Translation Flashcards
Translation of RNA into protein
Convert mRNA sequence to protein sequence, uses specific code, 4 bases, 20 amino acids, three bases for one amino acid, codon in mRNA matches anticodon in tRNA
Start codon
AUG (ATG in DNA)
Stop codons
UAG, UGA, UAA
Total tRNA
Code is degenerate but unambiguous, 61 codons for amino acids, 3 stop codons, <61 tRNAs required
Reading the code
Start codon (AUG) sets the reading frame, each codon read in sequence, each signifies one amino acid or stop codon
Point mutations
Change of base pair, missense, nonsense, null, silent, can occur through DNA replication mistakes, repair mistakes, chemically altered base that mispairs
Deletion mutation
Removal of one or more bases, can occur through intercalating chemicals, DNA polymerase slips, mobile genetic elements
Insertion mutation
Addition of one or more bases, can occur through intercalating chemicals, mobile genetic elements
Frameshift mutation
Deletion or insertion of a number of bases that cannot be divided by three, occurs by intercalating chemicals, mobile genetic elements
Inversion mutation
Inversion of a sequence of bases (may cause frame shift), caused by mobile genetic elements
Silent mutation
A change that specifies the same amino acid
Missense mutation
Change that specifies a different amino acid
Nonsense mutation
Change that produces a stop codon
Consequences of frameshift mutation
Insert or delete bases, not a multiple of three, normal protein sequence before, completely changed after site of mutation, usually leads to premature termination by new stop codon in new reading frame
Splice site mutations
May decrease splicing efficiency, may alter splicing, inserting sequence from intron into mRNA or cause exon skipping, loss of one or more exons
Large insertions or deletions
Can remove large portion of the gene or entire gene, or interrupt gene with insertion
Triplet repeat expansion
Proteins with long run of same codon, may increase number of repeats with time, at some length repeat interferes with protein function and/or production
Amino acids and tRNAs
Amino acid is attached to specific tRNA by unique aminoacyl-tRNA synthetase, recognition site on tRNA varies for each, one aminoacyl-tRNA synthetase for each tRNA
Aminoacylation
Aminoacyl-tRNA synthetase, amino acid attached to terminal adenine of CCA, initial step uses ATP to form intermediate, carboxyl group on amino acid is joined to 3’ carbon of adenine by ester bond, amino group free to be added to polypeptide chain
Error correction
Incorrect amino acid is identified, transferred to editing site and removed, tRNA can be charged with correct amino acid
Initiation of translation
mRNA associates with small ribosomal subunit (30S/40S), charged met-tRNA binds to initiation factor and large subunit (50S/60S), components associate producing functional ribosomes, requires ATP and GTP hydrolysis for energy
Prokaryotic protein factors in initiation of translation
IF-1, IF-2, IF-3
Eukaryotic protein factors in initiation of translation
eIFs (eukaryotic initiation factors)
Translation elongation
Amino acids are added to the carboxyl end of the growing polypeptide, delivery of aminoacyl-tRNA is directed by elongation factors, peptide bond formation is catalyzed by peptidyl transferase
Elongation factors
EF-Tu (eEF1a bound to GTP), EF-Ts (eEF-1b), requires GTP
Peptidyl transferase
Not a protein, is a ribozyme, enzymatic activity is associated with large ribosomal subunit (50S or 60S)
Steps in elongation
Charged aminoacyl tRNA arrives at the aminoacyl (A) site, adjacent to previous tRNA in the peptide (P) site which is attached to the polypeptide chain, peptidyl transferase attaches the new amino acid to the chain, uncharged tRNA moves from P site to exit (E) site and is released, amino acid with the chain moves from A to P sites
Translocation
Movement of the ribosome along the mRNA molecule, causes the net movement of the peptidyl-tRNA from the A site to the P site, uncharged tRNA is moved from the P to the E site where it is released
Requirements for translocation
Requires EF-G (eEF2), GTP
Termination of translation
Termination at first in frame stop codon, no tRNA with complementary anticodon, release factors bind, peptidyl transferase hydrolyzes the bond between polypeptide and final tRNA, ribosome subunits dissociate, release mRNA
Polysomes
Each mRNA can associate with more than one ribosome, faster translation, as one ribosome translocates away from the 5’ end, another can be recruited
Prokaryotic fMet tRNA
Initial methionine in prokaryotes does not have free amino group, formyl transferase enzyme adds formyl group, resembles peptide bond, removed later by another enzyme
Post-translational modification
Proteins can be modified after synthesis, various groups can be added to specific amino acid residues, modify function, activity, or help target protein to sub-cellular compartments
Carboxylation
Glutamate, ex- coagulation cascade
Hydroxylation
Proline, lysine, ex- collagen stability
Phosphorylation
Serine, threonine, tyrosine, ex- enzyme activity
Glycosylation
Serine, asparagine, ex- secretion, membrane
Fatty acylation
Ex- membrane anchor
Prenylation
Ex- membrane anchor
ADP-ribosylation
Ex- enzyme activity
Collagen assembly
Procollagen is post-translationally modified, hydroxylation of proline and lysine, glycosylation, triple helix forms and is stabilized by hydrogen bonds from hydroxyproline and hydroxylysine residues
Osteogenesis imperfecta
Caused by defects in collagen assembly or stability due to missense mutations, defects in post-translational modifications
Protein targeting
Proteins for secretion, membranes, organelles are often modified in ER and Golgi, synthesized by ribosomes of rough ER
Ribosomes of rough ER
Signal peptide of nascent protein binds signal recognition particle (SRP), docks with receptor on ER, protein enters ER lumen via pore, can be modified and enter secretion pathway
Lysosomal enzymes
Enzymes to degrade large molecules, modified with mannose-6-phosphate in Golgi, without M6P enzymes go to secretory vesicles and out of cell
I-cell disease
Lack of phosphotransferase activity, lysosomal enzymes leak out of cells, lysosomes filled with debris (inclusion bodies), serious health effects
Streptomycin
Inhibitor of protein synthesis, binds to the 16S rRNA of the 30S subunit, inhibits translation initiation
Tetracycline
Inhibitor of protein synthesis, binds to the 30S ribosomal subunit, blocks binding of the aminoacyl-tRNA to the A site of the ribosome, many bacteria are resistant
Chloramphenicol
Binds to 50S ribosomal subunit, blocks binding of amino acid on aminoacyl-tRNA, blocks peptidyltransferase activity, can inhibit mitochondrial activity, only used in serious conditions
Erythromycin
Binds to 50S ribosomal subunit, prevents translocation
