Transcript Pierce chapter 15

Chapter 15 – The Genetic
Code and Translation
Beadle and Tatum
• Neurospora crassa
– Haploid orange bread mold
– Haploid?
• Prototrophs – grow on
minimal media;
auxotrophs can only grow
on media that has been
supplemented with
substance that it can’t
synthesize
• Irradiated spores to
induce mutations
Beadle and Tatum
• Irradiated spores first grown on complete
media, then transferred to minimal media
– Showed strains that had acquired mutations
• Series of tubes with minimal media PLUS
one additional ingredient
– If strain grew, indicated mutation affected
production of that substance
Sib and Horowitz
• Biochemical pathway for
arginine production
• I – grew in presence of
ornithine, citrulline,
arginine
• II – grew in presence of
citrulline and arginine
• III – grew in presence of
arginine
• Which type codes for
which enzyme?
One gene/One enzyme
• Each gene codes for one enzyme
• Many proteins are composed of multiple
polypeptides, each coded for by a different
gene
– One gene/one polypeptide
Protein Structure
• Polymer of 20 amino
acids
• Amino acid structure
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Central carbon atom
Hydrogen atom
Amino group
Carboxyl group
R/radical group
• What defines one amino
acid
• Each R group has
different sizes/properties
Amino acid linkage
• Two amino acids
covalently linked
together by process
of dehydration
synthesis
– Formed bond =
peptide bond
• Long chain called a
polypeptide
Levels of Protein Structure
• Primary structure
– Amino acid sequence
• Secondary structure
– Hydrogen bonds form
alpha helices or beta
sheets
Levels of Protein Structure
• Tertiary structure
– Interactions between R
groups
• Quaternary structure
– Only present in proteins
composed of multiple
polypeptide chains
– How polypeptide subunits
are associated to make a
complete protein
Genetic code
• Nucleotide sequence must code for specific
amino acids
• Francis Crick – 3 nucleotides code for one
amino acid
– codon
• 64 codons
– 61 code for an amino acid
– “degenerate”
• More than one codon can code for the same amino acid
Genetic code
• tRNA
– 30-50 tRNA for 20 amino acids
– Isoaccepting tRNA have different anticodons but still
carry the same amino acid
• Wobble
– 1st nucleotide in anticodon pairs with the 3rd codon of
mRNA
– Flexibility in bonding allows one tRNA to recognize
more than one codon
• Still codes for same amino acid
Reading frame
• Determined by the START/initiation codon
– AUG – also codes for methionine
• No overlapping or skipping of bases
• Termination/stop codons
– Also called nonsense codons
• Universality
– With rare exception, genetic code is read the
same by all organisms
tRNA charging
• Attachment of appropriate amino acid
• Aminoacyl-tRNA synthetase (20 different)
– Recognize specific sequences in certain regions of tRNA, and
binds the appropriate amino acid to 3′ acceptor arm of tRNA
– Forms aminoacyl-tRNA
Translation
• Occurs at ribosomes
– Attaches to 5′ end of
mRNA and moves
toward 3′ end
– Binding determined by
Shine-Dalgarno
sequence in
prokaryotic cells/
modifications in
eukaryotic cells
Initiation of translation
• Ribosome has two subunits – small (30S) and
large (50S)
– Complete ribosome (70S)
• Initiation factors bind to small unit, prohibiting
small unit from binding with large subunit
– Now free to bind to mRNA
• Aminoacyl-tRNAmet attaches to initiation/ start
codon
• Large ribosomal subunit attaches
Ribosome
• Has three sites for
tRNA
– A (aminoacyl) site
– P (peptidyl) site
– E (exit) site
• Initiator tRNAmet
enters P site; all other
tRNA first enter the A
site
– A→P→E
Elongation
• Next codon is exposed in the A site
– Charged tRNA enters A
• Peptide bond forms between the P site amino acid and A
site amino acid
– Peptidyl transferase
• Translocation
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ribosome moves down mRNA molecule
P site tRNA enters E site
A site tRNA with growing polypeptide enters P site
A site now has next codon exposed; ready for next tRNA
During next shift, E site tRNA is released into cytoplasm to be recharged with another specific amino acid
Termination
• A STOP codon enters
A site
– No appropriate tRNA
• Release factors enter
A site
– Ribosome subunits
dissociate
– Polypeptide is
released from last
tRNA
Post-translation modifications
• Methionine cleaved off; chain possibly
cleaved
• Carbohydrates attached (forms
glycoproteins)
• Folding into proper 3D shape
– Aided by chaperone proteins