Docstoc

ProtSynthMut

Document Sample
ProtSynthMut Powered By Docstoc
					Protein Synthesis

   DNA at work
         If DNA = recipe book
      Proteins = courses of a meal
• Recipes for all
  polypeptides are
  encoded by DNA
• mRNA is a copy of
  that recipe (DNA
  sequence)
• mRNA (recipes)
  travel to ribosomes
  for translation into
  polypeptides
  (proteins)
         Early developments
• 1909: A. Garrod suggests that “genes”
  create phenotypes via enzymes
  – Genes: heritable units of DNA
  – Phenotype: observable characteristic
  – People who lack particular enzymes have
    disease phenotypes (metabolic incompetence)
          Early developments
• 1940’s: Beadle & Tatum; Neurospora crassa
  (mold) produce thousands of offspring;
  some cannot grow on traditional food
  source = nutritional mutants
  – Could these mutants lack an enzyme?
        Early developments
• They do!
• It’s often one dysfunctional enzyme per
  mutant, and one dysfunctional gene
• One gene-one enzyme hypothesis
  – One gene-one protein
• One protein-one polypeptide
Protein recipe is written in genetic
code (genes)
• Genes lie along DNA
  – What are chromosomes?
• Genes are linear
  sequences of
  nucleotides
• One, three-nucleotide
  sequence = codon
        Genetic code & codons

• Each codon codes for
  a particular Amino
  Acid
• Each gene has many
  codons in it
• Codons also exist for
  “start translating” and
  “stop translating”
        Genetic code & codons
• Redundant – multiple
  codons specify same
  AA
• Unambiguous - NO
  codon specifies more
  than one AA
• Ancient – ALL
  organisms have same
  genetic code
  – AUG = Methionine
    whether you’re a
    redwood or a fruitfly
            How RNA is made
•   RNA polymerase
    adds RNA
    nucleotides to
    DNA template
•   RNA molecule
    peels away from
    DNA strand
How RNA is made
1. Initiation: RNA
   polymerase binds to a
   promoter (specific
   nucleotide sequence)
2. Elongation: Polymerase
   adds complementary
   nucleotides to DNA
   template; RNA peels
   away, DNA reconnects
How RNA is made
3. Termination: RNA
   polymerase reaches
   “terminator sequence”.
  3. RNA polymerase detaches;
     mRNA detaches
            Further processing
• Addition of caps (G) &
  tails (poly A) by RNA
  polymerase
  – Allow recognition by
    ribosomes (Cap, Tail)
  – Protect RNA from RNase
    attack (Cap)
  – Protect RNA from
    exonuclease attack (Tail)
  – Allow export by
    transporter molecules
           Further processing
• Introns spliced out
  – Intervening sequences;
    NOT transcribed into
    polypeptide
• Exons joined
  – Coding regions of DNA
    that are transcribed
    into Amino Acids
     tRNA brings
    appropriate AA
• tRNA is “cook’s helper”
   – Brings individual ingredients
     (AA) to make the recipe
     (protein)
• Binds appropriate AA (in
  cytoplasm)
• Recognizes the mRNA
  codon that specifies its
  AA
   – Complementary nucleotide
     sequence (Anticodon) for
     recognition
   tRNA binding
      sites
• Anticodons & AA
  attachment sites are
  themselves a string of
  three nucleotides
• One enzyme attaches
  each AA to any of its
  possible tRNA
  transporters
      Ribosomes & Translation

• rRNA plus proteins
  – 2 rRNA subunits
• Bind mRNA
• Bind tRNA with
  attached Amino Acids
                 Ribosomes
• Small subunit binds
  mRNA
• Large subunit, with
  tRNA binding sites,
  attaches to small
  subunit + mRNA
                  Translation
1. Initiation
  •   mRNA binds to small subunit.
  •   Initiator tRNA binds to start codon, always AUG ->
      first AA of all polypeptides is always Met
                 Translation*
2. Elongation
  •   Large subunit binds to small -> functional ribosome
  •   Initiator tRNA attaches to P site of ribosome.
      Holds growing polypeptide. Next tRNA attaches to
      A site
    Translation
2. Elongation
  1. Codon recognition:
     tRNA anticodon binds
     to mRNA codon in the
     A site
  2. Peptide bond
     formation:
     Polypeptide detaches
     from tRNA in P site &
     binds to AA & tRNA
     in A site
      Translation
2. Elongation
  3. Translocation: tRNA
     in P site detaches, A
     site tRNA & mRNA
     move, as unit, into P
     site. New tRNA
     attaches to A site.
3. Termination
  –    Stop codon is
       reached; no AA is
       added; polypeptide
       releases & subunits
       dissociate
DNA – RNA - Protein
  •   Gene expression
  Mutations
• Any change in
  nucleotide
  sequence
  – Substitutions
  – Insertions
  – Deletions
• Many alternative
  phenotypes result
  from single
  nucleotide changes
      Point
    Mutations
• Substitution:
  – A single base pair is
    changed.
  – Synonymous (silent):
    results in NO AA
    change…why not?
  – Nonsynonymous:
    results in single AA
    change
  – These are less likely
    to be deleterious.
    WHY?
Example*
• Hemoglobin mutations
  – HbE: Codon position 26;
    Replace GLU w/ LYS;
    reduced Hb production.
    Hemoglobin instability
    at low O2
  – HbC: Position 6; Replace
    GLU w/ LYS; RBC’s
    become rigid &
    crystalize
  – HbS: Position 6; Replace
    GLU w/ VAL; At low O2,
    Hb polymerizes & RBC’s
    collapse
      Point
    Mutations
• Indels: insertions/
  deletions
  – A single nucleotide
    is inserted or
    deleted
  – Far more likely to
    be deleterious
    because these shift
    the reading frame
    (triplet grouping)
       Sources of mutation*
• Mutagenesis: Production of mutations
• Spontaneous mutations:
  – Errors in replication coupled with subsequent
    errors in proofreading
  – Errors in chromosome (DNA) separation
    during cell division
• Mutagens: Physical or chemical agents
  – X-rays, UV light (high energy photons)

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:4
posted:2/26/2012
language:
pages:27