Translation Flashcards
Triplet codon
1 triplet codon or 3 bases codes for 1 amino acid.
Note it is in RNA code so UGAC
Translation involving mRNA structure
In 5' to 3' direction Ribosomal binding site Start codon Coding sequence Stop codon
Ribosomal binding site
Site for ribosome binding
Start codon
A codon that specifies the first amino acid in a polypeptide sequence to be translated
Coding sequence
A series of codons from start codon to stop codon that determine amino acid sequence of a polypeptide.
Stop codon
Specifies the end of translation
List the 5 components for translation
Ribosome mRNA Charged tRNA Initiation and elongation factors Energy from hydrolysis of GTP
Ribosomes
Site of protein synthesis
Consists of rRNA and proteins
Arranged as 2 subunits
List the 2 subunits that make up the ribosomes
Small subunit - associates with mRNA
Large subunit - associates with charged tRNA
List the 2 important tRNA binding sites in a large subunit
Contains 3 important tRNA binding sites
1) P site
2) A site
3) Exit site
P site function
During protein translation, the P-site holds the tRNA which is linked to the growing polypeptide chain
A site function
The A site accepts an incoming tRNA bound to an amino acid.
Ribosome large subunit contains peptidyl transferase enzyme
it catalyzes the addition of an amino acid residue in order to grow the polypeptide chain in protein synthesis
Ribosomal subunits and rRNA naming after their S value
Named using parameters called S value or swedburg value.
S value refers to sedimentation value
Larger ribosomal subunits vs smaller ribosomal subunits
Larger ribosomal subunits sediment quicker than smaller ribosomal subunits.
tRNA function
Adapts each mRNA triplet codon to tis correct amino acid
2 characteristics of tRNA
- Short, composed of 75 to 90 nucleotides
- Single stranded RNA, folds on itself to form secondary structure
List 2 structures and their functions of tRNA
1) Anti-codon loop
Each tRNA contains 3 unpaired bases (triplet anticodon) at the center of the middle loop which can base pair with the complementary mRNA triplet codon.
2) tRNA 3’ acceptor region
Covalently binds to a specific amino acid and brings it to mRNA at ribosome
What is required by tRNA 3’ acceptor region to bind free amino acid
Aminoacyl tRNA synthetase and ATP are required to covalently bind free amino acid to the 3’ acceptor region of tRNA.
List the 3 Protein factors of Initiation of translation
IF1, IF2, IF3
IF1 function
Stabilize 30S subunit
IF2 function
Binds tRNA fmet to 30S-mRNA: binds to GTP and stimulate hydrolysis
IF3 function
Binds 30S subunit to mRNA, dissociate monomers to subunit at termination
List the 3 Protein factors of elongation of polypeptides
EF-Tu, EF-Ts, EF-G
EF-Tu
Binds GTP, brings aminoacyl-tRNA to the site of ribosome
EF-Ts
Generates active EF-Tu
EF-G
Stimulates translocation, GTP-dependent
List the 3 protein factors to cause the termination of transcription and release of polypeptide
RF-1, RF-2, RF-3
RF-1
Catalyzes release of polypeptide chain from tRNA
and dissociation of translocation complex;
specific for UAA & UAG termination codons
RF-2
Behaves like RF-1; specific for UGA and UAA
RF-3
F-3 Stimulates RF-1 & RF-2
Initiation step
- Small ribosomal subunit binds to a specific sequence on the mRNA
- A specific tRNA with the complementary anticodon
(UAC), binds to the start codon (AUG) on the mRNA - This tRNA which functions as the initiator tRNA, carries a modified form of methionine called N-formylmethionine (fMet)
- Large ribosomal unit then binds to ribosome small unit + mRNA
Shine dalgarno sequence
Shine-Dalgarno (SD) sequence (prokaryotes)
- AGGAGG consensus sequence located before mRNA start codon (AUG)
- Associate with 16S rRNA of small ribosome subunit
List the 3 steps In prokaryotes regarding fMet
- > The first amino acid inserted into all proteins is a modified form of methionine called N-formylmethionine (fMet) using tRNAf Met
- > When translation is completed, the formyl group or the entire fMet residue is removed
- > Normal methionine (Met) is added when AUG appears internally (using tRNAm Met)
fMet in bacteria
H
Formyl group I
C = O
I
NH O
II II
CH3 - S - CH2 - CH2 - C - C - O
I
H
List the 6 steps in Elongation in prokaryotes
- First charged tRNA is tRNAf Met which occupies the P site of ribosome and base-pairs with AUG (CODON 1) of mRNA.
- Second charged tRNA is attracted to the second mRNA codon (CODON 2) by complementary base-pairing and occupies the A site of the ribosome.
- Once both P site and A site are filled with charged tRNAs, peptidyl transferase catalyzes the formation of a peptide bond between the two amino acids.
- Concurrently, the covalent (ester) bond between the amino acid and the tRNA occupying the P site is hydrolysed, and the uncharged tRNA is released from the P site.
- Dipeptide is formed, which is attached to the tRNA at the A site.
- Elongation proceeds as ribosomes moves down 1 codon (5’ to 3’ direction).
List the 3 steps Termination in prokaryotes
- Ribosomes translate mRNA until it reaches a stop codon (UAG, UAA, UGA).
- When a stop codon occupies the A site of the ribosome, it will be recognized by the ribosome and the finished polypeptide (at P site) is then cleaved from the terminal tRNA.
- Polypeptide is released from the translation complex and the ribosome dissociates into large and small subunits
List 9 difference in translation between prokaryotes and eukaryotes
- > Ribosomes are larger and protein components are more complex.
- > mRNAs are much longer lived – most exist for hours (rather than minutes as in prokaryotes).
- > mRNA is processed to have a 5’ 7mG cap and 3’ poly-A tail and splicing of introns.
- > The 7mG cap is important for translation, mRNAs without 5’ cap are translated poorly.
- > Instead of the Shine-Delgarno sequence, there is a Kozak sequence GCCRCCAUGG (a consensus sequence, where R is a purine - A or G).
- > The start codon AUG is located within the Kozak sequence.
- > The Kozak sequence marks the initiation of translation, to facilitate the binding to the small subunit of ribosome to mRNA.
- > There is no need for fMet at the start codon (AUG).
