Question

An mRNA encodes a polypeptide that is 312 amino acids in length. The 53 rd codon in this polypeptide is a tryptophan codon. A mutation in the gene that encodes this polypeptide changes this tryptophan codon into a stop codon. How many amino acids would be in the resulting polypeptide: \(52,53,259\), or 260 ?

Short answer

The resulting polypeptide will contain 52 amino acids.

Step-by-step solution

Identify the nature of a stop codon

Understand that a stop codon is a special codon that causes translation to be terminated. This means that upon reaching a stop codon, the production of the polypeptide chain halts, and any subsequent amino acids that might have been part of the chain are not included.

Identify position of the stop codon

In this problem, the 53rd codon is mutated into a stop codon. So, all the previous codons, from the first to the 52nd, will result in producing the polypeptide.

Determine the number of amino acids in the resulting polypeptide

Since a polypeptide is formed by the linkage of amino acids and the production stops at the 53rd codon (which does not form its corresponding amino acid because it is a stop codon), the resulting polypeptide will have 52 amino acids.

Key concepts

mRNA translation

mRNA translation is a crucial process in cells that converts genetic information from messenger RNA (mRNA) into proteins, which are necessary for various cellular functions. The process begins with mRNA binding to a ribosome, the molecular machine that facilitates protein synthesis. The ribosome reads the mRNA sequence in sets of three nucleotides, known as codons.
Each codon on the mRNA corresponds to a specific amino acid or signals the start or stop of protein synthesis. For example, the codon AUG is the signal to start translation and also codes for the amino acid methionine. Translation continues as the ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain one by one, based on the sequence of codons.
In essence, mRNA translation is a translation of nucleotide sequence language into the protein language of amino acids, orchestrating genetic expression into functional proteins.

amino acids

Amino acids are the building blocks of proteins. They play a vital role in virtually all biological processes. Each amino acid consists of a central carbon atom, an amino group, a carboxyl group, and a unique side chain that determines its properties.
There are 20 standard amino acids that are encoded by the genetic code. Each amino acid is linked to a corresponding codon, a trio of nucleotides in the mRNA. These amino acids are assembled into a polypeptide chain during translation.
The sequence and specific properties of the amino acids in a polypeptide determine the protein's structure and function. Some amino acids like tryptophan have unique roles, such as serving as key components for brain neurotransmitters. The interplay between different amino acids is what gives proteins their vast range of functions and structures.

stop codon

A stop codon is a specific sequence of three nucleotides on mRNA that signals the end of protein synthesis. Unlike codons that specify amino acids, stop codons do not code for any amino acids and instead instruct the ribosome machinery to halt translation. The three stop codons are known as UAA, UAG, and UGA.
The presence of a stop codon marks the conclusion of a polypeptide chain, signifying that the ribosome should detach and release the synthesized protein. This ensures that proteins are formed with correct lengths and amino acid sequences.
Mutations that convert an amino acid-coding sequence into a stop codon, as seen in the exercise, lead to truncated proteins. Such premature stop codons can have significant effects, like shortening the polypeptide chain and potentially altering its intended function, which may lead to various genetic disorders.

polypeptide synthesis

Polypeptide synthesis is the biological process of creating a chain of amino acids that will fold into a functional protein. This process begins in the cell's ribosome, following the instructions encoded by the mRNA.
During translation, each codon on the mRNA is matched with a corresponding amino acid. The ribosome facilitates the linkage between these amino acids, which are connected by peptide bonds. As the ribosome proceeds down the mRNA, the polypeptide chain elongates until a stop codon is encountered.
Upon reaching a stop codon, the ribosome releases the newly synthesized polypeptide. The chain then undergoes folding and modifications to attain its functional form. Polypeptide synthesis is critical as it ensures the production of correct proteins necessary for cell function and survival.