From Gene to Protein

1. From Gene to Protein • ie: Transcription & Translation

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

3. The Triplet Code • The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ called…. • Codons: the basic unit of genetic code • Don’t forget! • ‘U’ (uracil) replaces ‘T’ in RNA • Xn= number of amino acids that can be coded for…. • where X= # of bases ; n=bases per codon For life on Earth: 43 = 64

4. Transcription: • Only one strand of DNA is transcribed: “The Template Strand” • RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA code • Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins • Terminator region: sequence that signals the end of transcription • Transcription unit: stretch of DNA transcribed into an RNA molecule

5. Transcription continued… • Initiation~ transcription factors mediate the binding of RNA polymerase II to an initiation sequence (TATA box) • Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end.. Uses ATP this time! • Termination~ RNA polymerase reaches terminator sequence… signals the RNA polymerase to stop transcription and release the mRNA

6. Transcription: overview • See Campbell animations!!! D:\bc_campbell_biology_7\0,7052,3117225-,00.html

7. mRNA modification • 1) 5’ cap: modified guanine; protection; recognition site for ribosomes • 2) 3’ tail: poly(A) tail (adenine); protection; recognition; transport • 3) RNA splicing: exons (expressed sequences) kept,introns (intervening sequences) spliced out; spliceosome D:\bc_campbell_biology_7\0,7052,3117225-,00.html

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

9. Translation continued • Ribosome: made of rRNA)site of mRNA codon & tRNA anticodon coupling • P siteholds the tRNA carrying the growing polypeptide chain • A siteholds the tRNA carrying the next amino acid to be added to the chain • E site discharged tRNA’s

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

11. Translation • See Campbell animations!! D:\bc_campbell_biology_7\0,7052,3117225-,00.html

12. Mutations: genetic material changes in a cell • Point mutations…. • Changes in 1 or a few base pairs in a single gene • Base-pair substitutions: • Silent mutations no effect on protein • Missense mutations ∆ to a different amino acid (different protein) • Nonsense mutations ∆ to a stop codon and a nonfunctional protein • Base-pair insertions or deletions:additions or losses of nucleotide pairs in a gene; alters the ‘reading frame’ of triplets~frameshift mutation • Mutagens: physical and chemical agents that change DNA. Examples include: • Radiation…or • Base Analogs