From Gene to Protein

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Chapter 17

• From Gene to Protein

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Researchers

• Archibald Garrod (1909)
• genes dictate phenotypes
• diseases are inborn errors of metabolism

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Researchers, cont.

• George Beadle Boris Ephrusi
• 1930s, mutant eye color blocks pigment production
  in Drosophila

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Researchers, cont.

• George Beadle and Edward Tatum worked with
  Neurospora (bread mold)
• one gene-one enzyme hypothesis
• one gene-one polypeptide hypothesis

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Expressing the Genotype

• bridge between genotype and phenotype is protein
• genes give instructions for making specific
  proteins, but they do not make it directly
• gene (DNA) RNA polypeptide

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RNA Types

• mRNA (messenger)
• tRNA (transfer)
• rRNA (ribosomal)
• snRNA (small nuclear)
• hnRNA (heterogenous nuclear)

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RNA Types, cont.

• mRNA - codons, processed version
• tRNA - anticodons, carries a.a.
• rRNA - protein synthesis
• snRNA - involved with processing the RNA
• hnRNA - pre-mRNA, at nucleus

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Protein Synthesis5 3

• Transcription
• Translation

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TranscriptionDNA RNA

• DNA RNA complimentary N-bases
• A U
• T A
• G C
• C G

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DNA in TranscriptionDNA contains

• promoter region -
• (TATA box and initiation site)
• terminator site
• transcription unit -
• (from initiation to terminator, transcribed
  into RNA)
• coding (sense) strand template
• noncoding (antisense) strand

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Transcription Enzymes

• transcription factors
• proteins aid RNA polymerases attach to promoter
• initiation factors
• RNA polymerases
• unwind DNA one turn at a time
• attach RNA nucleotides to DNA template

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RNA Splicing pre-mRNA mRNA

• in euk only, at nucleus
• pre-mRNA
• introns - noncoding sequences
• exons - coding regions
• mRNA with exons only
• to prevent degradation
• add 5 cap (guanosine triphosphate)
• add 3 poly A tail (150 or more)

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RNA Splicing Processing the RNA

• transcription unit averages about 8000
  nucleotides
• pre-mRNA 8000
• mRNA 1200
• average protein 400 a.a.

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Translation mRNA polypeptide
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Chemical Language

• one-letter code 41 4
• two-letter code 42 16
• three-letter code 43 64
• triplet code specifies a certain a.a.

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Reading Frame

• 3 N-base sequence codon
• no overlapping
• start at 5 end of mRNA

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Ribosomes

• processed mRNA leaves nucleus
• goes to cytoplasm, to ribosomes
• ribosomes with 2 subunits
• small and large
• 3 binding sites
• peptidyl-tRNA site
• aminoacyl-tRNA site
• mRNA binding site

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• tRNA structure
• genetic code
• lists codons (mRNA)
• corresponding a.a.

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TranslationmRNA polypeptide

• 3 stages
• initiation
• elongation
• termination

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Initiation

• mRNA reads AUG
• AUG start codon
• codes for methionine (Met)
• tRNA carries Met at 3 end

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Elongation

• a.a. are added one by one
• elongation factors are proteins aid in
• codon recognition,
• peptide bond formation,
• translocation of tRNA from A site to P site

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Termination

• stop with nonsense (stop) codons
• UAA, UAG, UGA
• release factor
• protein that frees polypeptide

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Polyribosome

• cluster of ribosomes
• synthesize multiple copies of same polypeptide
  using same mRNA

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Degeneracy of the Code(redundancy of the genetic
code)

• several codons code for same a.a. to allow for
  errors
• example alanine is coded for by
• codons GCU, GCC, GCA, GCG

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Degeneracy of the Codeexample

• mRNA
• 5 AAA GGG GCU 3
• 5 AAA GGG GCG3
• GCU, GCG code for alanine

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Wobble Concept

• 3rd N-base wobbles on tRNA
• only on tRNA
• A can be modified to I (inosine)
• I complimentary to U, C, A
• anticodon CCI complimentary to GGU, GGC, GGA
• 61 sense codons, 45 tRNA

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Ribosomes

• free
• synthesize protein that dissolve in cytosol and
  function there
• bound
• synthesize protein of membranes
• synthesize protein secreted from cell

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Sites

• euk prok
• Replication nucleus cytoplasm
• Transcription nucleus cytoplasm
• Translation cytoplasm cytoplasm

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Chapter 16

• The END