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BIOB10Y3 Lecture Notes - Marshall Warren Nirenberg, Sickle-Cell Disease, Phenylalanine

Biological Sciences
Course Code
Aarti Ashok

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Translation I
Encoding Genetic Information
how is the info in nucleic acids (DNA, then mRNA) converted into the order of amino acids incorporated to build
a protein?
o Genetic code
discovered that 3 nucleotides likely specified the ‘code’ for a single amino acid
o these nucleotides or codons are non-overlapping
examining the outcome of diseases such as sickle cell anemia (single base substitution) revealed
that the code is non-overlapping
only affected one amino acid = one codon
o the genetic code is degenerate
64 possible codons encode 20 amino acids and ‘stop’
more than one codon per type of amino acid
BUT what does each codon specify?
Identifying the codons
Marshall Nirenberg and colleagues synthesized an mRNA in vitro solely made up of U’s poly(U)
Incubated this with bacterial extract (contains all amino acids and factors necessary for protein synthesis)
Examined the new polypeptide synthesized it was polyphenylalanine
Therefore UUU codon = phenylalanine
By similar methods, the coding specificities of each of the 64 codons was discovered
The genetic code deciphered
Mitochondrial mRNAs and transcripts in some protists and fungi are not translated according to this standard
code some deviations exist in nature
Properties of the genetic code
codons for a specific amino acid tend to be clustered within a particular region of the chart
o similarity of codons that encode the same amino acid
o protects from some spontaneous mutations
similar amino acids tend to be encoded by similar codons - ex. hydrophobic amino acids
o base substitution would lead to incorporation of an amino acid with similar properties
greatest similarity b/w codons for the same amino acid is in the first 2 nucleotides; greatest variability in the last
nucleotide - ex. glycine codons
Decoding the codons: tRNAs
each tRNA is linked to a specific amino acid
each tRNA is able to recognize a particular codon on the mRNA
tRNAs: structure
composed of nucleotide sequences that are complementary sequences in other parts of the molecule folds
over onto itself cloverleaf structure
enzymatic modifications of some of bases is seen: cluster in the loop regions binding sites for specific proteins
all mature tRNAs have triplet sequence CCA at their 3’ end amino acid acceptor arm
the codons are recognized by a region of the tRNA known as the anticodon
tertiary structure forms an L-shape
anticodon loop is at one end of the L-shaped molecule
amino acid is attached to the tRNA at the opposite end
if there are 61 different amino acid encoding codons, then are there 61 different tRNAs? NO
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