations
A and C
copolymer
Ratio:
1A:5C
Deciphering the code – triplet binding assay
Genetic code
Example of base-pairing wobble – best explanation for third b
redundancy