Download The Genetic Code

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)
phe
val
arg
STOP
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…

1.
2.
3.
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
http://click4biology.info/c4b/3/images/3.5/table.gif
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”
Let’s practise…

1.
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 