Chapter 17 Genes to Protein

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Chapter 17Genes to Protein
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ObjectivesThe Connection Between Genes and
Proteins

• Explain what led Archibald Garrod to first
  suggest that genes dictate phenotypes through
  enzymes
• Describe Beadle and Tatums experiments with
  Neurospora
• Distinguish between the one gene, one enzyme
  hypothesis and the one gene, one polypeptide
  hypothesis
• Explain how RNA differs from DNA
• Explain how information flows from gene to
  protein
• Define codon
• Explain the early techniques used to identify
  what amino acids are specified by the triplets
  UUU, AAA, GGG, and CCC
• Why do polypeptides begin with methionine when
  they are synthesized
• Explain what it means to say that the genetic
  code is redundant and unambiguous
• Explain the significance of the reading frame
  during translation
• Explain the evolutionary significance of a nearly
  universal genetic code.   

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The Synthesis and Processing of RNA

• Distinguish between transcription and translation
• Explain how RNA polymerase recognizes where
  transcription should begin. Describe the
  promoter, the terminator, and the transcription
  unit
• Explain the general process of transcription,
  including the three major steps of initiation,
  elongation, and termination
• Explain how RNA is modified after transcription
  in eukaryotic cells
• Define and explain the role of ribozyme
• Describe the functional and evolutionary
  significance of introns.  

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The Synthesis of Protein

• Describe the structure and functions of tRNA
• Explain how tRNA is joined to the appropriate
  amino acid
• Describe the structure and functions of ribosomes
• Describe the process of translation (including
  initiation, elongation, and termination) and
  explain which enzymes, protein factors, and
  energy sources are needed for each stage
• Describe the significance of polyribosomes
• Explain what determines the primary structure of
  a protein and describe how a polypeptide must be
  modified before it becomes functional
• Describe two properties of RNA that allow it to
  perform so many different functions
• Compare protein synthesis in prokaryotes and in
  eukaryotes
• Define point mutations.
• Distinguish between base-pair substitutions and
  base-pair insertions. Give an example and explain
  the significance of each
• Describe several examples of mutagens and explain
  how they cause mutations

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Genes Specify Proteins

• A. Garrod First to propose relationship between
  genes and proteins (1909)
• Studied alkaptonuria black urine
• Proposed that afflicted persons lack enzymes that
  breaks down alkapton

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Beadle and Tatum Experiments

• Neurospora (bread mold) wild type can survive on
  minimal medium other molecules needed are
  produced through metabolic pathways
• Auxotrophs mutants that cannot survive on
  minimal medium as they cannot synthesize
  essential molecules
• Minimal Medium contains salts, sucrose, and
  vitamin biotin
• Complete growth medium minimal medium plus 20
  amino acids and some other nutrients
• Conclusion 1 gene 1 enzyme. Now modified to 1
  gene 1 polypeptide

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Protein Synthesis

• Transcription Synthesis of RNA using a DNA
  template
• A genes unique nucleotide sequence is
  transcribed from DNA to a complimentary
  nucleotide sequence in mRNA
• mRNA carries transcript of protein-building
  instructions to ribosome in cytosol

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Translation Synthesis of a polypeptide under
the direction of mRNA

• Linear sequence of bases is translated into a
  linear sequence of amino acids
• Occurs on ribosomes

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Genetic Code Codon Chart 3 mRNA bases codon 1
codon 1 amino acid
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Difference between RNA and DNA

• Ribose vs. Deoxyribose sugars
• Uracil vs. Thymine
• Single vs. Double stranded

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Transcription A closer look

• Three main steps
• RNA polymerase binding and initiation
• Transcription factor binds to TATA box in
  promoter
• RNA polymerase binds to promoter
• Enzyme separates 2 strands of DNA at initiation
  site and transcription begins

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Elongation

• RNA polymerase untwists and opens short segment
  of DNA (10 bases) at a time and links nucleotides
  from 5 to 3 direction

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Termination

• RNA stops at termination site

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Translation A closer look

• Transfer RNA (tRNA) interpreter between base
  sequence in mRNA and amino acid sequence
• tRNA aligns the appropriate amino acid to forma a
  new polypeptide
• One end attached to amino acid
• One end contains anti-codon

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• Aminoacyl-tRNA Synthetase enzyme that catalyzes
  the attachment of an amino acid to its tRNA

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• Ribosomes coordinate pairing of tRNA anticodons
  to mRNA codons
• Made up of 2 subunits large and small
• Constructed in nucleus
• Become functional when attached to RNA
• P site holds tRNA carrying growing peptide chain
• A site holds tRNA carrying next amino acid to be
  attached

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Eukaryotic Ribosome is 80 S (protein
weight) Consists of 2 subunits (60 S and 40 S)
60 S
40 S
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Building a polypeptide

• Initiation Brings together mRNA, the first
  amino acid attached to its tRNA (AUG Met) and
  two ribosomal subunits

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Elongation

• Codon recognition mRNA codon in A site forms H
  bonds with anticodon of tRNA carrying the next
  amino acid
• Peptide bond formation Peptidyl transferase
  catalyzes the formation of peptide bond between
  polypeptide in P site and new amino acid in A
  site (GTP)
• Translocation tRNA in P site releases from
  ribosome, and tRNA in A site is translocated to P
  site mRNA and tRNA move as a unit (GTP)

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Termination Stop codons (UAA, UAG, UGA) do not
code for amino acids

• Proteins release factor binds to codon (stop/A
  site)
• Peptidyl transferase hydrolyzes bond between
  polypeptide and tRNA in P site
• Frees polypeptide and tRNA
• mRNA and ribosomal subunits dissociate

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• Polyribosome cluster of ribosomes
  simultaneously translating mRNA

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Protein Synthesis in Prokaryotes and Eukaryotes

• Prokaryotes lack nuclei, so transcription is not
  segregated from translation consequently,
  translation may begin as soon as 5 end of mRNA
  peels away from template DNA, before
  transcription is complete

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RNA processing in Eukaryote

• 5 cap protection and recognition
• Poly-A tail protection, facilitate export
• Intron non-coding sequence in DNA that
  intervene between coding sequences (exons are
  initially transcribed, but not translated,
  because they are excised from transcript before
  leaving the nucleus)
• Exons coding sequences of a gene that are
  transcribed and expressed

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• RNA splicing RNA processing removes introns and
  joins exons from eukaryotic hnRNA, produces
  mature mRNA
• Spliceosome large molecular complex that
  catalyzes RNA splicing composed of small
  nuclear ribonucleoproteins (snRNPs) and other
  proteins

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Mutations and Protein Effects

• Mutation A permanent change in DNA that can
  involve large chromosomal regions or a single
  nucleotide pair

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Point Mutations A mutation limited to about 1
or 2 nucleotides in a single gene
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Types

• Substitutions
• Base-pair substitutions substitution of one
  base pair for another
• Can result in little or no change if redundant or
  if place in sequence is not essential to activity
• Can result in detectible changes in proteins
  usually bad, rarely good
• Missense mutation base pair substitution that
  alters an amino acid codon to a new codon that
  codes for a different amino acid
• Nonsense Mutation Base pair substitution that
  changes and amino acid codon to a chain
  termination codon, or vice versa

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• Insertion insertion of one or more nucleotide
  pairs into a gene
• Deletion deletion of one or more nucleotide
  base pairs into a gene
• Frameshift Mutation A base-pair insertion or
  deletion that causes a shift in the reading
  frame, so that codons beyond the mutation will be
  the wrong grouping of triplets and will specify
  the wrong amino acids

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Mutagenesis

• Mutagen Physical or chemical agents that
  interact with DNA to cause mutations
• Radiation
• Chemical mutagens