Start Codon Example: 5 Facts You Should Know!

Codons are a set of three nucleotides that code for amino acids. Let us see examples of the start codon and a few facts about it.

The start codon is the first codon in the mRNA, that is transcripted into amino acid in the ribosomes. Ribosomes are made up of two subunits and are present in the cell’s cytoplasm. The start codon usually codes for methionine in eukaryotes and N-formyl methionine in bacteria.

Examples of Start Codon:

  • AUG
  • GTG
  • ATT

AUG Codon

AUG is considered a universal start codon as it is recognized as a start codon in most of the organisms.

GTG Codon

GTG codon codes for Valine amino acid and is recognized as a start codon in few bacteria

ATT Codon

ATT codon codes for isoleucine and studies show that under certain conditions ATT is considered as a start codon in a few invertebrates.

In this article, let us discuss whether GTG and ATT are start codons, why AUG is a start codon and many other related questions.

Is GTG a start codon?

The process of conversion of mRNA to protein is known as translation and it takes place in the cytosol of the cytoplasm. Let us see if GTG is a start codon.

In the bacterial organism, GTG codon is used as an alternative start codon but it usually codes for valine. This codon is not extensively used as an initiation codon in mammalian mRNA sequences.

Is ATT a start codon?

The DNA gets converted into mRNA through a process called transcription and it reaches the cytoplasm. Let us see if ATT is a start codon.

ATT is recognized as a start codon in certain invertebrates and among some vertebrates like amphibians, Aves, and Squamata. The special initiator mt-tRNA differs in structure compared to the normal tRNA and is recognized by the IF-2 initiation factors.

Why is AUG a start codon?

The synthesis of proteins takes place in the ribosomes present in the cytosol- the matrix part of the cytoplasm. Let us see why AUG is considered a start codon.

AUG is considered the start codon because it is the initial codon that is recognized in the mRNA and undergoes translation. It is the most common start codon in all species hence it is termed a Universal Start Codon. It usually codes for methionine in eukaryotes and formyl methionine in prokaryotes.

The translation process from Wikipedia

How many start codons are there?

The codon that code for amino acid is present in the mRNA sequence. For each codon, an anticodon is present in the transfer RNA- tRNA. Let us see how many start codons are present.

There are basically four different types of start codons such as ATG, TTG, GTG, and CTG. The universally accepted start codon- AUG is present in most of the species. The start codon marks the beginning of protein synthesis and the stop codon stops the process.

Why does mRNA start with AUG?

There are three types of ribonucleic acid –RNA; messenger RNA- mRNA, ribosomal RNA-rRNA and transfer RNA-tRNA. Let us see why mRNA starts with AUG.

In the initiation phase of translation, the ribosome attaches to the mRNA strand and identifies the start codon AUG and begins the translation process. The codons are written from the 5’ end to the 3’ end on the mRNA sequence. The mRNA consists of the codon and the tRNA consist of the anticodon.

Non-AUG start codon?

The initiation starts with AUG codon and the termination takes place on recognition of stop codon. Let us see in details about non-AUG start codon.

Certain prokaryotes start the translation process using non-AUG start codons. The start of the translation process depends upon the type of initiation mechanism used. If the translation process starts with a non-AUG codon in human beings, it results in cancer and neuron degradation.


The AUG codon is considered the universal start codon as most of the species start the translation process using this codon. The translation process is stopped using the three stop codon; UAA, UAG, and UGA.

Shravanthi Vikram

Hi....I am Shravanthi Vikram, I have completed my master's in Bioinformatics and have 10 years of teaching experience in Biology. Let's connect through LinkedIn-

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