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From DNA to Proteins Chapter 13 Steps from DNA to Proteins Same two steps produce all proteins: 1) DNA is transcribed to form RNA – Occurs in the nucleus – RNA moves into cytoplasm 2) RNA is translated to form polypeptide chains, which fold to form proteins Three Classes of RNAs • Messenger RNA (mRNA) – Carries protein-building instruction • Ribosomal RNA (rRNA) – Major component of ribosomes • Transfer RNA (tRNA) – Delivers amino acids to ribosomes A Nucleotide Subunit of RNA phosphate group base (uracil) sugar (ribose) Base Pairing during Transcription • A new RNA strand can be put together on a DNA region according to basepairing rules • As in DNA, C pairs with G • Uracil (U) pairs with adenine (A) Transcription & DNA Replication • Like DNA replication – Nucleotides added in 5’ to 3’ direction • Unlike DNA replication – Only small stretch is template – RNA polymerase catalyzes nucleotide addition – Product is a single strand of RNA Base Pairing Compared DNA base pairing during transcription RNA DNA base pairing during DNA replication DNA Promoter • A base sequence in the DNA that signals the start of a gene • For transcription to occur, RNA polymerase must first bind to a promoter promoter region Gene Transcription newly forming RNA transcript DNA template winding up DNA template at selected transcription site DNA template unwinding Adding Nucleotides direction of transcription 3´ 5´ 3´ 5´ growing RNA transcript Transcript Modification unit of transcription in a DNA strand exon intron exon intron exon transcription into pre-mRNA poly-A tail cap snipped out snipped out mature mRNA transcript Genetic Code • Set of 64 base triplets DNA • Codons mRNA – Nucleotide bases read in blocks of three mRNA codons • 61 specify amino acids • 3 stop translation amino acids threonine proline glutamate glutamate lysine Code Is Redundant • Twenty kinds of amino acids are specified by 61 codons • Most amino acids can be specified by more than one codon • Six codons specify leucine – UUA, UUG, CUU, CUC, CUA, CUG Near-Universal Genetic Code tRNA Structure codon in mRNA anticodon in tRNA amino acid Ribosomes funnel small ribosomal subunit + large ribosomal subunit intact ribosome Three Stages of Translation Initiation Elongation Termination Initiation • Initiator tRNA binds to small ribosomal subunit • Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon • Large ribosomal subunit joins complex Binding Sites on Large Subunit binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA) Elongation • mRNA passes through ribosomal subunits • tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA • Peptide bonds form between the amino acids and the polypeptide chain grows Elongation Termination A STOP codon moves into the area where the chain is being built. It is the signal to release the mRNA transcript from the ribosome. The new polypeptide chain is released from the ribosome. It is free to join the pool of proteins in the cytoplasm or to enter rough ER of the endomembrane system. The two ribosomal subunits now separate also. Polysome • A cluster of many ribosomes translating one mRNA transcript • Transcript threads through the multiple ribosomes like the thread of bead necklace • Allows rapid synthesis of proteins What Happens to the New Polypeptides? • Some just enter the cytoplasm • Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified Overview transcription Pre mRNA transcript processing Assembly of RNA on unwound regions of DNA molecule mRNA rRNA tRNA protein subunits mature mRNA transcripts translation At an intact ribosome, synthesis of a polypeptide chain at the binding sites for mRNA and tRNAs ribosomal subunits mature tRNA Convergence of RNAs cytoplasmic pools of amino acids, ribosomal subunits and tRNAs final protein For use in cell or for transport Gene Mutations Base-Pair Substitutions Insertions Deletions Frameshift Mutations • Insertion – Extra base added into gene region • Deletion – Base removed from gene region • Both shift the reading frame • Result in many wrong amino acids Mutations in Genes part of DNA template mRNA transcribed from DNA THREONINE PROLINE GLUTAMATE GLUTAMATE LYSINE resulting amino acid sequence base substitution in DNA altered mRNA THREONINE PROLINE VALINE GLUTAMATE LYSINE altered amino acid sequence deletion in DNA altered mRNA THREONINE PROLINE GLYCINE ARGININE altered amino acid sequence Transposons • DNA segments that move spontaneously about the genome • When they insert into a gene region, they usually inactivate that gene Mutation Rates • Each gene has a characteristic mutation rate • Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation • Only mutations that arise in germ cells can be passed on to next generation Causes of Mutations • Exposure to harmful radiation and chemicals in the environment can cause DNA mutations