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Chapter 17: From Gene to Protein One gene one polypeptide Overview Transcription o DNA RNA o DNA acts as a template for the new RNA molecule o Carries DNA “message” to protein-synthesizing machine (mRNA = messenger RNA) o Occurs in nucleus Translation o RNA polypeptide o Translate base sequence to amino acid sequence o Occurs at the ribosomes Genetic Code 4 nucleic acids 43 = 64 combinations 20 amino acids in transcription, one strand of DNA is the template strand mRNA is complementary to the template in RNA code, Uracil takes the place of Thymine o ex: template strand of DNA has ACC … mRNA complement is UGG codon= triplet of nucleotides that codes for one amino acid o # nucleotides on an mRNA is 3x the number of amino acids o see chart on p. 299 o start codon is AUG also codes for Met polypeptide starts being made here o stop codons are UAA, UAG, UGA; these indicate the end of translation o reading frame: grouped sequence in which the code is read ALL LIVING THINGS SHARE THE SAME GENETIC CODE! Transcription (transcribe = copy) RNA polymerase o first pulls 2 DNA strands apart o also adds new RNA nucleotides (to 3’ end only) o transcription unit = piece of DNA that is transcribed onto RNA (the whole DNA molecule isn’t transcribed at once!!) Stages 1. Initiation Promoter region o Initial site of RNA polymerase attachment o Includes start codon and several dozen nucleotide pairs “upstream” Transcription factors o Mediate binding of RNA polymerase and initiation of transcription o Transcription factors + RNA polymerase = transcription initiation complex TATA box o Transcription factor binds here TATAAAA ATATTTT 2. Elongation RNA polymerase moves down DNA sequence, unwinding it 10 – 20 bases at a time Rate of 60 nucleotides / second Several molecules of polymerase may work on transcribing the same gene at the same time in order to speed things up 3. Termination Terminator = RNA sequence that signals the end of transcription Transcription produces a precursor to the polished RNA molecule called pre-RNA Modifications to RNA o Alterations of ends 5’ cap = modified G is added to 5’ end, protecting RNA from being degraded by enzymes and serving as a signal for ribosome attachment poly (A) tail = 30-200 amino acids on the 3’ end that prevents enzyme degradation and helps ribosome attachment; also helps transport RNA out of nucleus o RNA splicing Exons are coding regions of the RNA that are eventually expressed Introns are noncoding regions in between the exons that must be removed Introns are removed from pre-RNA sequence before export to the cytoplasm Sequences at the end of introns signal snRNP’s (small nuclear ribonucleoproteins) to splice at those locations snRNP’s work with other proteins to cut and paste together the exons o ribozymes intron RNA catalyzes its own removal NOT ALL CATALYSTS ARE PROTEINS!! Translation tRNA (transfer RNA) translates the mRNA code by transferring amino acids within the cytoplasm to the ribosome o each type of tRNA links a specific codon wth its corresponding amino acid o anticodon: sequence of tRNA complementary to mRNA codon o made in the nucleus during transcription o strand of 80 nucleotides folded into cloverleaf-like shape o 45 different tRNA’s the base-pairing rules relax a bit on tRNA molecules; this is known as wobble o ex: U in 3rd position of anticodon can pair with A or G o ex: I (inosine) in 3rd position of anticodon can pair with U, C, or A o aha! This is why 1 amino acid may correspond with multiple codons, only differing in the last letter aminoacyl-tRNA synthetase joins amino acids to their respective tRNA’s (20 types) ribosomes consist of 2 subunits (large and small) o both subunits are made in nucleolus and combine in cytoplasm o proteins + rRNA (ribosomal RNA) o binding sites P site: holds tRNA carrying growing polypeptide A site: holds tRNA carrying the on-deck amino acid E site: where tRNA molecules exit the ribosome Stages (same as transcription) 1. Initiation Requires GTP (source of energy) mRNA, tRNA, 2 ribosomal subunits come together 2. Elongation Requires GTP Amino acids added single file, with peptide bonds forming in between Translocation: tRNA in A site moves to P site, then E site 3. Termination Release factor binds to the site where the stop codon is “read” Causes H2O to bind to pp, hydrolyzing pp from ribosome Everything dissasembles, leaving completed pp behind Mutations Changes in genetic material Point mutations can change only a tiny piece in the DNA and may or may not have dramatic results Base pair substitutions o Replacement of nucleotide pair with another pair o Silent mutations do not change which amino acid is synthesized o Missense mutations still codes for an amino acid, but the wrong one o Nonsense mutations code for stop codon, causing production of nonfunctional protein Insertions and Deletions o + or – one or more nucleotide pairs o usually have a more profound effect than bp substitutions o frameshift mutation occurs if # of nucleotides added or deleted 3 alters all codons located “downstream”