DNA holds the instructions for building and operating our bodies. These instructions are organized into units called genes, which direct the creation of proteins, the molecules that perform most of life’s functions. Do all alterations in our DNA necessarily change the building blocks of proteins?
The Journey from Gene to Protein
The process of converting genetic information into a functional protein involves two main stages: transcription and translation. First, within the cell’s nucleus, a gene’s DNA sequence is copied into a messenger RNA (mRNA) molecule, a process known as transcription. This mRNA molecule serves as a temporary working copy of the genetic instructions.
Once formed, the mRNA travels out of the nucleus to the ribosomes, which are cellular structures responsible for protein synthesis. Here, during translation, the mRNA sequence is read in three-nucleotide units called codons. Each codon corresponds to a particular amino acid, the building blocks that link together to form a protein chain. Transfer RNA (tRNA) molecules assist by bringing the correct amino acid to the ribosome for each codon.
Understanding Nucleotide Mutations
A nucleotide mutation refers to a change in the DNA sequence. These alterations can occur spontaneously or be induced by environmental factors. Such changes are categorized by how they modify the original DNA strand.
One common type is a substitution, where one nucleotide base is replaced by another. An insertion involves the addition of one or more nucleotides, while a deletion occurs when one or more nucleotides are removed. These changes can have varying consequences for the resulting protein.
The Genetic Code and Its Redundancy
The genetic code translates nucleotides into amino acids. There are 64 possible three-nucleotide combinations, or codons, but only 20 common amino acids. This means most amino acids are specified by more than one codon, a property called degeneracy or redundancy.
For example, the amino acid leucine can be encoded by six different codons, and serine by six others. This redundancy acts as a safeguard against changes in the DNA sequence. Of the 64 codons, 61 specify amino acids, while three serve as “stop” signals, ending protein synthesis. One “start” codon, AUG, initiates protein synthesis and codes for methionine.
Different Outcomes of Nucleotide Mutations
Not all nucleotide mutations result in a change to the amino acid sequence or the final protein. This is primarily due to the redundancy of the genetic code.
One outcome is a silent mutation, where a nucleotide substitution leads to a new codon that still codes for the same amino acid. Since the protein’s amino acid sequence remains unchanged, these mutations often have no observable effect on its function.
Another is a missense mutation, where a nucleotide substitution results in a codon that specifies a different amino acid. The impact of a missense mutation can range from minor to significant, potentially altering the protein’s structure and function. A nonsense mutation occurs, transforming an amino acid-coding codon into a stop codon. This prematurely terminates protein synthesis, usually leading to a shortened and often non-functional protein.
Finally, insertions or deletions of nucleotides that are not in multiples of three can cause a frameshift mutation. This shifts the reading frame of the codons, altering the downstream amino acid sequence and typically resulting in a non-functional protein. Frameshift mutations are more disruptive than substitutions because they change every subsequent codon.