Our bodies are intricate systems, with instructions for every function encoded within deoxyribonucleic acid, or DNA. DNA, a complex molecule found in nearly all living organisms, is composed of long sequences of chemical units called nucleotides. Changes to this genetic material are known as mutations. These alterations can occur in various ways, affecting how genetic information is read and utilized by the cell.
Defining Insertion Mutations
An insertion mutation is a genetic alteration where one or more nucleotide base pairs are added into a DNA sequence. This addition can range in size from a single base pair to thousands of base pairs, or even entire segments of DNA. The presence of these extra nucleotides disrupts the original sequence of a gene.
If the number of inserted base pairs is not a multiple of three, it causes a “frameshift” mutation. Imagine reading a sentence where every three letters form a word; if an extra letter is inserted, all subsequent words become scrambled, changing the entire meaning. A frameshift changes how the cell reads the DNA sequence, leading to a completely different protein product or no functional protein at all.
How Insertion Mutations Happen
Insertion mutations can arise through spontaneous cellular processes and exposure to external factors. One common spontaneous mechanism is an error during DNA replication, where the DNA polymerase enzyme can accidentally add extra nucleotides.
Replication slippage is another spontaneous event, particularly in regions of DNA with repetitive sequences. Here, DNA strands can temporarily separate and misalign, leading to the insertion of extra copies of the repeat sequence. Mobile genetic elements, known as transposable elements or “jumping genes,” can also move and insert themselves into existing genes. Certain viruses can also integrate their genetic material into the host’s DNA, causing insertions.
Consequences for Genetic Information
The impact of an insertion mutation depends on its size and location within the DNA sequence. If an insertion occurs within a protein-coding gene and is not a multiple of three base pairs, it alters the reading frame for all subsequent codons. This often leads to a non-functional protein or a premature stop signal that truncates the protein, which renders it unable to perform its intended function.
When an insertion involves a multiple of three base pairs, it is called an “in-frame” insertion. This adds extra amino acids to the protein sequence without shifting the entire reading frame. While the protein may still function, its activity could be altered, reduced, or even enhanced. The specific outcome of any insertion mutation is influenced by its location and how it affects the final protein structure and function.
Real-World Implications
Insertion mutations have tangible effects on living organisms, contributing to genetic diversity and causing serious diseases. In humans, specific insertion mutations are linked to several inherited disorders. For instance, cystic fibrosis, a condition affecting the lungs and digestive system, can be linked to specific insertions in the CFTR gene, which encodes a protein involved in chloride ion transport.
Huntington’s disease, a neurodegenerative disorder, is another example, caused by an expansion of a CAG trinucleotide repeat within the HTT gene. While often associated with negative outcomes, insertions are also a source of genetic variation. Over long evolutionary timescales, these changes can introduce new traits into a population, occasionally providing a selective advantage and contributing to the diversity of life.
References
Cystic Fibrosis Foundation. What is Cystic Fibrosis?
Huntington’s Disease Society of America. What is Huntington’s Disease?
National Institutes of Health. Genetic Variation.