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Protein Synthesis AP Biology Ch. 17 “One Gene-One Polypeptide” Theory George Beadle and Edward Tatum (late 40’s to early 50’s) used X-rays to induce mutations in Neurospora crassa (bread mold) Auxotrophs—nutritional mutants unable to synthesize certain amino acid and vitamins. They traced the defect to the enzymes involved in their synthesis. Conclusion: One gene encodes one enzyme. Gene products encode both protein and RNA Central Dogma of Molecular Biology In eukaryotic cells, a message (mRNA) carries the genetic information (DNA) from the nucleus to the cytoplasm Transcription An adapter (tRNA) translates the nucleic acid (mRNA) into amino acid (protein) Translation Information flow is unidirectional GCTGCTAACGTC AGCTAGCTCGTA GCGCTAGCGCTT GCGTAGCTAAAG TCGAGCTCGCTT GCGTAGCTAAAG TCGAGCTGCTGC TAACGTCAGCTA GCTCGTAG RNA Proteins Structure of RNA Single strand of nucleotides [instead of 2 strands in DNA] Nucleotides contain 5-carbon sugar ribose [instead of deoxyribose in DNA] 4 different nitrogenous bases Adenine (A) Uracil (U) [instead of thymine in DNA] Cytosine (C) Guanine (G) Forms of RNA 3 Main Types of RNA mRNA (messenger RNA) brings DNA message out of nucleus to the cytoplasm Each 3 bases on mRNA is a “codon” tRNA (transfer RNA) –The anticodon that matches with the codon from mRNA to determine which amino acid joins the protein chain rRNA (ribosomal RNA) – make up the ribosomes—RNA that lines up tRNA molecules with mRNA molecules Transcription: Synthesis and Processing of RNA RNA Polymerase is an enzyme that synthesizes RNA using one strand of the DNA as a template Separates 2 strands of DNA and links RNA nucleotides as they base-pair along the DNA template in an overall is 5’ to 3’ direction Transcription: Initiation RNA polymerase attaches to promotor sequence (TATA box) of DNA sequence Prokaryotes— enzyme binds directly to sequence Eukaryotes— enzyme binds after transcription factors bind to the site Transcription: Elongation RNA polymerase Synthesizes in 5’ 3’ direction, antiparallel to DNA template Unwinds about 20 base pairs as it reads and synthesizes complementary RNA Transcription: Termination RNA polymerase terminates transcription at special DNA sequences, terminator Prokaryote—stops right at the end of termination signal Eukaryote—continues past termination signal, to a poly(A) tail (AAUAAA) in the pre-mRNA Transcription The code on DNA tells how mRNA is put together. Example: DNAACCGTAACG mRNAUGGCAUUGC Each set of 3 bases is called a triplet or codon (in mRNA) UGG CAU UGC RNA Splicing Pre-mRNA never leaves the nucleus Introns—noncoding regions Exons—coding regions snRNPs join with other proteins to form spliceosome, which releases introns Translation Genetic information in mRNA is read by codons 64 different codons Genetic code is redundant—more than one codon codes for many of the 20 amino acids AUG—universal start codon UAA, UAG, UGA— stop codons Structure of tRNA Aminoacyl-tRNA Formation Structure of a Ribosome Translation: Initiation Small ribosomal subunit binds to start codon (AUG) on mRNA Initiator tRNA (UAC) base-pairs with mRNA Large ribosomal unit binds so initiator tRNA is in the P site GTP provides energy for intiation process Translation: Elongation Translation: Termination Review entire process Coupled Transcription and Translation in Bacteria Point Mutations Substitution Mutations Missense mutation: altered codon still codes for an amino acid, although maybe not the right one Nonsense mutation: altered codon is a stop codon and translation is terminated prematurely Leads to nonfunctional proteins Insertions and Deletions Frameshift mutation: addition or loss of one or more nucleotide pairs in a gene shifts the reading frame for translation and incorrect protein is made The end Reading direction Ribosome 5’ mRNA U U C 3’ UG G A U G U A C Met U G G A C C Trp U U C U GG Incoming tRNA charged with amino acid Ribosome Reading direction 5’ 3’ mRNA U U C UG G A U G U G G A C C Outgoing “empty” tRNA Trp Met U U C A A G Phe U GG Ribosome Reading direction 5’ mRNA U U C 3’ UG G A U G U G G U U C A A G U GG A C C A C C Phe Trp Met Trp Ribosome Reading direction 5’ 3’ mRNA U U C UG G A U G U G G U U C U GG A C C A C C Phe Trp Met Trp Reading direction Ribosome 5’ 3’ mRNA U U C UG G A U G Outgoing “empty” tRNA Met Anticodon A A G A C C Phe Trp Amino acid U G G A C C Trp U U C U GG Incoming tRNA charged with amino acid