Nucleotide triplets:

NA QUESTION 1982: L. PETERSON/AP BIOLOGY A portion of a specific DNA molecule consists of the following sequence of nucleotide triplets: TAC GAA CTT CGG TCC This DNA sequence codes for the following short polypeptide: methionine – leucine – glutamic acid – proline – arginine Describe the steps in the synthesis of this polypeptide. What would be the effect of a deleltion or an addition in one of the DNA nucleotides? What would be the effect of a substitution in one of the nucleotides? STANDARDS: In the transcription phase of protein synthesis, students were given a point for a correct definition of transcription. They were awarded one point for mentioning each of the following: __ DNA as the template molecule for messenger RNA __ The proper base pairing (including the uracil substitution) __ The chemical characteristics of nucleotides __ A comparison of RNA and DNA (other than uracil substitution) __ The triplet arrangement of codons and/or anticodons __ The control of transcription (Operon, etc.) __ Promoters __ The role of polymerase __ Intervening sequences in eukaryotic cells __ Factors involved in the release of mRNA from DNA __ 5′ – 3′ arrangement with attachment at -OH end A definition of translation was worth an additional point with one point given for mention of each of the following: __ Movement of mRNA from nucleus to ribosome __ Association of mRNA with the ribosome and ribosomal RNA __ Location of ribosomes __ Description of ribosome/polyribosome structure __ Role of transfer RNA in transporting amino acids __ Specific characteristics of tRNA __ Amino acid-tRNA complex activation __ Role of ATP and enzymes __ tRNA initiator molecules (met or f-met) __ Initiation factors (GTP, etc.) resulting in union of 30s and 50s __ Bonding of tRNA to 50s (A site) __ Role of amino transferase __ Initiation codons __ Examples of initiator codons (AUG, GUG) __ mRNA codon exposed at A site __ Peptide bond formation (dehydration synthesis) between amino acids __ Translocation of ribosome (shift from A to P site) __ Termination (nonsense) codons __ Examples of termination codons (UGA, UAA) __ Factors that function in the release of the polypeptide A maximum of twelve points could be earned from this part of the question. In order for a student to score fifteen points, three or more points must have been from the discussion of changes in the code. A student was given one point for recognizing that an addition or deletion may alter a large portion of the resulting polypeptide. An additional point was given for explaining the importance of where the deletion or addition occurred, and another point for demonstrating knowledge of the frame shift concept. Mentioning the role of repair DNA was worth another point. A student who explained that a substitution of a nucleotide alters only one amino acid was given a point. Additional points could be earned by stating that, in some cases, the polypeptide may not be altered, that the position of the nucleotide substituted is important (” wobble” effect) and that these changes in the code are important to natural selection and evolution.