A mutation represents a change in the DNA sequence, the blueprint of life. These alterations can range from subtle single-unit changes to large-scale rearrangements of genetic material. A deletion mutation involves the removal of genetic information from this sequence, altering biological processes at their basic level.
What is a Deletion Mutation?
A deletion mutation involves the removal of one or more nucleotide base pairs from a DNA sequence. DNA is organized as a double helix, resembling a twisted ladder, with each rung composed of two chemical building blocks called nucleotides. These nucleotides—adenine (A), guanine (G), cytosine (C), and thymine (T)—form the genetic code, acting like an instruction manual for building and maintaining an organism. When a deletion occurs, it’s akin to removing letters or entire sentences from this manual, fundamentally altering the code.
The size of a deletion can vary significantly. Small deletions might involve the loss of just a single nucleotide base pair, often called a microdeletion. Larger deletions can encompass multiple base pairs, entire genes, or even substantial regions of a chromosome. These larger-scale deletions are categorized as chromosomal deletions, affecting broader genomic regions and potentially multiple genes.
How Deletion Mutations Occur
Deletion mutations can arise through several mechanisms, often linked to errors during cellular processes. One common way they occur is during DNA replication, the process where a cell makes copies of its DNA. The enzyme DNA polymerase can sometimes “slip” on the template DNA strand, leading to the accidental omission of one or more nucleotides from the newly forming strand. This slippage is particularly common in repetitive DNA sequences.
Errors in DNA repair mechanisms also contribute to deletions. Cells have systems to correct DNA damage, but if these repair pathways fail to fix an error, a deletion can become permanent.
Additionally, issues during genetic recombination, a process where DNA segments are exchanged between chromosomes, can lead to deletions. Unequal crossing over during meiosis, the cell division that produces reproductive cells, can result in one chromosome gaining genetic material while the other experiences a deletion. External factors can also induce DNA damage that leads to deletions, including exposure to certain chemicals or radiation.
Impact on Genes and Proteins
The molecular consequences of deletion mutations depend on their size and location within the DNA sequence. When a deletion occurs within a coding region of a gene and the number of deleted nucleotides is not a multiple of three, it results in a “frameshift mutation.” DNA is read in groups of three nucleotides, called codons, with each codon specifying a particular amino acid. A frameshift mutation shifts this reading frame, causing all subsequent codons to be read incorrectly. This leads to an altered sequence of amino acids downstream from the deletion, often resulting in a premature “stop” signal. The resulting protein is usually much shorter than normal, known as a truncated protein, and is often non-functional.
In contrast, if a deletion involves a multiple of three nucleotides (e.g., 3, 6, or 9 base pairs), it is termed an “in-frame” deletion. While this type of deletion does not alter the reading frame, it still results in the loss of one or more specific amino acids from the protein sequence. The protein produced will be shorter than the original but might retain some or altered function, depending on the location and importance of the missing amino acids. For instance, a deletion of three base pairs can remove a single amino acid, potentially affecting the protein’s overall structure or ability to perform its normal role.
Deletion Mutations and Health
The molecular impact of deletion mutations directly translates into their effects on human health, leading to various genetic disorders. When a deletion causes a protein to be non-functional, missing, or significantly altered, the biological processes that protein normally facilitates can be disrupted. The severity of the health impact is influenced by the deletion’s size, its precise location within the genome, and the specific function of the affected gene or genes.
A well-known example is a specific type of Cystic Fibrosis, where a common cause is a three-base pair deletion in the CFTR gene. This in-frame deletion leads to the loss of a single amino acid, phenylalanine, at position 508 of the CFTR protein, affecting its ability to transport chloride ions and resulting in the disease’s symptoms. Larger chromosomal deletions can have more widespread effects, as seen in conditions like Cri-du-chat syndrome. This syndrome is caused by a deletion of a segment on the short arm of chromosome 5, leading to a range of symptoms including a characteristic high-pitched cry, intellectual disability, and distinctive facial features. The extent of the deletion on chromosome 5 can influence the severity of the syndrome’s symptoms.