The Genetic Code

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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
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