Translation Flashcards
Nirenberg and Matthaei
breaking of genetic code resulted from studies of protein synthesis using cell-free extracts of E. coli and synthetic mRNAs
complete mRNA genetic code noteworthy facts
- codons refer to base triplets in mRNAs
- 61 of 64 trinucleotides are referred to as sense codons. others are nonsense
- codons are 5’ to 3’
- amino acids can be encoded by more than one codon
- aug = start
- uaa, uag, uga = stop
given a protein sequence, can you tell what the DNA/RNA sequence was?
no
degenerate
multiple codons can specify a single amino acid
specific/unambiguous
each codon specifies only one amino acid
non-overlapping
one a start codon is recognized, there is only one correct “reading frame” of triplets
no punctuation
once initiated, bases are read continuously (without pausing) until a stop codon is reached
universal
translation process and coding in all species is always the same
tRNA
- each tRNA has an anticodon loop that has 3 bases that can base pair with the mRNA
- we don’t have one tRNA per codon
wobble hypothesis
- proposed by Crick
- base-pairing at 3rd position is not as as constrained as other 2 bases
- permits flexibility in codon; anticodon recognition, allowing mRNA translation with < 64 tRNAs
wobble rules
- the first 2 base pairing in a codon-anticodon interaction confer most of the specificity required for translation
- the interactions between the third codon and anticodon nucleotides are less stringent
aminoacyl-tRNAs
-formation is two step reaction
class 1 aminoacyl-tRNAs
function as monomers
class 2 aminoacyl-tRNAs
function as polymers or dimers
requirements for protein synthesis
- mRNA transcript
- assorted supply of aminoacyl-tRNAs
- large and small ribosomal subunits
- assorted initiation and elongation factors
- GTP-energy
shine-dalgarno sequences
- in prokaryotes
- help align mRNA to ribosome so translation begins at appropriate position
components of bacterial ribosomes
70s made up of 50s and 30s subunits
components of eukaryotic ribosomes
80s ribosome is similar in structure to bacterial ribosome but composed of 60s and 40s subunit
In eukaryotes, which RNA polymerase would be responsible for transcribing the rRNA in the large subunit of the ribosome?
RNA Pol I
In eukaryotes, which polymerase would be responsible for transcribing rRNA in the small subunit?
RNA Pol III
In prokaryotes, which RNA polymerase is responsible in transcribing the large subunit rRNA?
They only have one RNA polymerase
initiation of translation
- binding of mRNA, ribosomal subunits, and first aminoacyl-tRNA
- IF1 and IF3 bind to 30s subunit
- 30s subunit binds to Shine-Dalgarno sequence upstream from AUG
- 30s subunit slides 5’ to 3’ to position AUG start codon in P site
- Ensures first AUG is located
- IF2 w GTP binds to 30s A site
- GTP hydrolyzed
elongation of translation
- sequential formation of all peptide bonds
- before it begins, peptide chain is attached to P site and E and A sites are empty
- recognized aminoacyl-tRNA then binds in A site with help of EF-Tu elongation factor
- peptide bond is formed by peptidyltransferase region
- ribosome translocates to move P site tRNA to E site, and newly added peptide-tRNA from A site to P site
termination of translation
- stop codon recognition, release of polypeptide and dissociation of ribosome subunits
- when ribosome encounters stop codon, no tRNA complementary to codon
- release factor binds the A site
- peptidyl-tRNA is attacked by a water molecule, releasing peptide
- release factor RF3 releases process through GTP binding and hydrolysis
- ribosome then dissociates from mRNA
poly-ribosomes
- multiple ribosomes attached to a single mRNA
- up to 50 can be attached to one mRNA
difference in ribosome subunits prok and euk
prok: 30S + 50S = 70S
euk: 40S + 60S = 80S
difference in mRNA processing prok and euk
prok: rRNas and tRNAs
euk: rRNAs, tRNAs, and mRNAs
difference in transcript recognition prok and euk
prok: Poly-G/A shine-dalgarno sequence
euk: 5’ cap and poly A tail
difference in rate of transcription prok and euk
- prok: 20 amino acids per sec, coupled to transcription
- euk: 1 amino acid per sec, not coupled to transcription
