The Genetic Code
Essential Questions
1. Use the Genetic Code to identify condons, anti-codons and amino acids from a DNA sequence. (Essential Skill 2-2)
The Universal Genetic Code
• Codon = every 3 bases on mRNA that codes for a specific amino acid. – Start Codon = AUG =
Methionine
– Stop Codons = (UAA, UAG, UGA)
• This is what the tRNA reads
The Genetic Code is the same for all organisms!!!
tRNA – “reads” the codons
• Anticodon = 3 bases on tRNA that are complementary to the codon on mRNA.
• tRNA brings the correct amino acid to the ribosome/protein.
• The correct amino acid depends on the mRNA codons
How to Use the Genetic Code• Each codon has 3 letters
(nucleotides: G, C, A & U)– 1st letter can be found along
the left side of the chart– 2nd letter can be found along
the top of the chart– 3rd letter can be found along
the right side of the chart • it must be in the same row as
the first letter
• Where all three letters intersect is the name of the amino acid this codon codes for!– Each code always starts with
AUG (start) and ends with a stop codon!
• NOTE: The Genetic Code always uses the mRNA sequence!!!
tDNA TAC TCT GGG CTG ACT
mRNA
AA
Letter
Basic Decoding Example
tDNA TAC TCT GGG CTG ATC
mRNA AUG AGA CCC GAC UAG
AA Met Arg Pro Asp
Letter (start) W H S !
How Mutations Affect the Amino Acid Sequence
• A mutation (change in the nucleotide sequence) can affect protein structure in 3 ways:
– No Change: It doesn’t change the amino acid sequence
– Small Change: It changes only 1 amino acid
– Large Change: It changes the whole protein
Protein Synthesis Review
DNA mRNA protein
Transcription vs. Translation
• Transcription = DNA into mRNA (occurs in nucleus)
• Translation = mRNA into Protein (occurs at ribosome)
– tRNA carries the amino acids
– rRNA makes up the ribosomes
Protein Synthesis Review