# DNA Profiling ## Principles of DNA Profiling - **DNA profiling** (genetic fingerprinting) enables scientists to identify individuals for criminal cases and identify human remains because every person (apart from identical twins) has unique repeating short non-coding regions of DNA called **variable number tandem repeats (VNTRs)** - These regions consist of 20 to 50 base sequences that repeat

VNTRs are repeating DNA sequences in non-coding regions that vary in length between individuals

- The **number** of VNTR regions are inherited from biological parents - Closely related individuals are more likely to have similar patterns of repeats - Think of it like inherited physical traits—just as you might have your dad's nose, you inherit specific patterns of DNA repeats from both parents - The **length** of VNTR regions is unique to each individual (except identical twins) - This uniqueness is what makes DNA profiling so powerful for identification - When DNA testing occurs in forensic investigations or paternity cases, the pattern of these repeats (shown as bars on a gel) creates a DNA profile or fingerprint that can be analysed to draw conclusions (e.g., identifying suspects) ## Creating a DNA Profile - To create a DNA profile from DNA being tested, scientists complete the following steps in sequence: ### 1. Obtain the DNA - DNA can be extracted from biological samples like: - Hair roots (not the shaft—the follicle contains cells with nuclei) - Blood spots - Semen - Saliva (e.g., from drinking a beverage or licking an envelope)

Care must be taken to avoid contamination of samples during collection and handling

### 2. Increase the quantity of DNA using PCR - Use **PCR** to produce **large quantities** of the required DNA fragment from very small samples (even just one molecule of DNA or RNA) - This amplification step is critical—imagine trying to read a sentence written in microscopic letters; PCR is like photocopying that sentence until it's large enough to analyse easily ### 3. Cut the DNA using restriction endonucleases - Use **restriction endonucleases** (enzymes) to cut the DNA into fragments - Different restriction enzymes cut at different base sequences (restriction sites) - Scientists select enzymes that will cut close to the **VNTR** regions - These VNTR regions may be referred to as 'satellite' or 'microsatellite' DNA ### 4. Separate fragments using gel electrophoresis - Separate the DNA fragments according to size using **gel electrophoresis** - DNA is negatively charged (due to phosphate groups), so fragments move towards the positive electrode (anode) - Smaller fragments travel faster and further through the gel than larger fragments - Think of it like runners in a race through an obstacle course—lighter, smaller runners can weave through obstacles more quickly than larger ones ### 5. Visualise the fragments using probes - The fragments are not visible to the naked eye, so must be transferred onto absorbent paper or nitrocellulose membrane - The membrane is heated to separate the two DNA strands - **Probes** are then added - Probes are **single-stranded DNA sequences** that are **complementary** to the VNTR regions sought by scientists - Probes contain a label for identification, either: - A **radioactive label** (e.g., phosphorus isotope) which emits radiation that makes X-ray film go dark, creating a pattern of dark bands - A **fluorescent stain/dye** (e.g., ethidium bromide) which fluoresces (glows) when exposed to ultraviolet (UV) light, creating a pattern of coloured bands ### 6. Analyse the pattern - An X-ray image is taken or UV light is shone onto the paper, producing a pattern of bands - This pattern (the DNA profile) is then compared to control DNA samples - In criminal cases: compared to suspect DNA, victim DNA, and control samples from unrelated individuals - In paternity cases: child's bands are compared to potential parents' bands

Each band on the gel represents a DNA fragment of a specific length; matching bands indicate matching VNTR sequences

## Uses of DNA Profiling ### Forensic Medicine / Criminal Investigations - DNA profiling has been used by forensic scientists to **identify suspects** of crimes **Process:** 1. Samples of body cells or fluids (e.g., blood, saliva, hair, semen) are collected from: - The crime scene - The victim's body (e.g., in sexual assault cases) 2. DNA is extracted and profiled 3. The profile is compared to samples from: - The suspect (or criminal DNA database) - The victim - People with no connection to the crime (control samples) 4. The individual whose profile shows the most bands in common with the crime scene sample is likely the perpetrator - DNA profiling can also be used in forensics to **identify bodies or body parts** that are unidentifiable - E.g., badly decomposed remains - E.g., body parts remaining after an explosion or natural disaster

Contamination during sample collection can lead to false matches or inconclusive results

### Other Applications **Disease Risk Identification:** - DNA profiling (along with next-generation DNA sequencing) can identify individuals at **risk of developing particular diseases** - Research shows certain VNTR sequences are associated with increased incidence of specific diseases - E.g., several genes have been linked to increased risk of certain cancers - E.g., specific genes linked to Alzheimer's disease development **Determining Familial Relationships:** - DNA profiling can determine **familial relationships** for: - **Paternity cases** (to determine who the biological father is) - A child inherits half their VNTRs from each parent, so their profile should show a combination of bands from both mother and father - **Immigration cases** (to determine if family members are genuinely related) - Authorities can verify claimed family relationships by comparing VNTR patterns **Species Conservation:** - Can be used in **captive breeding programmes** to: - Reduce chances of inbreeding by identifying genetic diversity - Track lineages and ensure genetic variation is maintained - Match breeding pairs to maximise genetic health of offspring - It's like creating a dating app for endangered species—ensuring the best genetic matches to keep populations healthy!

In paternity testing, look for a combination of parental bands that covers all the child's bands—the child's profile should be a mix of both parents

## Interpreting DNA Profiles | Application | What to Look For | Interpretation | |-------------|-----------------|----------------| | Criminal Investigation | Most bands in common between crime scene and suspect | Suspect with most matching bands is likely the perpetrator | | Paternity Testing | Child's bands are combination of mother's and father's bands | All child's bands should match either mother or potential father | | Body Identification | Exact match between unidentified remains and reference sample | Identical profiles confirm identity | | Conservation | Degree of similarity between individuals | More different bands = more genetic diversity = better breeding match | **Key principle:** The more bands that match between two samples, the more closely related or likely to be the same individual they are (except when specifically looking for diversity in conservation breeding).