DNADeoxyribonucleic Acid — the molecule that carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms., or deoxyribonucleic acid, is the hereditary material found in nearly every cell of a living organism. It is often called the "blueprint of life" because it contains the instructions needed to build and maintain all living things. In humans, each cell contains approximately 3 billion base pairs of DNA tightly packed into 23 pairs of chromosomes located in the cell's nucleus.
The structure of DNA resembles a twisted ladder, which scientists call a double helixA double helix is the shape that DNA takes — two long chains of nucleotides twisted around each other, like a spiral staircase.. The sides of the ladder are made of alternating sugar (deoxyriboseDeoxyribose is a five-carbon sugar that makes up part of the backbone of DNA. It is distinct from ribose sugar found in RNA.) and phosphate molecules. The rungs of the ladder are made of pairs of nitrogenous basesNitrogen-containing molecules that form the "rungs" of the DNA ladder: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).. There are four of these bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Adenine always pairs with Thymine, and Guanine always pairs with Cytosine. This consistent pairing is known as complementary base pairing and is essential to how DNA copies itself.
DNA extraction is the process of isolating DNA from an organism's cells so it can be studied. During extraction, scientists must first break open the cells (cell lysis), then separate the DNA from proteins and other cellular materials, and finally collect the DNA in a purified form. Common household materials, such as dish soap, salt, and cold alcohol, can be used to demonstrate this process. Dish soap dissolves the cell membrane's lipid bilayer, salt causes proteins to clump together and precipitate out, and cold alcohol allows the DNA to precipitate into a visible, stringy white mass.
Once DNA is extracted, scientists use a variety of techniques to analyze it. One of the most powerful is gel electrophoresisA laboratory technique used to separate DNA fragments by size using an electric current through a gel matrix. Smaller fragments travel farther., a process in which an electric current moves DNA fragments through a gel. Because DNA carries a negative charge, it migrates toward the positive end of the gel. Smaller fragments travel farther and faster, while larger fragments remain closer to the starting point. The resulting pattern of bands — called a DNA profileA unique pattern of DNA bands used to identify individuals. Often called a "DNA fingerprint," no two people (except identical twins) have the same profile. or DNA fingerprint — is unique to each individual (except identical twins).
In forensic science, DNA profiles are compared between samples found at crime scenes and reference samples from suspects. In biomedicine, DNA analysis is used for genetic disease diagnosis, cancer screening, paternity testing, and the development of personalized medicine. Technologies such as PCRPolymerase Chain Reaction — a technique that rapidly makes millions of copies of a specific DNA segment, allowing very small samples to be analyzed. (Polymerase Chain Reaction) allow scientists to amplify even tiny amounts of DNA — sometimes just a single cell — into quantities large enough to analyze.
DNA technology has revolutionized both criminal justice and medicine. The CODISCombined DNA Index System — the FBI's national DNA database used to match DNA profiles from crime scenes to known offenders and missing persons cases. database, maintained by the FBI, contains millions of DNA profiles used to solve crimes and identify missing persons. As of 2024, CODIS has produced over 650,000 criminal investigations matches, demonstrating the power of molecular biology in serving justice.
The structure of DNA is often compared to a twisted ladder. The sides of the ladder represent the sugar-phosphate backbone, while the rungs represent the . This pairing follows a specific rule: Adenine always pairs with , and Guanine always pairs with .
During DNA extraction, dish soap is used to break apart the cell membrane. Explain why dish soap is effective for this step, using your knowledge of the chemical composition of cell membranes.
A forensic scientist collects a blood sample at a crime scene that contains very little DNA. Identify and describe the technology that would allow the scientist to produce a large enough DNA sample for analysis. Explain how this technology works.
During gel electrophoresis, DNA fragments are separated by size. Based on the reading, predict where a very large DNA fragment would appear on the gel relative to the starting point, and explain your reasoning.
State one forensic application and one biomedical application of DNA analysis. For each application, describe how it benefits society.
Record your observations for each step of the extraction procedure.
| Step | Reagent Added | Observation | Scientific Reason |
|---|---|---|---|
| 1 | Strawberry + Buffer | ||
| 2 | Dish Soap | ||
| 3 | Salt (NaCl) | ||
| 4 | Filtration | ||
| 5 | Cold Ethanol | ||
| 6 | DNA Collection |
Answer one question for each step of the extraction procedure. Select the best answer.
Gel electrophoresisA laboratory method that uses electricity to separate molecules — like DNA fragments — by size through a gel matrix. is one of the most important tools in molecular biology. It allows scientists to separate DNA fragments by size so they can be visualized, measured, and compared. Understanding how to read a gel is an essential skill for forensic scientists, biomedical technicians, and researchers.
How the Gel Works
A gel is made from a material called agarose — a porous, jelly-like substance derived from seaweed. Think of it like a molecular obstacle course. DNA samples are loaded into small wells at the negative end (top) of the gel. When an electric current is applied, DNA migrates toward the positive end (bottom) because DNA is negatively charged. Smaller fragments move quickly and travel farther from the wells, while larger fragments are slowed down by the gel and remain closer to the starting point. After the current is stopped, the gel is stained so the bands become visible under UV light.
Reading the Gel — The DNA Ladder
Every gel includes a DNA ladderA size standard loaded into the first lane of a gel. It contains DNA fragments of known sizes (in base pairs) and is used to estimate the sizes of unknown fragments. in Lane 1. The ladder contains fragments of known sizes, measured in base pairs (bp)A base pair is one rung of the DNA ladder — one Adenine-Thymine pair or one Guanine-Cytosine pair. Fragment sizes are measured in how many base pairs long they are.. By comparing where an unknown band lines up relative to the ladder bands, scientists can estimate the size of each unknown fragment. For example, if an unknown band sits at the same level as the 1,000 bp ladder band, that fragment is approximately 1,000 base pairs long.
Comparing Profiles — What to Look For
To compare two DNA samples, scientists examine whether the bands in both lanes appear at the same position on the gel. If all bands match in position, the DNA fragments are the same size, suggesting the samples may have come from the same individual. If even one band is at a different height, the profiles are different and the samples do not match. In forensic cases, scientists compare the crime scene sample (loaded in its own lane) to reference samples from suspects, loaded in separate lanes side by side on the same gel. A true match requires every band to align precisely.
Important Limitations
Gel electrophoresis alone is not sufficient to make a definitive identification. Scientists use it alongside other methods, including STR (Short Tandem Repeat) profiling analyzed with advanced software, to increase certainty. Additionally, samples must be handled carefully to prevent contamination — any foreign DNA introduced to a sample could produce misleading results. Scientists also consider whether the crime scene sample had enough DNA to produce clear bands, or whether degradation may have caused some bands to appear faint or missing.
In this scenario, scientists are performing a paternity test. A child's DNA is compared against the mother's and two possible fathers. In a paternity test, each band in a child's DNA profile must have come from either the mother or the biological father. If a child has a band that is not present in the mother's profile, it must have come from the biological father. This is called a obligate paternal bandA band in the child's gel profile that did not come from the mother — it must have been inherited from the biological father.. Run the gel below, then determine which male is the biological father.
In some forensic cases, a crime scene sample contains DNA from more than one contributor — this is called a mixed DNA sampleA DNA sample containing genetic material from two or more individuals. Mixed samples are more complex to interpret and require advanced analysis to separate contributors.. Mixed samples are more difficult to interpret because the gel will display bands from all contributors overlapping in the same lane. Scientists must determine which bands belong to which individual. In this cold case, a cotton swab collected from a door handle shows five distinct bands — investigators believe the sample contains DNA from two people. Five suspects are profiled. Your job is to identify which two suspects are most likely the contributors, and eliminate the others with evidence.
Additionally, one of the suspect samples shows a degraded bandA band that appears faint, smeared, or incomplete because the DNA was damaged by heat, moisture, bacteria, or UV light before it was collected. Degraded DNA can complicate interpretation. — appearing faint and smeared rather than sharp. Consider what this might mean when evaluating the evidence.
A hair follicle was recovered from the scene of a laboratory break-in. PCR was used to amplify the DNA, and four STR (Short Tandem Repeat) markers were analyzed. The table below shows the STR profiles for the crime scene sample and four suspects. Each marker has two allele values (one from each chromosome). A match occurs when all four markers match exactly.
| Sample | Marker D3S1358 | Marker vWA | Marker FGA | Marker D8S1179 |
|---|---|---|---|---|
| CRIME SCENE | 15, 17 | 16, 19 | 22, 25 | 13, 15 |
| Suspect A: Alex M. | 15, 17 | 16, 19 | 22, 25 | 13, 15 |
| Suspect B: Jordan T. | 14, 18 | 16, 20 | 22, 24 | 13, 14 |
| Suspect C: Casey R. | 15, 17 | 14, 19 | 21, 25 | 13, 15 |
| Suspect D: Morgan K. | 12, 15 | 16, 19 | 22, 25 | 10, 15 |
Compare each suspect's STR profile to the crime scene. Select the individual whose profile is an exact match.
In 1989, Marcus Daley was convicted of armed robbery and sentenced to 18 years in prison. The prosecution's case relied heavily on eyewitness testimony from two individuals who claimed to have seen Daley near the scene. No physical DNA evidence was tested at the time — the technology was still in its early stages and not widely used in courts.
In 2004, Daley's attorney filed a motion to have biological material from the crime scene re-examined using modern DNA profiling techniques. A cigarette butt and a partial glove recovered from the scene had been preserved in the evidence locker. Both were subjected to STR analysisShort Tandem Repeat analysis — comparing the number of repeated DNA sequences at multiple locations in the genome to create a unique genetic profile for identification. and compared to a reference sample from Daley.
The results were definitive: the DNA recovered from both items did not match Marcus Daley. A database search through CODIS identified a second individual — already serving time for a different crime — as the source of both samples. After 15 years of wrongful imprisonment, Daley was exonerated and released.
This case highlights both the power and the limitation of forensic evidence. When DNA evidence is available but not tested, or when courts rely too heavily on eyewitness accounts alone, injustice can occur. Post-conviction DNA testing has now exonerated over 375 individuals in the United States through organizations such as the Innocence Project.
| Sample | D3S1358 | vWA | FGA | D8S1179 | Match to Scene? |
|---|---|---|---|---|---|
| Crime Scene (Glove + Cigarette) | 14, 16 | 17, 20 | 20, 23 | 11, 14 | — |
| Marcus Daley (Convicted) | 15, 18 | 16, 19 | 22, 25 | 13, 15 | ✗ NO MATCH |
| CODIS Database Hit — Inmate R.V. | 14, 16 | 17, 20 | 20, 23 | 11, 14 | ✓ FULL MATCH |
DNA analysis is not limited to criminal investigations — it is one of the most powerful tools in modern medicine. In biomedical technology, scientists use DNA sequencing and mutation analysis to detect genetic changes that can cause disease, including cancer. One of the most well-known examples is the BRCA1/BRCA2 geneBRCA1 and BRCA2 are human genes that produce proteins which help suppress tumor growth. Mutations in these genes significantly increase a person's lifetime risk of developing breast and ovarian cancer. test for hereditary breast and ovarian cancer risk.
In this case study, a 34-year-old patient named Elena Vasquez came to her physician after learning that her mother and maternal aunt had both been diagnosed with breast cancer before age 45. Her doctor ordered a genetic panel — a blood test in which DNA is extracted from white blood cells and analyzed for mutations across multiple cancer-related genes.
Elena's DNA was amplified using PCR, then sequenced using next-generation sequencing (NGS) technology. The results identified a pathogenic mutationA mutation that has been scientifically confirmed to cause or significantly increase the risk of a disease. "Pathogenic" means disease-causing. in the BRCA2 gene — specifically, a deletion of two base pairs at position 6174 of the gene. This type of mutation causes a frameshiftA frameshift mutation occurs when a deletion or insertion changes the reading frame of the genetic code. This disrupts the production of the normal protein, often resulting in a nonfunctional or absent protein product., disrupting the production of the BRCA2 tumor suppressor protein and significantly increasing Elena's lifetime risk of developing breast and ovarian cancer.
Armed with this information, Elena was able to work with her medical team to develop a personalized surveillance plan including enhanced MRI screenings, risk-reduction medications, and a discussion of preventive surgical options. This case illustrates the concept of precision medicineAn approach to medical treatment that uses an individual's genetic information to tailor prevention, diagnosis, and treatment strategies specifically to that person. — using an individual's unique genetic profile to guide healthcare decisions before disease even develops.