Theme 2: The Gene*c Code
4.2.2 The gene*c code shows the correspondence between
codons and amino acids.
• AUG ini(ates the process of transla(on and specifies the amino acid
methionine (Met) by base pairing with the an(codon of a charged tRNA,
denoted tRNAMet
◦ AUG codon is also used to specify the incorpora(on of Met at
internal sites within the polypep(de chain
• The polypep(de is synthesized from the amino acid end to the carboxyl
end, and so Met forms the amino end of any polypep(de being
synthesized
◦ BUT, the Met is usually cleaved by an enzyme once synthesis is
complete
• AUG codon that ini(ated transla(on is preceded by a region in the
mRNA that isn't translated and so, establishes the reading frame for how
the codons will be read in their groups of three
• At each step, ribosome binds to a tRNA with an an(codon that can base
pair with the codon, and the amino acid on that tRNA is aIached to the
growing chain to become the new carboxyl end of the polypep(de chain
◦ Con(nues un(l one of three “stop” codons is encountered: UAA,
UAG, or UGA
• Gamow- Suggested the number of nucleo(des that would be necessary
to code for one amino acid
◦ 1954- Founded the RNA Tie Club which consisted of 20 members
(one for each amino acid)
◦ First reasoned that if one nucleo(de coded for one amino acid,
then DNA could only code for four dis(nct amino acids
• BUT, decided that a 1-base code (or even 2-base code or
doublets) would not be enough to code for all 20 amino
acids
, ◦ Determined that a 3-base code would provide (4x4x4) 64
combina(ons which, as it is more than 20, means that many
unique triplets will code for the same amino acid
• Nirenberg and MaBhaei- Used a cell-free system to decipher the first
leIer of the code in 1961
◦ Placed all components needed for protein synthesis (including an
RNA template, nucleo(des, ribosomes, amino acids and an energy
source)
◦ Determined what specific amino acids a par(cular RNA nucleo(de
template could give rise to
• Made a string of uracils and found that the polypep(de
sequence consisted of iden(cal amino acids
• Crick, BarneB, Brenner and WaBs-Tobin- In 1961, confirmed that the
code was wriIen out in trplets of nucleo(des
• How was the gene(c code deciphered? (Also confirmed number of
nucleo(des that code for a single amino acid)
◦ Gene*c Code- The correspondence between three-leIer
nucleo(des codons in RNA and amino acids in a protein
◦ Khorana made RNAs of known sequence; Added these synthe(c
RNAs to a solu(on containing all other components needed for
transla(on
• By adjus(ng concentra(ons of magnesium and other
factors, Khorana got the ribosome to ini(ate synthesis with
any codon
◦ When a synthe(c poly(U) was used as the mRNA, the resul(ng
polypep(de was polyphenylalanine
• The codon UUU corresponds to Phe
◦ When a synthe(c mRNA with alterna(ng U and C was used, the
resul(ng polypep(de had alterna(ng serine (Ser) and leucine
(Leu)
◦ Redundancy of gene(c code has strong paIerns:
• The redundancy results almost exclusively from the third
codon posi(on
• When an amino acid is specified by two codons, they differ
either in whether the third posi(on is a U or a C (both
4.2.2 The gene*c code shows the correspondence between
codons and amino acids.
• AUG ini(ates the process of transla(on and specifies the amino acid
methionine (Met) by base pairing with the an(codon of a charged tRNA,
denoted tRNAMet
◦ AUG codon is also used to specify the incorpora(on of Met at
internal sites within the polypep(de chain
• The polypep(de is synthesized from the amino acid end to the carboxyl
end, and so Met forms the amino end of any polypep(de being
synthesized
◦ BUT, the Met is usually cleaved by an enzyme once synthesis is
complete
• AUG codon that ini(ated transla(on is preceded by a region in the
mRNA that isn't translated and so, establishes the reading frame for how
the codons will be read in their groups of three
• At each step, ribosome binds to a tRNA with an an(codon that can base
pair with the codon, and the amino acid on that tRNA is aIached to the
growing chain to become the new carboxyl end of the polypep(de chain
◦ Con(nues un(l one of three “stop” codons is encountered: UAA,
UAG, or UGA
• Gamow- Suggested the number of nucleo(des that would be necessary
to code for one amino acid
◦ 1954- Founded the RNA Tie Club which consisted of 20 members
(one for each amino acid)
◦ First reasoned that if one nucleo(de coded for one amino acid,
then DNA could only code for four dis(nct amino acids
• BUT, decided that a 1-base code (or even 2-base code or
doublets) would not be enough to code for all 20 amino
acids
, ◦ Determined that a 3-base code would provide (4x4x4) 64
combina(ons which, as it is more than 20, means that many
unique triplets will code for the same amino acid
• Nirenberg and MaBhaei- Used a cell-free system to decipher the first
leIer of the code in 1961
◦ Placed all components needed for protein synthesis (including an
RNA template, nucleo(des, ribosomes, amino acids and an energy
source)
◦ Determined what specific amino acids a par(cular RNA nucleo(de
template could give rise to
• Made a string of uracils and found that the polypep(de
sequence consisted of iden(cal amino acids
• Crick, BarneB, Brenner and WaBs-Tobin- In 1961, confirmed that the
code was wriIen out in trplets of nucleo(des
• How was the gene(c code deciphered? (Also confirmed number of
nucleo(des that code for a single amino acid)
◦ Gene*c Code- The correspondence between three-leIer
nucleo(des codons in RNA and amino acids in a protein
◦ Khorana made RNAs of known sequence; Added these synthe(c
RNAs to a solu(on containing all other components needed for
transla(on
• By adjus(ng concentra(ons of magnesium and other
factors, Khorana got the ribosome to ini(ate synthesis with
any codon
◦ When a synthe(c poly(U) was used as the mRNA, the resul(ng
polypep(de was polyphenylalanine
• The codon UUU corresponds to Phe
◦ When a synthe(c mRNA with alterna(ng U and C was used, the
resul(ng polypep(de had alterna(ng serine (Ser) and leucine
(Leu)
◦ Redundancy of gene(c code has strong paIerns:
• The redundancy results almost exclusively from the third
codon posi(on
• When an amino acid is specified by two codons, they differ
either in whether the third posi(on is a U or a C (both
