Translation and Proteins

1. Translation and Proteins Sections 14.1-14.4 & 14.7-14.11

2. The genetic code is _________, meaning that an amino acid may be coded by more than one codon. • unambiguous • degenerate • commaless • universal • nonoverlapping

3. The genetic code is _________, meaning that an amino acid may be coded by more than one codon. Answer:2. degenerate Explanation:Degeneracy is a term for redundancy in the genetic code; 61 codons code for 20 amino acids.

4. The wobble hypothesis predicts that codons coding for the same amino acid • may differ at the first position. • may differ at the second position. • may differ at the third position. • may differ at the first two positions. • may differ at all three positions.

5. The wobble hypothesis predicts that codons coding for the same amino acid Answer:3. may differ at the third position. Explanation:The wobble hypothesis suggests that pairing may be less stringent at the third codon position. This allows the same tRNA molecule to pair with two or more codons that are identical at the first two codon positions.

6. A nonsense suppressor mutation is a mutation • that alters the reading frame. • that changes the amino acid sequence of the gene product. • that creates a termination codon. • in a tRNA gene that allows it to recognize a termination codon. • that allows a ribosome to bypass termination codons.

7. A nonsense suppressor mutation is a mutation Answer:4. in a tRNA gene that allows it to recognize a termination codon. Explanation:A nonsense mutation changes an amino acid coding codon to a termination codon. A second suppressor mutation in a tRNA gene restores the function of the nonsense mutation by allowing an amino acid to be added to the polypeptide.

8. The sigma subunit of bacterial RNA polymerase • binds to a bacterial gene’s promoter. • is composed of both polypeptide and RNA molecules. • is required for RNA polymerization. • is required for termination of transcription. • is required for ribosomal binding.

9. The sigma subunit of bacterial RNA polymerase Answer:1. binds to a bacterial gene’s promoter. Explanation:Sigma is a polypeptide subunit of RNA polymerase that binds to the Pribnow and TATA boxes of the promoter, putting the catalytic center of the enzyme in contact with the transcription initiation site of the gene.

10. The poly(A) tail of mRNAs • is added to the 3 end of mRNAs. • is found on most mature eukaryotic mRNAs. • is found on some prokaryotic mRNAs. • helps prevent degradation of eukaryotic mRNAs. • All of the above.

11. The poly(A) tail of mRNAs Answer:5. All of the above. Explanation:A string of many adenine residues is added posttranscriptionally to the 3 end of most eukaryotic mRNAs. The primary function is to stabilize mRNAs against degradation by exonucleases.

12. TRANSLATION Charging tRNA A. 20 Aminoacyl tRNA synthetases {aminoacylation} B. Energy from ATP links (cov) 5-phosphate group of ATP to carboxyl end of the amino acid and loses two phosphates

13. TRANSLATION • Aminoacyl tRNA synthetase transfers amino acid onto tRNA D. AMP is lost from the charged amino acid and the amino acid is attached to the tRNA via the 3’-OH or 2’OH group of the ribose of the adenine of the tRNA

15. Characteristics of the Genetic Code • Triplet code • Continuous • Nonoverlapping • Universal (almost) • Degenerate • Start and Stop • Wobble

16. Initiation of Translation Initiation (prokaryotes) A. Formylmethionine (fMet)- brought to small ribosome by tRNA with codon 5’-CAU-3’ B. Shine-Dalgarno Sequence – 5’-AGGAGG-3’ binds to 16S rRNA of small ribosome (5’CCUCCU3’) C. IF1, IF2, & IF3 (bound to ribosome) D. GTP & Mg (bound to ribosome)

17. Initiation of Translation • When fMet binds 30S-mRNA complex, IF3 leaves • 50S binds, GTP hydrolysis, and IF1 and IF2 leaves • 70S Initiation Complex formed • fMet resides in the P site and A site is vacant

19. Differences between Prokaryotic and Eukaryotic Translation • No fMet in Eukaryotes • No Shine-Dalgarno • Cap-binding protein finds the end of mRNA in Eukaryotes • AUG embedded in the Kozak sequence • Poly A tail associates with the eIF-4f protein

20. TRANSLATION Elongation A. Two sites A and P – initiator tRNA goes directly to P site B. Peptidyl transferase – peptide bond between amino acids C. E site – uncharged tRNA D. EF’s

22. TRANSLATION Termination A. Termination triplets UAG, UGA, UAA B. GTP-dependent release factors – cleave polypeptide chain C. Polyribosome

24. TRANSLATION Prokaryotes vs. Eukaryotes 1. F-met vs. 5-cap 2. Euk. larger ribosomes 3. Euk. longer lived RNA 4. Kozak Sequence – “ACCAUGG”

25. Posttranslational Modification • N-terminus/C-terminus amino acids removed (f-met) or acetylated • Amino acids within peptide chain modified • Carbohydrates attached • Trimmed • Signal peptides removed • Cofactors (metals)

26. Amino Acid Structure • Carboxyl group, amino group, R-group • R-group may be a) nonpolar, b) polar, c) negatively charged, and d) positively charged • N-terminus, C-terminus

29. Protein Structure • Primary Structure – amino acid sequence • Secondary Structure – helix, sheets • Tertiary Structure – 3-D structure • Quaternary Structure – assembling more than one polypeptide chain

33. One Gene : One Protein • Sickle Cell Anemia • HbA, HbS, HbAHbS • Hemoglobin composed of 4 chains • Glutamic acid (negative charge, polar) changed to valine (uncharged, nonpolar)

35. tRNAs • have double-stranded regions. • contain unusual nucleotides coded by genes with nonstandard bases. • can be recognized by several aminoacyl-tRNA synthetases. • can carry more than one type of amino acid per molecule. • consist of two subunits.

36. A tRNAs Answer:1. have double-stranded regions. Explanation:Regions of tRNA molecules fold back and base pair with other regions of the same molecules, creating secondary structures that are double stranded.

37. Peptidyl transferase • is a polypeptide subunit of the small ribosomal subunit. • is not found in eukaryotes. • is a function of an rRNA in the large ribosomal subunit. • catalyzes the reaction that joins a tRNA to its amino acid. • is active during initiation of translation.

38. Peptidyl transferase Answer:3. is a function of an rRNA in the large ribosomal subunit. Explanation:Though once believed to be a protein enzyme, peptidyl transferase is a ribozyme (an RNA molecule with catalytic activity). In bacterial ribosomes, this function is assigned to the 23S rRNA. It catalyzes the formation of peptide bonds between amino acids during elongation of translation.

39. The phrase “one-gene:one-polypeptide” is more accurate than “one-gene:one-enzyme” because • most genes code for nontranslated RNAs. • most proteins are not enzymes. • not all enzymes are encoded by genes. • most enzymes have multiple subunits coded by different genes. • not all enzymes are composed of polypeptides.

40. The phrase “one-gene:one-polypeptide” is more accurate than “one-gene:one-enzyme” because Answer:4. most enzymes have multiple subunits coded by different genes. Explanation:Although most proteins coded by genes are enzymes, individual genes often code for polypeptides that are folded to become subunits of functional enzymes.