From Gene to Protein n ie: Transcription & Translation

Protein Synthesis: overview n One gene-one enzyme hypothesis »(Beadle and Tatum) n One gene-one polypeptide (protein) hypothesis n The linear sequence of DNA determines the sequence of Amino Acids n Transcription: synthesis of mRNA under the direction of DNA n Translation: actual synthesis of a polypeptide under the direction of mRNA n Difference in types of cells: n Prokaryotes- DNA not segregated from ribosomes. Transcription & Translation occurs in rapid succession n Eukaryotes- mRNA is modified and translocated to the cytoplasm for translation

The Triplet Code n The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ called…. n Codons: the basic unit of genetic code n Don’t forget! –‘U’ (uracil) replaces ‘T’ in RNA n X n = number of amino acids that can be coded for…. n where X= # of bases ; n=bases per codon For life on Earth: 4 3 = 64

Transcription: n Only one strand of DNA is transcribed : “The Template Strand” 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 continued… n Initiation~ transcription factors mediate the binding of RNA polymerase II to an initiation sequence (TATA box) n Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end.. Uses ATP this time! n Termination~ RNA polymerase reaches terminator sequence… signals the RNA polymerase to stop transcription and release the mRNA

Transcription: overview n See Campbell animations!!! D:\bc_campbell_biology_7\0,7052, ,00.html

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 D:\bc_campbell_biology_7\0,7052, ,00.html

Translation n mRNA from nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome n Each codon on the mRNA has a complimentary … n tRNA anticodon (nucleotide triplet) n Each tRNA has been previously linked to an amino acid using the enzyme… n “Aminoacyl-tRNA-synthetase” n Wobble : the ability of one tRNA to recognize 2-3 different mRNA codons. Example: n CC _ anticodon recognizes GGC, GGA, GGG, and GGU but all code for Glycine. “Redundancy not Ambiguity!”

Translation continued n Ribosome: made of 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 continued (switch to transparency?) 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)

Translation n See Campbell animations!! D:\bc_campbell_biology_7\0,7052, ,00.html

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: n Silent mutations no effect on protein n Missense mutations ∆ to a different amino acid (different protein) n Nonsense mutations ∆ 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. Examples include: n Radiation…or n Base Analogs