The Genetic Code

Genetic code: the dictionary of molecular meaning

• Universal: same code used by all organisms on earth

• Triplet: 3 bases = one “word”

• Unambiguous: each triplet has only one meaning

• Degenerate: individual amino acids may be called for by more than one triplet (this is also referred to as redundant)

• Puntuationless: no spaces between words

• Colinear: sequence in mRNA is perfectly compatible with amino acid sequence in finished protein

• Specific signnals– “Start” = AUG– “Stop” = UAA, UAG, and UGA

• During transcription, one DNA strand, the template strand, provides a template for ordering the sequence of nucleotides in an RNA transcript.– The complementary RNA

molecule is synthesized according to base-pairing rules, except that uracil is the complementary base to adenine.

• During translation, blocks of three nucleotides, codons, are decoded into a sequence of amino acids.

Fig. 17.3

• By the mid-1960s the entire code was deciphered.– 61 of 64 triplets code

for amino acids. – The codon AUG not

only codes for the amino acid methionine but also indicates the start of translation.

– Three codons do not indicate amino acids but signal the termination of translation.

Fig. 17.4

• To extract the message from the genetic code requires specifying the correct starting point.– This establishes the reading frame and

subsequent codons are read in groups of three nucleotides.

– The cell’s protein-synthesizing machinery reads the message as a series of nonoverlapping three-letter words.

What Does DNA Say?

• Coding strand: provides info for the template strand

• Template strand: provides the template for mRNA

• Template strand is read 3’→5’

• mRNA is made 5’→3’

• Amino acids are coded for on the

mRNA!!

Steps

1. Write template DNA strand from coding strand (it is complementary)

2. mRNA is complementary to template DNA strand (remember U intead of T)

3. tRNA strand is complementary to mRNA

4. Use mRNA to get amino acids

5. Start means start and stop means stop