Difference between Start Codon and Stop Codon

What is a start codon?

The start codon is the initial set of codons in an mRNA transcript that is translated by a ribosome. In eukaryotes, the most common start codon is AUG which codes for methionine. The non-AUG start codons are rare in eukaryotes. For instance, the Candida albicans (fungus) uses CAG as a start codon.

Usually, the starting sequence is preceded by the 5’UTR (untranslated region).

What is a stop codon?

The stop codon is a termination codon that signals to stop a protein translation process. This codon disassociates the ribosomal subunits, and thus disrupts the amino acid chain. UAG, UAA and UGA are the standard stop codons.

Difference between Start Codon and Stop Codon

Start Codon

Stop Codon

Start codon is a site where the protein translation initiates.

The stop codon marks the termination of the translation process.

A start codon interacts with initiation factors or nearby sequences to initiate the translation process.

A stop codon can individually initiate the termination.

The standard start codon is AUG.

The standard stop codon is UAG, UGA and UAA.

Usually codes for methionine.

It terminates the amino acid coding sequence.

tRNA with anticodons are present for it.

tRNA with anticodons are absent here.

Also Read: Genetic Code – Codons for Amino Acids

Frequently Asked Questions on Difference between Start Codon and Stop Codon


What are codons?

Codons are triplet nucleotide sequences that code for a protein (except stop codon). The AUG triplet is considered the standard start codon that codes for methionine. There is a total of 64 such codons out of which 61 code for proteins. The remaining three are stop codons. The full set of codons is termed a genetic code.


What is an ochre codon?

The stop codon UAA is known as the ochre codon. Likewise, UAG is called amber and UGA is called opal. The ochre mutations are nonsense mutations seen in the UAA codon. The amber and ochre mutations are almost similar to each other.


What are anticodons?

A unit of triplet nucleotide sequence that is complementary to the coding sequence of mRNA is called an anticodon. The tRNA carries distinct anticodon sequences to the corresponding codon. Sometimes an anticodon can pair with more than one codon. This happens in wobble base pairing.

Extended Reading: Difference between Codon and Anticodon

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