Protein Synthesis

Making proteins from genes One gene-one enzyme hypothesis (Beadle and Tatum) Later revised to the one gene-one polypeptide hypothesis Transcription synthesis of RNA under the direction of DNA (mRNA) Translation synthesis of a polypeptide under the direction of mRNA

DNARNAProtein ‘U’ (uracil) replaces ‘T’ in RNA The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ Triplet code/codons More than one triplet for each Amino Acid

The Genetic Code

Initiation of Transcription Transcription factors attach to TATA box Contains sequences of only adenine and thymine RNA polymerase attaches to begin transcription

Elongation and Termination RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end Termination RNA polymerase reaches terminator sequence Polyadenylation signal--AAUAAA

Transcription Helpers 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

mRNA modification 5’ cap: modified guanine molecule Protection recognition site for ribosomes 3’ tail: poly (A) tail (up to 250 adenine nucleotides) Recognition transport

RNA Splicing Exons (expressed sequences) kept Introns (intervening sequences) spliced out Splicing done by snRNPs which bind together to form a large compound known as a spliceosome Made of snRNA RNA that acts as an enzyme= ribozyme

Basics of Translation mRNA moved through ribosome “code” read in triplets tRNA transfers correct amino acid to the polypeptide chain

tRNA Has an anticodon (nucleotide triplet) Codes for a specific amino acid

Picking the right Amino Acid Aminoacyl-tRNA synthetase Joins a specific amino acid to its tRNA molecule Wobble Relaxation in the base-pairing rules Third nucleotide in a codon can be different in many cases and still code for the same amino acid

Ribosomes rRNA (ribosomal RNA) P site A site E site site of mRNA codon & tRNA anticodon coupling P site holds the tRNA carrying the growing polypeptide chain A site holds the tRNA carrying the next amino acid to be added to the chain E site discharged tRNA’s leave the ribosome

Initiation of Translation Union of mRNA, tRNA, and small ribosomal subunit Start codon = AUG (codes for methionine) Large subunit then attaches

Elongation Codon recognition Peptide bond formation Translocation Anticodon of tRNA binds with the complementary mRNA codon in the A site Peptide bond formation Large subunit forms a peptide bond between the amino acid in the A site and the growing polypeptide Translocation tRNA in A site moves to P site… the one in the P site moves to the E site

Termination When a stop codon (UAG, UAA, or UGA) is reached, the A site accepts a “release factor” Release factor cuts the polypeptide free from the tRNA molecule Ribosome dissociates

Polyribosomes Translation of mRNA by many ribosomes many copies of a polypeptide very quickly

Where does it happen? Protein synthesis always begins in the cytosol Will continue there unless it receives a signal peptide telling it to bind to the ER SRP’s (signal-recognition particles) Binds to a ribosome and brings it to the membrane of the ER Attaches to a receptor molecule

Roles of RNA

Mutations Point mutation- The change of just one nucleotide Base-pair substitution One nucleotide substituted for another Sometimes a “silent mutation” if it still codes for the same amino acid Missense mutation- still codes for an amino acid, but not the right one Nonsense mutation- changes to a stop codon and terminates translation

Frame-shift Mutations Insertion Nucleotide is added Deletion Nucleotide is lost Causes the codons from that point on to be incorrectly grouped Mutagen- anything that causes a mutation (chemicals, X-rays, carcinogens, etc.)