The Genetic Code – A Level Biology Revision

The Genetic Code - A Level Biology Revision

Beyond Science returns with this A Level Biology blog on The Genetic Code. Get ahead with KS5 revision today. Here, we’ll cover the different aspects of the genetic code; sequence of nucleotides in DNA and RNA. We will also look at how an amino acid can be coded for by more than one codon.


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The genetic code is the sequence of nucleotides in DNA and RNA.

During transcription in eukaryotic cells, DNA is copied into mRNA. This is because mRNA can move out of the nucleus, whereas DNA cannot.

A sequence of three nucleotides is called a codon. Each codon codes for a specific amino acid or stop signal. The first amino acid coded for by mRNA is the start codon. In eukaryotes this is methionine (AUG).

The sequence is non-overlapping. This means that each nucleotide is only used in one codon. During translation, the mRNA is used as a template to assemble the amino acids in a protein. In a tRNA molecule, a sequence of three nucleotides is called an anticodon. These base pair with each codon on the mRNA. A molecule of tRNA with a particular anticodon always attaches to the same specific amino acid.

An amino acid can be coded for by more than one codon; the genetic code is degenerate.

The table below shows the possible combinations of bases and the corresponding amino acid or signal.

mRNA is translated from the 5′ (5 prime) end towards the 3′ (3 prime) end. From the start codon, each 3 nucleotides is translated into a particular amino acid, until a stop codon is reached:

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