Your DNA is a molecular fingerprint unlike anyone else's on Earth. Scientists can read it, compare it and use it to solve crimes, identify remains, trace ancestry and predict disease risk. This lesson explores the technologies that make DNA readable.
You have probably seen crime shows where detectives use DNA evidence to catch a killer. How do you think this actually works? Is there really a test that can say "this DNA belongs to this person" with 100% certainty?
Now answer: List three situations where identifying someone's DNA might be useful, and one situation where you would NOT want your DNA to be identified.
With the exception of identical twins, no two humans share the same DNA sequence. DNA profiling exploits this uniqueness to create a genetic fingerprint that can identify an individual or establish biological relationships.
DNA profiling does not sequence your entire genome. Instead, it analyses specific highly variable regions of DNA called short tandem repeats (STRs). These are short sequences of DNA that repeat a variable number of times at specific locations. One person might have 8 repeats at a particular location, while another has 13. By examining multiple STR locations (usually 13-20), scientists can create a profile so specific that the chance of two unrelated people matching is less than one in a billion.
The process works conceptually as follows:
Imagine trying to sort a mixture of paper clips, staples and nails by size. You could pour them through a sieve with holes just large enough for the smallest items to fall through. Gel electrophoresis does the same thing — but for DNA fragments.
Gel electrophoresis is a laboratory technique used to separate DNA fragments based on their size. The concept is straightforward:
In DNA profiling, each band represents a DNA fragment of a particular size. When samples from a crime scene and a suspect are run side by side, matching band patterns suggest the DNA came from the same person.
DNA profiling tells you who — DNA sequencing tells you what. While profiling compares patterns at specific locations, sequencing reads the actual order of A, T, G and C bases in a DNA molecule.
DNA sequencing determines the exact nucleotide sequence of a DNA segment, a gene, a chromosome or an entire genome. The first human genome sequence, completed in 2003 by the Human Genome Project, took 13 years and cost approximately $3 billion. Today, thanks to advances in technology, a human genome can be sequenced in days for under $1,000.
At Stage 5, you need to understand sequencing conceptually:
Sequencing is not typically used in forensics (profiling is faster and cheaper), but it is essential in medicine, research and evolutionary biology.
DNA technologies have transformed criminal justice, family law, medicine and our understanding of human history. Here are the major applications you need to know.
DNA profiling is one of the most powerful tools in criminal investigations. A single hair, drop of blood or skin cell left at a crime scene can be enough to identify a suspect or exonerate an innocent person. Since 1989, DNA evidence has been used to overturn hundreds of wrongful convictions in the United States alone, including some death row cases. In Australia, forensic DNA analysis is conducted by state police laboratories and the Australian Federal Police (AFP).
A child inherits half their DNA from their mother and half from their father. By comparing STR profiles, scientists can determine with near-certainty whether a man is the biological father of a child. DNA testing can also confirm sibling relationships, identify remains after disasters and reunite separated families.
Commercial DNA testing services analyse your DNA and compare it to reference databases from populations around the world. This can reveal your genetic ancestry, migratory history and even distant relatives. From a scientific perspective, these databases also help researchers study human migration patterns and genetic diversity.
DNA sequencing can identify mutations that cause or increase the risk of genetic diseases such as cystic fibrosis, Huntington's disease and certain types of cancer. In Australia, newborn screening programs test babies for treatable genetic conditions. As sequencing becomes cheaper, personalised medicine — tailoring treatments to a person's genetic makeup — is becoming a reality.
The NSW Forensic DNA Database is one of Australia's most important criminal justice tools. Under the Forensic Procedures Act 2000 (NSW), police can collect DNA samples from suspects and convicted offenders. These profiles are stored in a database and can be matched against DNA from crime scenes. The database has solved thousands of cases, including cold cases decades old. However, it also raises privacy concerns: your DNA contains information about your health, ancestry and relatives. In 2023, Australian researchers at the Garvan Institute of Medical Research in Sydney sequenced the genomes of thousands of Australians as part of the Genomics Health Futures Mission, aiming to improve diagnosis and treatment of rare diseases and cancer.
1 The crime scene DNA and suspect DNA show identical band patterns. What does this suggest? Why is this not absolute proof on its own?
2 Why do forensic scientists examine 13-20 different STR locations rather than just one?
3 Explain why identical twins cannot be distinguished by standard DNA profiling.
1 A police DNA database stores genetic profiles of convicted criminals. Some civil liberties groups argue this violates privacy. Provide one argument FOR and one argument AGAINST storing DNA in a police database.
2 Distinguish between DNA profiling and DNA sequencing. Give one application where each is most appropriate.
3 The Garvan Institute in Sydney is sequencing Australian genomes to improve cancer treatment. Explain how knowing a patient's exact DNA sequence could lead to better medical outcomes than standard treatment.
1. What does DNA profiling analyse to create a unique genetic fingerprint?
2. In gel electrophoresis, why do smaller DNA fragments travel further through the gel?
3. A forensic scientist compares DNA from a crime scene with DNA from three suspects. The crime scene DNA matches Suspect 2 at all 20 STR locations. What is the most reasonable conclusion?
4. Which technology would be most appropriate for identifying a genetic mutation that causes a rare disease in a patient?
5. Why might a person's DNA profile stored in a police database raise privacy concerns?
6. Explain what DNA profiling is and why it uses short tandem repeats (STRs) rather than sequencing the entire genome. 3 MARKS
7. Describe the process of gel electrophoresis conceptually and explain how it is used in DNA profiling. 4 MARKS
8. Distinguish between DNA profiling and DNA sequencing. For each, identify one application where it is the most appropriate technology and justify your choice. 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. Identical band patterns: Identical patterns strongly suggest the crime scene DNA came from the suspect [1 mark]. However, this is not absolute proof because DNA evidence must be considered alongside other evidence (motive, opportunity, alibi), and contamination or mishandling of samples is possible [1 mark].
2. Multiple STR locations: Examining many locations dramatically reduces the chance of a random match [1 mark]. While one location might match by coincidence, the probability of matching at 20 independent locations is less than one in a billion [1 mark].
3. Identical twins: Identical twins develop from the same fertilised egg and therefore share virtually identical DNA [1 mark]. Standard DNA profiling cannot distinguish them because it examines the same DNA regions [1 mark].
1. Police database arguments: FOR: DNA databases have solved thousands of crimes, including cold cases, and can exonerate innocent people [1 mark]. AGAINST: DNA contains sensitive health and ancestry information; storing it indefinitely raises concerns about surveillance, data breaches and potential misuse by governments or hackers [1 mark].
2. Profiling vs sequencing: DNA profiling analyses specific variable regions (STRs) to create a unique pattern for identification [1 mark]. Best for forensics and paternity testing because it is fast, cheap and highly discriminating [1 mark]. DNA sequencing reads the exact order of all bases in a DNA segment [1 mark]. Best for medical diagnosis and research because it can reveal mutations and disease-causing changes [1 mark].
3. Genome sequencing for cancer: Knowing a patient's exact DNA sequence can reveal specific mutations driving their cancer [1 mark]. This allows doctors to choose targeted therapies that attack those specific mutations [1 mark]. It also helps predict how a patient will respond to different drugs, avoiding treatments that are unlikely to work and reducing side effects [1 mark].
1. B — DNA profiling analyses STRs, not the whole genome. Option A describes genome sequencing. Option C is too narrow. Option D is incorrect — while mitochondrial DNA can be used, standard profiling uses nuclear STRs.
2. C — Smaller fragments move more easily through gel pores. Option A is wrong — DNA is negatively charged. Option B is backwards — smaller fragments are lighter, not heavier. Option D is incorrect — dye absorption does not affect movement.
3. A — A match at 20 STR locations makes it extremely likely the DNA came from the same person. Option B overstates the case — guilt requires more than DNA. Option C confuses DNA presence with timing. Option D contradicts established science.
4. D — Sequencing reads exact base orders and can find mutations. Option A is wrong — profiling does not read sequences. Option B is wrong — gel electrophoresis only separates by size, it does not identify mutations. Option C is irrelevant.
5. B — DNA contains health, ancestry and relative information beyond criminal identity. Option A is false — profiles are unique. Option C is an unsupported generalisation. Option D is false — DNA does not change significantly over a person's lifetime.
Q6 (3 marks): DNA profiling is a technique that analyses specific variable regions of DNA to create a unique pattern that can identify an individual [1 mark]. It uses STRs because these short tandem repeats vary greatly between individuals, making them excellent genetic markers [1 mark]. Sequencing the entire genome would be unnecessary, far more expensive and much slower for identification purposes [1 mark].
Q7 (4 marks): Gel electrophoresis separates DNA fragments by size using an electric field and a gel matrix [1 mark]. DNA is loaded into wells at one end of the gel, and an electric current pulls the negatively charged DNA toward the positive electrode [1 mark]. Smaller fragments travel further through the gel pores than larger fragments [1 mark]. In DNA profiling, this technique is used to separate amplified STR fragments so that the band pattern — which is unique to each individual — can be visualised and compared between samples [1 mark].
Q8 (5 marks): DNA profiling analyses specific highly variable regions (STRs) to produce a unique genetic fingerprint for identification [1 mark]. It is most appropriate for forensic investigations because it is fast, relatively inexpensive and can match crime scene DNA to suspects with extremely high confidence [1 mark]. DNA sequencing determines the exact order of nucleotide bases in a DNA molecule [1 mark]. It is most appropriate for medical diagnosis because it can identify specific mutations that cause or increase the risk of genetic diseases, enabling personalised treatment plans [1 mark]. Profiling tells you who; sequencing tells you what is in the DNA [1 mark].
Test your knowledge of DNA profiling, gel electrophoresis and sequencing in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of DNA technologies, forensics and genome sequencing. Pool: Lesson 8.
Tick when you have finished all activities and checked your answers.