Cell Structure and Organelles for Forensic Scientists

The most fundamental division in cell biology separates prokaryotes from eukaryotes, and this division has direct operational consequences in forensic analysis. A prokaryotic cell, such as Staphylococcus epidermidis shed from skin along with a touch DNA deposit, has no nuclear envelope. Its single circular chromosome sits freely in the cytoplasm, its ribosomes are smaller than those of eukaryotes, and it has no mitochondria, no endoplasmic reticulum, and no membrane-bounded organelles at all. A human skin cell deposited at the same location is a eukaryote: it has a nucleus, a nuclear envelope with nuclear pore complexes, multiple mitochondria, an endoplasmic reticulum, a Golgi apparatus, and a cytoskeletal network.

When a forensic swab is submitted for DNA analysis, the laboratory targets eukaryotic (human) nuclear DNA. Bacterial DNA present on the same swab will be co-extracted and can compete with human DNA in PCR amplification if present in very high copy numbers. Swabs taken from decomposed remains or from outdoor scenes frequently carry heavy bacterial loads. DNA extraction protocols optimised for low-copy human material must account for this, and some differential lysis steps exploit size and structural differences between prokaryotic and eukaryotic cells to enrich for the human fraction.

The nucleus is bounded by a double-membrane envelope perforated by nuclear pore complexes that regulate molecular traffic between the nucleus and cytoplasm. Inside, the DNA is not free: it is wound around octamers of histone proteins to form nucleosomes, and these nucleosomes are further compacted into the higher-order chromatin fibre. The degree of compaction varies with the cell cycle and with gene activity. Transcriptionally active regions are in loosely coiled euchromatin; silenced regions occupy tightly packed heterochromatin at the nuclear periphery.

For forensic DNA extraction, chromatin compaction is both a protection and an obstacle. The histone packaging slows the entry of environmental nucleases and some chemical degradants, which is why nuclear DNA survives longer in protected tissues such as tooth pulp or compact bone than in exposed soft tissue. At the same time, extraction protocols must disrupt the chromatin complex to release the DNA. Chelex extraction works by chelating divalent cations needed by nucleases and by boiling to denature proteins; organic phenol-chloroform extraction removes proteins including histones as an organic layer; solid-phase extraction uses silica to bind and wash the DNA away from cellular debris. Each method disrupts chromatin through different mechanisms.

The nucleus also contains the nucleolus, a non-membrane-bounded region where ribosomal RNA is synthesised. The nucleolus has no direct forensic role, but its size and number are sometimes noted in histological examination of tissue for species identification or pathological assessment. More significant is the sex chromatin (Barr body) visible in female somatic cells as a condensed mass of the inactive X chromosome at the nuclear periphery. Before PCR-based methods, Barr body counts in buccal smears were used as a provisional indicator of biological sex, a technique still occasionally described in older forensic textbooks.

Mitochondria are double-membrane organelles in the cytoplasm of eukaryotic cells. The outer membrane is permeable to small molecules; the inner membrane is highly folded into cristae and is the site of the electron transport chain and ATP synthesis. The mitochondrial matrix, enclosed by the inner membrane, contains the mitochondrial genome: a circular, double-stranded DNA molecule of approximately 16,569 base pairs in humans, encoding 13 proteins, 22 transfer RNAs, and 2 ribosomal RNAs. Each mitochondrion typically contains two to ten copies of this genome, and each cell contains between a few hundred and several thousand mitochondria depending on cell type and metabolic demand. The practical consequence is that a single human cell contains only two copies of any given nuclear locus but potentially tens of thousands of copies of the mitochondrial genome.

The hypervariable regions (HV1 and HV2) in the non-coding control region of the mitochondrial genome accumulate substitutions at a higher rate than the nuclear genome and are the standard targets for forensic mitochondrial DNA sequencing. HV1 spans approximately nucleotide positions 16024 to 16365; HV2 spans approximately positions 73 to 340. An additional region, HV3 (positions 340 to 576), is sometimes included in extended sequencing. Population databases such as EMPOP (European DNA Profiling Group Mitochondrial Population Database) hold tens of thousands of verified mitochondrial haplotypes and are used to estimate the frequency of a typed sequence in defined reference populations.

Heteroplasmy complicates mitochondrial DNA interpretation. Because a cell's mitochondrial genomes are present in large copy numbers and replicate semi-independently, two slightly different mitochondrial sequences can coexist in the same individual: a condition called heteroplasmy. It occurs as a low-level variant in some tissues and can appear as a mixture signal in the sequence trace. Analysts must distinguish true heteroplasmy from sequencing artefact, and reporting guidelines from bodies such as SWGMAT (Scientific Working Group for Materials Analysis) and the European Network of Forensic Science Institutes address this specifically.

The cell membrane is a fluid mosaic of phospholipid bilayer and embedded proteins. The phospholipid bilayer, with its hydrophilic head groups facing both the extracellular and cytoplasmic surfaces and its hydrophobic fatty acid tails forming the interior, acts as the primary permeability barrier. Integral membrane proteins span the bilayer and carry out transport, signalling, and cell-cell recognition. Peripheral membrane proteins attach to the bilayer surface.

The most forensically significant membrane components are the blood group antigens. ABO antigens are carbohydrate chains attached to glycoproteins and glycolipids on the red blood cell membrane surface, with the terminal sugar determining the A, B, or O specificity. The Rh antigens, including the D antigen that gives the positive or negative designation, are integral membrane proteins. These antigens are the basis of classical serological blood grouping, which remains in use for scene reconstruction and evidence linking even where DNA profiling is available. The secretor status of an individual, governed by the FUT2 gene, determines whether ABO-active oligosaccharides are also secreted in body fluids such as saliva and semen, which is relevant when typing body fluid stains that contain no intact cells.

After cell death, the membrane loses its regulated permeability. Phospholipases from microorganisms and from the cell's own lysosomes begin to degrade membrane lipids. This is one pathway by which environmental microbes infiltrate the cell and reach the DNA. Freeze-thaw cycles, ultraviolet radiation, and certain fixative chemicals also disrupt membrane integrity. In practice, a bloodstain that has been exposed to repeated wetting and drying cycles on an outdoor surface will have lost most membrane integrity, leaving behind cellular debris and potentially degraded DNA but few intact cells.

The cytoskeleton is composed of three polymer systems: actin microfilaments (7 nm diameter), tubulin microtubules (25 nm), and intermediate filaments (10 nm). Each system has distinct subunit proteins, dynamics, and cellular functions. For forensic purposes, the most important are the intermediate filaments, specifically keratins. Type I and type II keratins polymerise into heterodimeric coiled-coil structures that form the main structural component of epithelial cells, hair, and nails.

During hair growth, cortical cells in the follicle bulb undergo programmed keratinisation: their nuclei and organelles are degraded, and the cell volume becomes packed with densely crosslinked keratin filaments. The result is a physically durable, chemically resistant shaft that can survive burial for thousands of years in dry conditions. Forensic hair examination exploits the preserved architecture: the cortex houses the melanin granules whose colour, shape, and distribution pattern vary between individuals and populations; the medulla, if present, shows a species-specific internal pattern; the cuticle scales exhibit species-characteristic morphology under scanning electron microscopy. These features are compared in microscopic hair examination, which is a presumptive identification technique. Nuclear and mitochondrial DNA can be typed from the hair root if present; the shaft provides mitochondrial DNA only.

Keratin's chemical stability also makes it a protein target in archaeological and highly degraded forensic samples. Proteomics approaches using liquid chromatography-mass spectrometry can retrieve keratin peptide sequences from samples where DNA is unrecoverable, providing species identification and, in some cases, individual-level variation through single-amino-acid polymorphisms. This is an emerging technique being evaluated in wildlife forensics and in cases involving aged skeletal material. For routine casework, nuclear STR typing from the hair root or pulp, and mitochondrial DNA typing from the shaft or degraded bone, remain the standard approaches.

After cell death, degradation proceeds through autolysis and putrefaction. Autolysis begins immediately: lysosomal enzymes, normally sequestered in membrane-bounded vesicles, leak into the cytoplasm and begin digesting cellular proteins and nucleic acids. Nucleases, including DNase I and DNase II, cleave nuclear and mitochondrial DNA into fragments. The rate depends on temperature, pH, and moisture. At 37 degrees Celsius in a wet environment, significant DNA fragmentation can occur within hours. In dry, cold, or anaerobic conditions, the same process may take decades.

From a preservation standpoint, the most protective matrices for biological evidence are tooth pulp and compact cortical bone, both of which shield DNA mechanically from the environment, maintain low pH microenvironments that inhibit nucleases, and, in the case of bone mineral, may bind and stabilise DNA. Dried bloodstains on porous substrates are moderately stable in cool, dark conditions but degrade rapidly in humidity, heat, and UV exposure. Touch DNA deposits are among the most labile: the few hundred cells in a typical contact deposit are exposed on the substrate surface, and the nuclear copy number may already be below the STR profiling threshold before collection.

Formalin fixation, used in histopathological preparation, cross-links DNA to proteins and fragments it into short pieces. Formalin-fixed, paraffin-embedded (FFPE) tissue is a common sample type in medicolegal cases where tissue has been archived pathologically. Short amplicon STR typing and whole-genome sequencing libraries can be prepared from FFPE DNA with modified protocols, but success rates are lower than for fresh or frozen tissue and require validation specific to the degree of fixation. Jurisdictions differ in whether forensic typing results from FFPE material meet admissibility standards under their rules of evidence: under the Bharatiya Sakshya Adhiniyam 2023 in India, the UK's Criminal Procedure Rules, or the Daubert standard in US federal courts, the method's validation status and error rate must be demonstrated to the court.