Cell Structure and Organelles for Forensic Scientists

Bacteria and archaea are prokaryotes: they lack a membrane-bound nucleus. Their DNA lies in a nucleoid region, and additional DNA may occur in plasmids. A single circular chromosome is common in bacteria, but chromosome number and shape have exceptions. Eukaryotes include humans, other animals, plants and fungi; their cells generally have a nucleus and membrane-bound organelles.

Do not assume every shed human skin cell has an intact nucleus and organelles. Surface corneocytes have undergone differentiation and may be anucleate. Contact deposits may also contain extracellular DNA and material transferred from other sources. Human blood contains anucleate mature red cells alongside nucleated white cells.

A human-specific assay is designed to recognise its intended human targets, rather than amplify all bacterial DNA present. Heavy microbial contamination can affect extraction, total-DNA estimates and specimen quality, but does not automatically compete as an amplifiable target in a validated human STR assay. Assess inhibition and cross-reactivity for the actual method.

The nuclear envelope is a double membrane with pores that regulate exchange in living cells. Chromatin combines DNA with proteins, including histones. DNA wraps around histone cores to form nucleosomes, with further organisation that changes during the cell cycle. Euchromatin and heterochromatin describe different chromatin states, not an absolute active-versus-inactive map of every gene.

A typical diploid human nucleus before DNA replication has 46 chromosomes. One haploid set contains roughly three billion base pairs; a diploid cell contains approximately twice that amount. DNA amount changes during replication, and gametes and cells with unusual chromosome complements require different descriptions. Two copies of an autosomal locus are not two copies of the entire DNA molecule.

Extraction must release DNA and separate it from material that interferes with the assay. Detergents disrupt membranes; suitable proteolytic treatment digests proteins. In organic extraction, proteins partition largely into the organic phase or interphase while DNA remains in the aqueous phase under the appropriate conditions. Silica methods bind and wash DNA under controlled buffer conditions.

Chelex-based procedures use a chelating resin to bind metal ions and may include heating. They do not work by the same chemistry as silica purification. Chromatin can influence accessibility, but it does not explain specimen preservation alone: mineral association, exposure, temperature and microbial processes also matter.

The nucleolus is involved in ribosomal RNA production and ribosome assembly. An inactive X chromosome may form a Barr body in suitable cells; its visibility and number depend on chromosome complement and cell state. Barr-body observation is not a definitive measure of a person’s sex, gender or identity.

Mitochondria have outer and inner membranes; the inner membrane forms cristae involved in oxidative phosphorylation. The human mitochondrial reference genome is about 16.6 thousand base pairs and contains 37 genes: 13 protein-coding genes, 22 transfer-RNA genes and two ribosomal-RNA genes. Sequence length can vary, so a reference length is not an invariant measurement for every person.

Copy number differs by tissue, cell state and specimen. It is misleading to assign a fixed number of mitochondria or mitochondrial genomes to every human cell. Higher starting copy number can make mitochondrial targets recoverable when a chosen nuclear assay fails, without establishing that no nuclear DNA remains.

Forensic testing may sequence defined control-region intervals or the whole mitochondrial genome. The compared range, quality and interpretation rules must be specified. A population database such as EMPOP supports appropriately qualified sequence-frequency assessment; a simple observed count is not automatically the probability that a named person is the donor.

Mitochondrial DNA usually follows maternal inheritance. Maternal relatives may share a sequence, and unrelated people can also share the tested sequence. Heteroplasmy means more than one mitochondrial sequence variant occurs within a person or specimen. Its level can differ between tissues; contamination and sequencing artefacts must also be considered.

Use relevant DNA guidance, including SWGDAM publications, for the actual method. SWGMAT was a materials-analysis group and must not be cited as the source of mitochondrial DNA interpretation rules. Selection of mitochondrial or nuclear testing depends on the specimen, available methods and comparison question, rather than a blanket rootless-hair rule.

Cell membranes contain a phospholipid bilayer, proteins and other components. Hydrophilic heads face aqueous surroundings, while hydrophobic tails form the interior. Membrane proteins have roles in transport, signalling and recognition. Loss of intact membranes after death does not imply that all DNA or every antigen is immediately lost.

ABO antigens are carbohydrate structures carried on glycoproteins and glycolipids. A and B transferases modify the H structure with different terminal sugars. Ordinary group O red cells lack A and B structures but retain H and other antigens; rare phenotypes require more specific treatment. Rh antigens, including D, are proteins and form a different blood-group system.

FUT2-related secretor status affects soluble ABH substances in certain secretions. This historical serological information must not be confused with a DNA result identifying a fluid. Blood-group agreement is shared by many people and does not uniquely link a person to an item or reconstruct the deposition event.

After death or environmental exposure, membrane integrity changes through chemical, enzymatic and physical processes. Freeze–thaw cycles can disrupt cells, but disruption alone is not equivalent to destruction of every DNA target. Interpretation should use the measured condition and assay performance rather than infer a fixed survival time from a damaged membrane.

The eukaryotic cytoskeleton includes actin filaments, microtubules and intermediate filaments. Keratins are intermediate-filament proteins important in epithelial cells, hair and nails. Hair formation also involves keratin-associated proteins and crosslinking that produce a durable shaft rather than a functioning nucleated cell.

During keratinisation, nuclei and organelles break down. Loss of visible nuclei does not prove the absence of all nuclear DNA: fragmented nuclear material can remain in a rootless shaft. Conventional STR profiles may be difficult to obtain, while appropriate mitochondrial or other sequencing methods may recover information. A visible root is not a guarantee of a full nuclear profile either.

Cuticle, cortex, pigment and medulla features inform hair examination and some species assessments. Similarity is not proof of a common individual source, and human population-group inference from morphology is limited and potentially misleading. Microscopy can guide comparison and sample selection without being described as an individual identification test.

Proteomic methods can examine peptides and genetically influenced protein variation in hair and other materials. Capability is method- and specimen-specific. Keratin is not the principal protein of bone or tooth dentine; collagen and other matrix proteins are important there. Emerging proteomic findings need validation and appropriate population information before forensic weight is assigned.

Autolysis is breakdown by the body’s own processes; putrefaction involves microbial activity. Nucleases can fragment DNA, while hydrolysis, oxidation and ultraviolet exposure can alter it. These processes interact with temperature, moisture, oxygen availability and the substrate. Neither anaerobic conditions nor a particular tissue guarantees preservation for a fixed interval.

Bone mineral and dental structures can shield or bind genetic material, but not by maintaining a universally protective low-pH environment. Acidic conditions can damage DNA and dissolve mineral. The condition of a specimen and the sampling location matter more than a universal ranking of bone, pulp and soft tissue.

Contact deposits do not contain a predictable few hundred cells. Yield varies with deposition, transfer, background, exposure, collection and extraction. A low human-DNA result cannot by itself identify which process caused the loss or tell when contact occurred.

Formalin fixation can create crosslinks and damage that complicate extraction and amplification. Archived formalin-fixed, paraffin-embedded tissue can sometimes yield useful DNA with appropriate methods, but results depend on fixation and storage history, target length and validation. Where permitted, a separate unfixed specimen may better support genetic work.

Preserve material for its intended examinations under specimen-specific procedures. Drying, cooling or freezing can be appropriate in different circumstances, but they are not interchangeable instructions. Laboratory and legal requirements vary by jurisdiction; a law’s name does not establish the scientific validity of a particular fixed-tissue result.