Genetic Code Flashcards
Base pair triplets in the coding region of a gene specify a _______ _______ of AAs in its polypeptide product
Collinear sequence
Genetic code is _____, _____, and _____
Degenerate, comma free, universal
Homopolymers
Synthetic mRNAs of repeated sequence
Random copolymers
mixed mRNAs of known sequences were tested in the same way and produced polypeptides with different AAs
E.g. 5’ UCCUCCUCCUCC 3’ tested and because of the 3 possible reading frames, the polypeptides produced serine (UCC), proline (CCU), and leucine (CUC)
Precipitation of tRNA
tRNA binds with specific codon on mRNA of known sequence, forms ribosomal complex, and is precipitated out of solution
Researchers able to analyze specific tRNA to determine which AA is attached
Doing that with each tRNA allowed researchers to determine what each codon specifies
Redundancy in genetic code exists to?
Reduce risk of mutations
Wobble position
3rd codon position is flexible
I (inosine)
can produce a A, U, or a G in the codon
Ribosome has 3 sites
Aminoacyl site(A), Peptidyl site (P), exit site (E)
Activation of tRNA
specialized enzyme called aminoacyl tRNA synthetase is needed
Enzyme has a pocket for the AA, a pocket for hydrolysis of ATP (drive reaction) and a catalytic binding site which incorporates tRNA
When the components are assembled and are in the presence of ATP, covalent bonds form between tRNA and AA and tRNA becomes charged and is able to function in translation
Cells contain at least one tRNA synthetase for each AA
Initiation in prokaryotic cells (TRANSLATION)
mRNA, large and small ribosomal subunits, initiation factors (IF1-3) and GTP (energy source) are all required to form the initiation complex
IF-3 binds to small ribosomal subunit, allowing small subunit to attach to mRNA
Interaction of small subunit with mRNA is facilitated by shine-dalgarno sequence
Formylmethionine tRNA (Fmet-tRNA, a modified form of methionine with a formate group attached to tRNA) forms a complex with IF-2 and GTP and binds to initiation codon
IF-1 binds small subunit and 30S initiation complex is formed
Then all initiation factors dissociate, GTP is hydrolyzed to GDP + Pi and large subunit joins the complex to form the 70S initiation complex
Elongation in prokaryotic cells
aminoacyl tRNA binds to A site of ribosome (requires complex formation of of aminoacyl tRNA, elongation factor Tu (EF-Tu) and GTP)
AA is transferred from tRNA in P site to tRNA in A site by formation of peptide bond by peptidyl transferase enzyme
Ribosome translocates along mRNA to position next codon in A site, resulting in polypeptide-tRNA being translocated from A site to P site
Ribosome translocation requires elongation factor G (EF-G) plus GTP
Uncharged tRNA is translocated from P site to E site and removed
A new aminoacyl tRNA binds in A site and cycle repeats, polypeptide grows
SUMMARY - tRNA binding (EF-Tu + GTP) → peptide bonds form (peptidyl transferase) → ribosome translocation (EF-G + GTP)
Elongation occurs until stop codon (UAA, UAG, UGA) is reached
Termination in prokaryotic cells
when a stop codon enters A site of ribosome, a release factor (RF) binds to A site
RF-1 recognizes UAG and UAA while RF-2 recognizes UAA and UGA
RF binding alters activity of peptidyl transferase, resulting in a water molecule being added to c-terminus if polypeptide, releasing it and leading to termination
Binding of RF-3 and GTP to ribosome assists in dismantling of entire complex
Do eukaryotes have fMET or shine dalgarno sequence
NO
Translation in eukaryotic cells
Eukaryotes initiation complex binds at 7-methylguanosine cap at 5’ end of mRNA and scans inwardly for AUG to begin translation
Kozak sequence - influences which AUG in vicinity of 5’ 7-MG starts translation
Sequence: 5’ GCC A/G CC AUG G 3’ (underlined nucleotides are absolutely required)
poly(A) tail of the eukaryotic mRNA interacts with the mRNA 5’ 7-MG cap structure via a cap-binding protein complex (CBC) to promote translation initiation
