Hair’s biological makeup often raises questions about DNA presence. While DNA is an individual’s unique blueprint, its distribution within a hair strand isn’t uniform. This article clarifies where DNA resides in hair and its implications.
The Hair Follicle and Root: Where DNA Resides
Hair growth originates from a structure embedded within the skin called the hair follicle. At the base of each follicle lies the hair root, a bulb-like structure that contains living, actively dividing cells. These cells contain nuclear DNA, an individual’s complete genetic code. Blood vessels nourish the hair root, supplying nutrients and hormones for rapid cell division and growth.
A small, cup-shaped structure within the hair bulb, known as the dermal papilla, contains specialized mesenchymal cells that play a role in hair growth. These cells signal to the surrounding epithelial cells, influencing hair development and regeneration. When a hair is plucked, the root, often accompanied by a small piece of tissue called a follicular tag, is removed, providing a source of these DNA-rich cells.
Nuclear vs. Mitochondrial DNA in Hair
Hair contains two primary types of DNA: nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Nuclear DNA is located in the nucleus of living cells and is inherited from both parents, making it unique to an individual (with the exception of identical twins).
Mitochondrial DNA (mtDNA) resides in mitochondria, cellular organelles responsible for energy production. Unlike nuclear DNA, mtDNA is inherited solely from the mother and is present in multiple copies within each cell, making it more abundant and robust to degradation. Mitochondrial DNA can be found in the hair root and also within the hair shaft itself, providing a genetic marker even when nuclear DNA is absent.
Hair Shafts: Lacking Nuclear DNA
The hair shaft, the visible part of hair, is primarily composed of dead, hardened keratin protein. As hair grows outward from the follicle, the cells undergo a process called keratinization, where they lose their nucleus and other organelles. Consequently, the hair shaft lacks nuclear DNA as it consists of non-living material.
While the hair shaft does not contain nuclear DNA, it can retain mitochondrial DNA. This is due to mtDNA’s higher copy number and the protective membranes surrounding mitochondria, which help them persist even in degraded samples. Analyses of the hair shaft usually focus on mitochondrial DNA.
The Importance of Hair DNA in Forensics
Hair evidence is common at crime scenes, offering valuable genetic information for investigations. For forensic analysis, hairs with an intact root are preferred. They contain nuclear DNA, allowing for specific individual identification through techniques like Short Tandem Repeat (STR) analysis.
If enough root tissue is present, a nuclear DNA profile can be compared to known samples or searched in databases to identify individuals. When hair strands lack the root or follicular tag, nuclear DNA analysis becomes more challenging due to the degraded and limited quantity of genetic material. In such cases, mitochondrial DNA analysis is often employed.
While mtDNA cannot uniquely identify an individual, as it is shared among maternally related family members, it can still provide significant investigative leads by excluding suspects or linking individuals to a crime scene through maternal lineage. Advances in DNA extraction methods are also improving the ability to recover nuclear DNA from rootless hair shafts, although it remains more degraded.