From DNA to Protein

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Chapter 13
From DNA to Protein
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From Genes to Proteins

• All proteins consist of polypeptide chains
• A linear sequence of amino acids
• Each chain corresponds to the nucleotide base
  sequence of a gene
• The Path From Genes to Proteins
• Transcription
• Enzymes use the base sequence of a gene as a
  template to make a strand of RNA
• Translation
• Information in the RNA strand is decoded
  (translated) into a sequence of amino acids
• Prokaryotes and Eukaryotes
• In prokaryotic cells (no nucleus)
• Transcription and translation occur in cytoplasm
• In eukaryotic cells
• Genes are transcribed in the nucleus
• Resulting mRNA is translated in the cytoplasm

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Transcription DNA to RNA

• Two DNA strands unwind in a specific region
• RNA polymerase assembles a strand of RNA
• Covalently bonds RNA nucleotides (adenine,
  guanine, cytosine, uracil) according to the
  nucleotide sequence of the exposed gene
• Three Types of RNA
• Messenger RNA (mRNA)
• Carries protein-building codes from DNA to
  ribosomes
• Ribosomal RNA (rRNA)
• Forms ribosomes (where polypeptide chains are
  assembled)
• Transfer RNA (tRNA)
• Delivers amino acids to ribosomes

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RNA and DNA Compared
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RNA Base Pairing
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Gene Transcription
Promoter region
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Gene Transcription
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Gene Transcription
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RNA Modification Alternative Splicing

• Before mRNA leaves the nucleus
• Introns are removed
• Some exons are removed along with introns
• Alternative splicing Either all exons remain in
  the mature RNA or some are removed and remaining
  exons are spliced together in different
  combinations
• Poly-A tail is added to 3 end of new mRNA
• The Poly-A Tail
• The longer its poly-A tail, the more time an mRNA
  transcript (and its protein-building message)
  will remain intact in the cytoplasm

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Post-Translational RNA Modification
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The Genetic Code

• Messenger RNA (mRNA) carries DNAs
  protein-building information to ribosomes for
  translation
• mRNAs genetic message is written in codons
• Sets of three nucleotides along mRNA strand
• Codons
• Codons specify different amino acids
• A few codon signals stop translation
• Sixty-four codons constitute a highly conserved
  genetic code

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From DNA to Polypeptide
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tRNA and rRNA Function in Translation

• Transfer RNA (tRNA)
• Anticodon binds to mRNA codon
• Also binds amino acid specified by codon
• Different tRNAs carry different amino acids
• tRNAs deliver free amino acids to ribosomes
  during protein synthesis

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rRNA

• Ribosome Ribosomal RNA (rRNA) and proteins make
  up the two subunits of ribosomes

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Three Stages of Translation

• mRNA-transcript information directs synthesis of
  a polypeptide chain during translation
• Translation proceeds in three stages initiation,
  elongation, termination
• Initiation
• One initiator tRNA, two ribosomal subunits, and
  one mRNA come together as an initiation complex
• Elongation
• tRNAs deliver amino acids to the ribosome in the
  order specified by mRNA codons
• Ribosomal rRNA catalyzes the formation of a
  peptide bond between amino acids
• Termination
• Translation ends when RNA polymerase encounters a
  STOP codon in mRNA
• New polypeptide chain and mRNA are released
• Ribosome subunits separate from each other

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Initiation
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Elongation
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Termination
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Mutated Genes and Their Protein Products

• Mutations are permanent, small-scale changes in
  the base sequence of a gene
• Common mutations include insertions, deletions,
  and base-pair substitutions

frameshift
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Transposable Elements

• Transposable elements Segments of DNA that can
  insert themselves anywhere in a chromosome

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Some Causes of Mutations

• Spontaneous mutations
• DNA replication error occurs spontaneously by DNA
  polymerase
• Natural and synthetic chemicals
• Cigarette smoke
• Environmental agents
• Ionizing radiation
• Free radical
• Nonionizing radiation
• T-T dimer by UV

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SUMMARY Protein Synthesis