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How do we read the exact order of bases in DNA, and how does that give each person a unique genetic fingerprint?

Describe the purpose of DNA sequencing and how genetic profiling produces a pattern unique to an individual

A focused answer to the O-Level outcome on sequencing and profiling. What sequencing reads, how genetic profiles use variable regions, and the uses in forensics and relationships.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
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What this dot point is asking

This outcome asks you to describe what DNA sequencing is for, and how genetic profiling produces a pattern unique to an individual. The two are often confused, so the key is to keep them apart: sequencing reads the exact order of bases, while profiling compares the sizes of variable regions to identify a person.

The answer

DNA sequencing

DNA sequencing reads the exact order of bases (A, T, C, G) along a piece of DNA.

  • Modern sequencing is fast and cheap enough to read whole genomes.
  • It is used to identify genes, find the exact mutation causing a genetic disease, and study how organisms are related.

Sequencing tells you the precise genetic message, letter by letter.

Genetic profiling (DNA fingerprinting)

Genetic profiling does not read every base. Instead it compares particular regions of DNA that vary a lot between people, called variable regions.

The steps are:

  1. Collect a DNA sample (for example from blood or cells).
  2. Amplify the variable regions using PCR, to make enough material.
  3. Separate the fragments by size using gel electrophoresis, producing a pattern of bands.

Why profiles are unique

The variable regions differ in length from person to person, so the fragment sizes, and therefore the positions of the bands, are different for almost every individual. The only people with matching profiles are identical twins. This is why a genetic profile acts like a fingerprint.

Sequencing versus profiling

  • Sequencing reads the whole base order, used to study genes and find mutations.
  • Profiling compares the sizes of variable regions, used to identify individuals and establish relationships.

Examples in context

Example 1. Establishing paternity. A child's genetic profile contains bands inherited from each biological parent. Comparing the child's profile with a possible parent's can confirm or rule out a biological relationship, a common everyday use of profiling.

Example 2. Diagnosing a genetic disease. Sequencing a patient's DNA can reveal the exact base change responsible for an inherited condition, guiding diagnosis and treatment. This shows sequencing's power to read the precise genetic message, not just compare patterns.

Try this

Q1. State what DNA sequencing determines. [1 mark]

  • Cue. The exact order of bases (A, T, C, G) along a piece of DNA.

Q2. Explain why a genetic profile is different for almost every individual. [2 marks]

  • Cue. Profiles are based on variable regions that differ in length between people, so the fragment sizes and band positions are essentially unique (except in identical twins).

Q3. Give one use of DNA profiling. [1 mark]

  • Cue. Forensic identification (comparing a crime-scene sample with suspects), or establishing biological relationships such as paternity.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDescribe how a genetic profile (DNA fingerprint) is produced and explain why it is different for almost every individual.
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Examiners want the procedure and the reason profiles differ between people.

A sample of DNA is collected, for example from blood or cells. Specific regions of the DNA that vary a lot between people, called variable regions, are copied by PCR to give enough material. The fragments are then separated by size using gel electrophoresis, producing a pattern of bands.

The pattern is different for almost every individual because the variable regions differ in length from person to person, so the fragment sizes, and therefore the band positions, are unique to each individual (except identical twins).

What markers reward: collecting DNA, amplifying variable regions with PCR, separating by gel electrophoresis to give a banding pattern, and the explanation that the variable regions differ in length between people so the pattern is essentially unique.

Original4 marksGive two uses of DNA profiling and one use of DNA sequencing, explaining each briefly.
Show worked answer →

The answer should distinguish profiling from sequencing and give correct uses.

DNA profiling can be used in forensic investigation, comparing DNA from a crime scene with that of suspects to see if the patterns match. It can also be used to establish biological relationships, such as paternity, by comparing the profiles of relatives.

DNA sequencing reads the exact order of bases, and can be used to identify the gene or mutation responsible for a genetic disease, or more broadly to read whole genomes for research and medicine.

What markers reward: two valid profiling uses (such as forensic identification and establishing relationships) and one valid sequencing use (such as finding a disease-causing mutation or reading a genome), each with a brief correct explanation.

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