Lecture #7Date _________ n Chapter 17~ From Gene to Protein

Protein Synthesis: overview n One gene-one enzyme hypothesis (Beadle and Tatum) n One gene-one polypeptide (protein) hypothesis n Transcription: synthesis of RNA under the direction of DNA (mRNA) n Translation: actual synthesis of a polypeptide 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 n ‘U’ (uracil) replaces ‘T’ in RNA

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) n Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end n Termination~ RNA polymerase reaches terminator sequence

mRNA modification n 1) 5’ cap: modified guanine; protection; recognition site for ribosomes n 2) 3’ tail: poly(A) tail (adenine); protection; recognition; transport n 3) RNA splicing: exons (expressed sequences) kept,introns (intervening sequences) spliced out; spliceosome

Translation, I n mRNA from nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome n tRNA anticodon (nucleotide triplet); amino acid

Translation, II n rRNA site of mRNA codon & tRNA anticodon coupling n P site holds the tRNA carrying the growing polypeptide chain n A site holds the tRNA carrying the next amino acid to be added to the chain n E site discharged tRNA’s

Translation, III n Initiation~ union of mRNA, tRNA, small ribosomal subunit; followed by large subunit n Elongation~ codon recognition peptide bond formation translocation n Termination~ ‘stop’ codon reaches ‘A’ site n Polyribosomes: translation of mRNA by many ribosomes (many copies of a polypeptide very quickly)

Mutations: genetic material changes in a cell n Point mutations…. n Changes in 1 or a few base pairs in a single gene n Base-pair substitutions:silent mutations no effect on proteinmissense ∆ to a different amino acid (different protein)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

Disorders n Nondisjunction- when the chromosomes don’t split from each other giving one cell more than the other. n Autosomal- these are all of your chromosomes except for your sex chromosomes. You have 44 autosomal chromosomes. n Down syndrome- produces mild to severe mental retardation. (nondisjunction) n Albinism- lack of pigment (recessive) n Cystic fibrosis- excess mucus in lungs, digestive tract, liver; increased susceptibility to infections (recessive) n Galactosemia- accumulation of galactose (sugar) in tissue, mental retardation; eye and liver damage (recessive)

n Phenylketonuria PKU- accumulation of phenylalanine in tissue; lack of normal skin pigment; mental retardation (recessive) n Tay-Sachs disease- lipids accumulation in brain cells; mental deficiency; blindness; death in early childhood (recessive) n Achondroplasia- Dwarfism (dominant) n Hunington’s disease- mental deterioration and uncontrolled movements (dominant) n Hypercholesterolemia- excess cholesterol in blood; heart disease (dominant) n Sickle cell- misshapen, or sickled, red blood cells; damage to many tissue (co-dominant)