Transcript Transcription & Translation

Transcription
• RNA Polymerase ‘reads’ template DNA strand to make
complement RNA (mRNA code for a polypeptide).
• Associated with a gene(s) is an up-gene promoter/operator
sequence for RNA polymerase binding and a down-gene
termination sequence.
• Gene transcription can be regulated (on/off switch)
negatively or positively by regulatory proteins (more later).
Translation
• Ribosome moves along the mRNA, codon-by-codon,
adding appropriate amino acids via specific tRNAs to its
growing polypeptide product.
Transcription
Genes +
p/operator 3’
= operon. 5’
Sigma-factor
Gene(s)
5’ antisense
3’ sense
Operator
region
Antisense (template) strand
Sense (complementary)
strand
Trailer
sequence
Promoter Region
• Site for RNA Polymerase recognition and binding.
• Recognition requires involvement of a sigma factor.
• Different promoter types have their own sigma factor.
• There can be an adjacent regulator binding site (= operator).
Pribnow Box
mRNA Elongation
(recall that Us replace Ts)
5’
3’
5’
3’
Transcription
Termination
• Rho – dependent: a protein
called “ρ” unwinds the DNA
template – mRNA complex; thus
weakening the association until
mRNA and RNA polymerase
falls off DNA.
• Rho-Independent: here two
G+C regions rich slow RNA
polymerase; a hairpin forms in
the mRNA weakening the
mRNA-DNA template
association; which is further
weakened by poly-A stretch.
rRNA and tRNA Products
• rrn-gene has multiple rRNA & tRNA products.
• Spacer RNA is spliced out by special nucleases.
• rrn-operons are often located close to Ori.
• Why might this have been selected? (think fast growth)?
mRNA product
• Within the leader is the Shine-Dalgarno sequence (5’AGGA3’). It
complements a site on 16SrRNA of ribosome; used to bind a ribosome to
mRNA for translation.
• Trailer at 3’ end of mRNA was required for proper ending of
transcription.
• Translation of mRNA information into protein starts at the initiation
codon (5’AUG3’); proceeds codon by codon until the end of gene, as
identified by a stop, or non-sense, codon.
• Coding region of mRNA can be polygenic (multiple genes = multiple
starts / stops).
Coding Region
Direction of Translation
Genetic Code:
(3 nonsense; 61 coding yet fewer tRNA)
start
Genetic code
degeneracy; more
than one codon for
some amino acids.
Results from tRNA
“wobble” of first
anticodon position;
sometimes a novel
nucleotide like inosine
(e.g. ICC anticodon
for Gly codons GGA,
GGC, GGU).
Codon written in
reverse (3’→5’)
3’-A U G-5’
Initiation Codon establishes the
proper “reading-frame”!
Translation
Translation
Initiation
Three initiation factor proteins
are involved (IF1, IF2, IF3).
30S subunit alone recognizes
Shine-Delgarno sequence of
mRNA, which binds initiation
tRNA, IF1 & IF2; IF3 released.
50S subunit binds 30S (=70S),
sandwiching mRNA and placing
initiation tRNA at the P-site.
A-site ready to receive tRNA for
next codon. Ribosomes slide
along mRNA in 5’→3’ direction.
Translation
Elongation
Peptidyl
transferase
Translation
Termination
Elongation of polypeptide
continues until a stop
codon is reached at the Asite.
Ribosome stalls, allowing
release factor proteins to
bind.
Upon dissociation of 50S
from 30S, the IF3 binds
30S to make it receptive
to a new mRNA start.
Stop
codon