Difference Between Start Codon and Stop Codon

The key difference between start codon and stop codon is that start codon is the trinucleotide sequence which marks the beginning of the sequence that translates into a protein while stop codon is the trinucleotide sequence which marks the end of the sequence that translates into a protein.

Genetic code of a gene contains the instruction needed to make a specific protein. It contains trinucleotide sequences which are known as codons. Each codon specifies an amino acid. Amino acids are the building blocks of a protein. Genetic code of each gene starts with a start codon and ends with a stop codon. Therefore, start codon marks the site at which translation into protein begins. Stop codon, on the other hand, marks the site at which translation into protein ends. Therefore, we can identify these two codons as two punctuation marks in the genetic code. Both are specific trinucleotide sequences.

1. Overview and Key Difference
2. What is Start Codon
3. What is Stop Codon
4. Similarities Between Start Codon and Stop Codon
5. Side by Side Comparison – Start Codon vs Stop Codon in Tabular Form
6. Summary

What is Start Codon?

The start codon is the first codon of the transcribed mRNA sequence that translates into an amino acid by the ribosome. Therefore, it marks the site at which translation into protein begins. It is a sequence composed of three nucleotides. During the translation, tRNA recognizes the start codon and begins the translation. AUG is the most common start codon. In eukaryotic genes, it specifies the amino acid methionin while in prokaryotic genes, it specifies the formyl methionine (fMet). However, there are some alternative start codons in both eukaryotes and prokaryotes which usually code for amino acids other than methionine. But, non-AUG start codons are rarely found in eukaryotes. Some alternative start codons are CUG, AUA and AUU in humans and GUG and UUG in prokaryotes.

What is Stop Codon?

The stop codon is a trinucleotide sequence which marks the site at which translation of the mRNA into protein ends. Hence, it is the last codon of transcribed mRNA. There are three stop codons. They are UAG, UAA, and UGA, and they are named as amber (UAG), opal or umber (UGA) and ochre (UAA). They signal the halt to protein synthesis. They are also known as termination codons or nonsense codons. They do not code for an amino acid.

During the translation, stop codon is responsible for releasing the newly formed polypeptide chain from the ribosome since no tRNA exists with an anticodon complementary to the stop codon.

What are the Similarities Between Start Codon and Stop Codon?

Start codon and stop codon are two punctuation marks in the genetic code.
They are trinucleotide sequences within a messenger RNA (mRNA) molecule.
Both codons are important to minimize coding errors.

What is the Difference Between Start Codon and Stop Codon?

Start codon is the codon that marks the site at which translation begins while stop codon is the site at which translation halts. So, this is the key difference between start codon and stop codon. Start codon is found at the 5’ end of the mRNA while stop codon is present at the 3’ end of the mRNA.

Moreover, start codon codes for methionine while stop codons do not code for amino acids.

The below infographic tabulates more differences between start codon and stop codon.

Summary – Start Codon vs Stop Codon

Start codon and stop codon are two punctuation marks of the genetic code of a gene. Start codon marks the site at which translation into protein sequence begins while stop codon marks the site at which translation ends. There are three stop codons as UAG, UAA, and UGA, and they do not code for an amino acid while the most common start codon AUG codes for methionine. In addition, no tRNA exists with an anticodon complementary to stop codon. Thus, stop codon is responsible for releasing the new polypeptide chain from the ribosome. Thus, this summarizes the difference between start codon and stop codon.

Reference:

1. Susha Cheriyedath, M.Sc. “START And STOP Codons”. News-Medical.Net, 2020, Available here.
2. “Molecular Biology Review”. Ncbi.Nlm.Nih.Gov, 2020, Available here.