n Chapter 17~ From Gene to Protein

Protein Synthesis: overview n One gene-one enzyme hypothesis (Beadle and Tatum) –The function of a gene is to dictate the production of a single enzyme n Not all proteins are enzymes n Some proteins are composed of more than 1 polypeptide chain (each of which is specified by its own gene) n Modified to : One gene-one polypeptide hypothesis n Transcription: synthesis of RNA under the direction of DNA (mRNA) n Translation: actual synthesis of a polypeptide at ribosomes under the direction of mRNA

The Triplet Code n The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ n Codons – base triplets n ‘U’ (uracil) replaces ‘T’ in RNA n Template strand – side of DNA strand to be transcribed n Read in the 5’→3’ direction n See page 314 for table

Transcription, I n RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA code n Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins n Terminator region: sequence that signals the end of transcription n Transcription unit: stretch of DNA transcribed into an RNA molecule

Transcription, II n Initiation~ transcription factors mediate the binding of RNA polymerase to an initiation sequence (TATA box – sequence at beginning of promoter) n Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end n Termination~ RNA polymerase reaches terminator sequence

mRNA modification - Eukaryotes n 1) 5’ cap: modified guanine; protection from hydolytic enzymes; recognition site for ribosomes; transport out of nucleus n 2) 3’ tail: poly-A tail (adenine); protection; recognition; transport n 3) RNA splicing: exons (expressed sequences) kept,introns (intervening sequences) spliced out; n – snRNPs (small nuclear ribonucleoproteins) – join to form spliceosomes – recognize splice sites – release introns/join exons

Translation, I n mRNA from nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome n tRNA (interpreter) – used repeatedly n – Anticodon complementary nucleotide triplet n – Transfers amino acids in cytoplasm to ribosome n – aminoacyl-tRNA synthetase – binds amino acids to tRNA

Translation, II n Ribosome – 2 subunits (large, small) – composed of rRNA and proteins – only join when attached to mRNA n – Site of mRNA codon & tRNA anticodon coupling n 1 mRNA binding site n 3 tRNA binding sites n 1. P (peptidyl-tRNA) site - holds the tRNA carrying the growing polypeptide chain n 2. A (aminoacyl-tRNA) site - holds the tRNA carrying the next amino acid to be added to the chain n 3. E (exit) site - discharged tRNA’s

Translation, III n Initiation~ union of mRNA, tRNA, small ribosomal subunit; followed by large subunit – helped by initiation factors n Elongation~3 steps codon recognition peptide bond formation – btw P and A site translocation – tRNAs move to next site n Termination~ ‘stop’ codon reaches ‘A’ site – helped by release factors n Polyribosomes: translation of mRNA by many ribosomes (many copies of a polypeptide very quickly) – 2 nd ribosome attaches

Mutations: genetic material changes in a cell n Point mutations - Changes in 1 or a few base pairs in a single gene n – Base-pair substitutions: n silent mutations - no effect on protein n missense - ∆ to a different amino acid (different protein) n nonsense - ∆ to a stop codon and a nonfunctional protein n Base-pair insertions or deletions: additions or losses of nucleotide pairs in a gene; alters the ‘reading frame’ of triplets~frameshift mutation n Mutagens: physical and chemical agents that change DNA – x rays, uv light