1. Deciphering the instructions
The Genetic Code
Deciphering the instructions

2. The code
The genetic code is written in the bases that exist on the nucleotides.
On DNA, these bases are:
Adenine
Guanine
Cytosine
Thymine

3. Triplet codes
DNA code is read in groups of three nucleotide bases. Each group of three is called a TRIPLET
Each triplet codes for ONE amino acid in the polypeptide chain.
For example, the following segment of DNA codes for 6 amino acids:
5’ ACTCCAGGGGTACTGTTC 3’

4. Conventions of a gene
Each gene (segment of coding material) starts with a START triplet (TAC)
Each gene ends with a STOP triplet (ACT)
Eg. Code:
5’ TAC AAA CAA GCT ACT 3’ Start Add Add Add STOP phe val arg
Start (met)
STOP
phe
val
arg

5. When does the code matter?
As DNA, the code cannot be made directly into a polypeptide. It must first be converted into mRNA.
The mRNA strand that is created from the DNA template is the COMPLEMENT. It differs from the DNA complement strand, as it contains Uracil (U) instead of Thymine (T)

6. Let’s practise… For the following DNA code, answer the questions:
5’ TACATGGCATCG 3’
How many amino acids are coded for?
What does the complementary DNA strand look like?
What does the mRNA strand built from this look like?

7. Codons and mRNA
Once the triplets are on mRNA, they are called CODONS (how confusing!)
We can use the codons to determine the amino acids that will be joined up.

8. mRNA amino acid codon table
Most amino acids are coded by more than one codon
There are 20 amino acids
YOU WILL NOT NEED TO LEARN THIS OFF BY HEART… YOU WILL BE GIVEN THE TABLE IF IT’S REQUIRED
There is only ONE start codon, but there are multiple STOP codons. If there is another start codon within the gene, it will mean “add amino acid met”

10. Let’s practise… Remember this DNA code? 5’ TACATGGCATCG 3’
What would the resulting polypeptide chain look like?

11. Properties of the code Non-overlapping
Universal (identical in almost all living things)
Set of instructions to assemble amino acids into proteins

12. Activity
Making DNA code using your name 