Deciphering the instructions The Genetic Code Deciphering the instructions

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

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’

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

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)

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?

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.

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” http://click4biology.info/c4b/3/images/3.5/table.gif

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

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

Activity Making DNA code using your name 