Difference between Start Codon and Stop Codon

Start and stop codons are two punctuation marks of the genetic code that signal the beginning and end of protein synthesis. The main difference between the start and stop codon is that the start codon marks the site at which translation into protein sequence begins whereas the stop codon marks the site at which translation ends. The most common start codon is AUG which codes for methionine while there are three stop codons UAG, UAA, and UGA, and they do not code for an amino acid.

Differences Between the Start codon and Stop codon

The differences between the Start codon and Stop codon are given below:

Feature	Start Codon	Stop Codon
Definition	It is a specific trinucleotide sequence where the protein translation process begins.	It is a specific trinucleotide sequence where the protein translation process halts.
Bases	The start codon is always AUG	There are 3 stop codons: UAG, UGA, and UAA.
Location	The start codon is located at the beginning i.e., the 5’ end of the transcribed mRNA sequence.	The stop codon is located at the end I.e., the 3’ end of the transcribed mRNA sequence.
Dependence	It works with initiation factors or nearby sequences to start the translation.	It independently signal the termination.
Code for Amino Acid	Yes. It codes for methionine most of the time.	No. Do not code for any amino acid but ends the amino acid coding sequence
Presence of tRNA with Anticodons	tRNA with anticodons is present for the start codon.	tRNA with anticodons is not present with stop codons.
Open Reading Frame (ORF)	Start codon is included in ORF.	Stop codon is not included in ORF.
Role of Ribosome	It begins the formation of polypeptide chain. Here, it halts the formation of polypeptide chain.	Here, it halts the formation of polypeptide chain.
Effect of Mutation	Mutation in the start codon can lead to the failure to initiate the protein synthesis or the production of abnormal protein.	Mutation in the stop codon can lead to premature termination of protein synthesis or the production of incomplete protein.
Significance	It marks the beginning site of the translation of the transcribed mRNA into a protein.	It marks the site where the process of translation ends. It also releases the newly formed polypeptide chain from the ribosome.

What is Genetic code?

The universal genetic code is made up of triplet bases or several codons. The standard code has evolved over time to reduce coding errors. There are total 64 codons in the genetic code created from the permutation and combination of the 4 bases in nucleic acids. The genetic code is degenerate that is more than one codon can code for a single amino acid. As a result, out of the 64 codons, 61 codons code for the 20 amino acids. There are two punctuation marks in the genetic code known as the Start and Stop codons which mark the end of protein synthesis in all organisms.

What is a Start Codon?

The initial set of codons in an mRNA transcript that is translated by a ribosome is known as the start codon. In eukaryotes, the start codon typically is AUG, as it the first codon in the transcribed mRNA that undergoes translation. It codes for the amino acid methionine (Met) in eukaryotes and formyl methionine (fMet) in prokaryotes. However, there are exceptions, such as in the case of the fungi, Saccharomyces cerevisiae, where GUG is used as a start codon and in case of Candida albicans where CAG is used as a start codon.

During protein synthesis, the tRNA recognizes the Start codon AUG with the help of some initiation factors and starts translation of mRNA. Generally, the starting sequence is preceded by the 5’UTR (untranslated region). Some alternative start codons are found in both prokaryotes and eukaryotes. Since they employ a different initiator tRNA when acting as start codons, alternative codons typically code for amino acids other than methionine. It has been discovered that E. Coli uses AUG 83%, GUG 14%, and UUG 3% as start codons in prokaryotes.

What is a Stop Codon?

The stop codon is a special termination codon that signals to stop a protein translation process. It disassociates the ribosomal subunits, and thus disrupts the polypeptide chain during translation effectively. The most common recognized stop codons in the genetic code are UAG, UAA, and UGA. These codons are also known as termination codons or nonsense codons as they do not code for an amino acid.

The three STOP codons are named as amber (UAG), opal or umber (UGA) and ochre (UAA). Charles Steinberg and Richard Epstein discovered “Amber” or UAG, and they named it after their friend Harris Bernstein’s last name in German. The remaining two STOP codons were then named “ochre” and “opal” so as to maintain the “color names” theme.

During protein synthesis, Stop codons cause the release of the nascent polypeptide chain from the ribosome. This happens because there exists no tRNAs with anticodons complementary to the Stop codons.

Similarities Between Start Codon and Stop Codon

Similarities between Start Codon and Stop Codon are:

• The genetic code contains two punctuation marks called start and stop codons.
• They are trinucleotide sequences found in messenger RNA (mRNA) molecule.
• Both codons are crucial to minimize coding errors.

Conclusion: Difference Between Start Codon and Stop Codon

In conclusion, start codons, such as AUG, initiate protein synthesis by indicating the beginning of translation, guiding the ribosome to assemble amino acids. In contrast, stop codons (UAA, UAG, UGA) serve as termination signals, instructing the ribosome to terminate protein synthesis and release the newly formed polypeptide. The distinction lies in their roles: start codons initiate the process, while stop codons bring it to a timely and precise conclusion, collectively ensuring the accurate translation of genetic information into functional proteins.

FAQs on Start and Stop Codon

What are Codons?

Codons are sequences of three nucleotides (adenine, guanine, cytosine, or thymine) in the DNA or RNA that encode specific amino acids or signal the end of protein synthesis.

What is an Ochre Codon?

The term “ochre codon” refers to one of the three stop codons that signal the termination of protein synthesis during translation. It specifically refers to the UAA stop codon.

What are Anticodons?

Anticodons are sequences of three nucleotides found in transfer RNA (tRNA) molecules. These sequences are complementary to the codons in mRNA (messenger RNA) during the process of translation.

What is an Example of a Start Codon and a Stop Codon?

An example of a start codon is AUG (adenine-uracil-guanine), which codes for the amino acid methionine and an example of a stop codon is UAA (uracil-adenine-adenine), which does not code for any amino acid.

Is the Start and Stop Codon Transcribed?

No, transcription doesn’t use stop codons, but it does create them. A start codon initiates translation, while a stop codon ends it.

What are the 3 Codons?

The three codons that signal the end of protein translation are UAG, UAA, and UGA. These codons are also known as stop codons, nonsense codons, or termination codons.