20 October, 2006

Chapter 15

The Genetic Code

Overview• The genetic code specifies which codons translate to which amino acids.• The genetic code minimizes the effects of mutations.• Wobble allows the 5’ base of the anticodon to pair more freely than

other bases.• There are three stop codons.• The code was deciphered using synthetic polynucleotides.• tRNA binding to synthetic trinucleotides at the ribosome was also used

in deciphering the code.• The message is read 5’ to 3’, with nonoverlapping, nondelimited codons

in a fixed reading frame.• Mutations may be missense, nonsense, or frameshift.• The genetic code is nearly universal.

Genetic Code

The code minimizes the effects of mutations.

Mutations in the first base give similar amino acids.

Pyr second = hydrophobic, Pur second = polar.(transitions are more copmmon than transversions.)

GCn = same, AUn = different (GC is stronger than AU)

The code minimizes errors.

tRNA

Wobble

Wobble in the first anticodon position

Anticodon Structure

Polynucleotide Phosphorylaseand the synthesis of polyA

Repeating Sequences

Generating Repeating Sequences

Deciphering the Code

Multiple Frameshifts

Second-Site Nonsense Suppression

Mitochondrial Genetic Code

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