DNA Fingerprinting

 

DNA Fingerprinting

1. What is DNA Fingerprinting?

DNA fingerprinting is a technique used to identify an individual by analyzing specific variable regions of their DNA, particularly highly polymorphic repetitive DNA sequences.

The fundamental idea is:

Different individuals have different patterns of repetitive DNA sequences, producing a characteristic DNA banding pattern. Thus, DNA fingerprinting as a rapid way of comparing DNA sequences between individuals without having to sequence their entire genomes.

Why is it possible?

Although humans are overwhelmingly similar genetically, some regions of DNA show considerable variation among individuals. These variable regions are particularly useful for identification.

Key concept

DNA fingerprinting → DNA polymorphism → repetitive DNA → VNTR → characteristic banding pattern

2. Historical Background

Alec Jeffreys

DNA fingerprinting was developed by Sir Alec Jeffreys in the 1980s. He discovered highly variable repetitive DNA sequences called minisatellites, which could be used to distinguish individuals.

Jeffreys = Fingerprints

Just as fingerprints identify a person through physical patterns, DNA fingerprints identify a person through characteristic DNA patterns.

3. Basis of DNA Fingerprinting

The technique is based primarily on DNA polymorphism.

What is polymorphism?

Polymorphism means the occurrence of genetic variation within a population.

In DNA fingerprinting, the important variation is the variation in the number of repeated DNA units at particular loci. Thus:

Variation in repeat number → variation in DNA fragment length → different banding patterns

Important distinction

Term	Meaning
Polymorphism	Genetic variation among individuals
Repetitive DNA	DNA containing repeated sequences
Satellite DNA	Highly repetitive DNA
Minisatellite	A class of satellite DNA
VNTR	Variable Number Tandem Repeat
DNA fingerprint	Characteristic pattern generated from variable DNA regions

4. Repetitive DNA

The human genome contains many DNA sequences that are repeated. These are called repetitive DNA.

A repetitive sequence can be represented simply as:

ABC ABC ABC ABC ABC

The same short DNA sequence occurs repeatedly. The number of repetitions can differ between individuals. For example:

Person A

ABC – ABC – ABC – ABC

→ 4 repeats

Person B

ABC – ABC – ABC – ABC – ABC – ABC

→ 6 repeats

The difference in repeat number produces DNA fragments of different lengths. That difference can ultimately be detected as different bands.

5. Satellite DNA

Highly repetitive DNA sequences are often referred to as satellite DNA.

Satellite DNA can be classified into different types based on the size of the repeated units and arrangement; the most important relationship is:

VNTR → minisatellite → satellite DNA

VNTR belongs to a class of satellite DNA called minisatellite.

Important Point

Do not confuse: VNTR with satellite DNA.

VNTR is a type of variable repetitive sequence; i.e. VNTR is a type of minisatellite DNA.

6. VNTR- Variable Number Tandem Repeat

This is the single most important term to remember for DNA fingerprinting.

Break the term down:

Variable
→ number of repeats differs between individuals

Number
→ the number of copies is being considered

Tandem
→ repeats occur one after another

Repeat
→ the same sequence is repeated

Therefore:

VNTRs are DNA sequences in which a particular sequence is repeated tandemly, with the number of repeats varying among individuals.

The number of repeats shows a very high degree of polymorphism, making VNTRs useful for DNA fingerprinting.

7. Why are VNTRs useful?

Suppose a particular VNTR consists of:

AGCT AGCT AGCT AGCT

One person may have 4 copies. Another person may have:

AGCT AGCT AGCT AGCT AGCT AGCT → 6 copies.

Because the number of repeats is different, the total length of the DNA region differs.

Therefore: Different repeat number → different fragment size

When these fragments are separated, they produce different positions of bands. This produces an individual's characteristic DNA pattern.

8. Why are VNTRs called highly polymorphic?

A DNA region is useful for identification if it varies significantly among individuals. VNTRs show a high degree of polymorphism because the number of tandem repeats can vary.

For example:

Individual	Number of repeats
A	4
B	7
C	5
D	10

Consequently, their DNA fragments can have different lengths. This variation makes VNTRs excellent genetic markers for identification.

9. Size of VNTR

The size of VNTR regions varies approximately from: 0.1 kb to 20 kb

This is a potentially testable factual point.

10. What is a Tandem Repeat?

"Tandem" means one after another. For example:

ATGC ATGC ATGC ATGC

The repeated sequence is arranged consecutively. It is therefore a: Tandem repeat

If the number of these repeats varies among individuals:

→ Variable Number Tandem Repeat (VNTR)

11. Basic Principle of DNA Fingerprinting

The entire concept can be understood in one chain:

Step 1

Individuals possess repetitive DNA sequences.

Step 2

Some repetitive regions are highly polymorphic.

Step 3

VNTRs are repetitive sequences where the number of repeats varies.

Step 4

Different numbers of repeats produce DNA fragments of different lengths.

Step 5

DNA fragments are separated according to size.

Step 6

Specific VNTR-containing fragments are detected using a probe.

Step 7

The detected fragments produce a characteristic pattern.

Step 8

The pattern can be compared between individuals. Therefore:

DNA fingerprinting identifies individuals by comparing characteristic patterns generated from highly polymorphic repetitive DNA sequences, especially VNTRs.

12. Classical DNA Fingerprinting- Overall Procedure

A simplified classical procedure is:

Isolation of DNA

↓

Digestion of DNA using restriction endonucleases

↓

Separation of DNA fragments by gel electrophoresis

↓

Transfer/blotting of DNA fragments onto a membrane

↓

Hybridisation with labelled VNTR probe

↓

Detection by autoradiography

↓

Characteristic banding pattern

13. Step 1- Isolation of DNA

DNA is first isolated from the biological sample.

Possible biological sources include:

• Blood
• Hair roots
• Saliva
• Semen
• Tissue
• Other nucleated cells

The important point is: DNA must first be obtained from the sample.

14. Step 2 — Digestion by Restriction Endonucleases

The isolated DNA is treated with restriction endonucleases. These enzymes cut DNA at specific recognition sequences. As a result:

Large DNA molecule

↓

Many DNA fragments

The resulting fragments contain regions of different lengths.

Why is this important?

Different individuals can have differences in the lengths of DNA fragments containing VNTRs. This produces different patterns after separation and detection.

15. Step 3- Separation by Gel Electrophoresis

DNA fragments are separated according to their size. DNA has an overall negative charge because of its phosphate backbone. Therefore, during electrophoresis: DNA moves towards the positive electrode (anode).

Important fact

DNA → negatively charged → moves towards positive electrode

Smaller DNA fragments generally migrate farther through the gel than larger fragments. Thus:

Smaller fragment → travels farther

Larger fragment → travels less

This allows fragments of different sizes to be separated.

16. Step 4- Southern Blotting

The separated DNA fragments are transferred from the gel onto a membrane. This is known as Southern blotting.

Important exam association

Southern blotting → DNA

Remember:

• Southern → DNA
• Northern → RNA
• Western → protein

17. Step 5- Hybridization with a VNTR Probe

The DNA fragments on the membrane are exposed to a labelled probe. The probe is complementary to particular repetitive DNA sequences.

What is a probe?

A probe is a labelled nucleic acid sequence used to detect a complementary DNA sequence through hybridization. In classical DNA fingerprinting, a VNTR probe is used.  The probe binds to complementary VNTR-containing DNA fragments.

18. What is Hybridization?

Hybridization is the pairing of complementary nucleic acid sequences. For example:

DNA sequence:

5′ — ATGC — 3′

Complement:

3′ — TACG — 5′

The probe binds to its complementary sequence. Thus:

VNTR-containing DNA + complementary labelled probe → hybridization

19. Step 6- Autoradiography

The hybridized fragments are detected using autoradiography in the classical method when a radioactive label is used. This produces visible bands. After hybridization with a VNTR probe, the autoradiogram gives many bands of differing sizes.

These bands collectively constitute the DNA fingerprint.

20. The Final DNA Fingerprint

The final result is a pattern of DNA bands. For example:

Individual A

│

│ █

│

│   █

│

│ █

│

│      █

│

│   █

│

Individual B

│

│ █

│

│ █

│

│      █

│

│ █

│

│        █

│

The positions of the bands are compared.

If the relevant banding pattern matches, the samples may have originated from the same individual, subject to appropriate forensic interpretation.

21. Why does each person have a different DNA fingerprint?

Because the number and arrangement of highly polymorphic repetitive DNA sequences vary among individuals. Thus:

Different VNTR pattern

→ different fragment sizes

→ different band positions

→ different DNA fingerprint.

The resulting pattern as characteristic of an individual and states that it differs among individuals in a population, except in the case of monozygotic (identical) twins.

22. Identical Twins- Important Exception

Monozygotic (identical) twins generally have the same DNA fingerprint pattern because they originate from the same zygote and have essentially the same nuclear DNA sequence. Therefore:

Normal individuals

Different DNA fingerprints

Monozygotic twins

Essentially identical DNA fingerprints

23. Why doesn't DNA fingerprinting require sequencing the entire genome?

The human genome is extremely large. Sequencing an entire genome just to distinguish two individuals would be unnecessary for the classical identification problem. Instead, DNA fingerprinting focuses on highly variable repetitive regions. Thus:

Whole genome sequencing

❌ unnecessary for classical DNA fingerprinting

Selected highly polymorphic repetitive regions

✅ useful for identification

24. Applications of DNA Fingerprinting

DNA fingerprinting has several important applications.

A. Forensic science

One of its most important applications is identification of individuals in forensic investigations. DNA obtained from a crime scene can be compared with DNA from suspects or other individuals. For example:

Crime-scene sample

vs.

Suspect A

vs.

Suspect B

The banding pattern can be compared.

B. Paternity and maternity testing

DNA fingerprinting can help establish biological relationships. A child inherits genetic material from both parents. Therefore, DNA patterns can be compared among:

• Child
• Mother
• Alleged father

The child's genetic markers should be consistent with inheritance from the biological parents.

C. Identification of missing persons

DNA profiles can be compared with:

• Biological relatives
• Previously collected samples
• Reference samples

to assist in identification.

D. Identification of deceased individuals

DNA fingerprinting can be useful when conventional identification is difficult, including certain mass-casualty or disaster situations.

E. Population studies

DNA fingerprinting has applications in determining population and genetic diversities.

25. DNA Fingerprinting and Genetic Diversity

Different populations may show different frequencies of genetic variants. Analysis of polymorphic DNA sequences can therefore help researchers study:

• Genetic variation
• Population structure
• Genetic diversity
• Relationships among populations

26. PCR and DNA Fingerprinting

The sensitivity of DNA fingerprinting has been enhanced by the use of PCR- Polymerase Chain Reaction.

PCR allows amplification of DNA. Therefore, even a very small amount of starting DNA can potentially provide enough material for analysis. The sensitivity of DNA fingerprinting has increased with PCR and that DNA from a single cell can be sufficient for DNA fingerprinting analysis.

PCR → amplification of DNA

Therefore:

Tiny DNA sample

↓

PCR amplification

↓

More DNA available for analysis

27. PCR- Quick Revision

Since PCR is linked to DNA fingerprinting, remember its three major cyclic steps:

1. Denaturation

DNA double strand separates into two single strands.

2. Annealing

Primers bind to complementary sequences.

3. Extension

DNA polymerase extends the primers and synthesizes new DNA.

Memory trick

D → A → E

Denaturation → Annealing → Extension

28. Modern DNA Profiling vs Classical DNA Fingerprinting

This distinction is useful for avoiding confusion. The classical DNA fingerprinting discussion focuses on:

VNTR/minisatellite DNA + probe + hybridization + banding pattern

Modern forensic DNA profiling often uses short tandem repeat (STR) markers, which are different from the classical minisatellite VNTR approach.

29. VNTR vs STR- Conceptual Difference

Feature	VNTR	STR
Full form	Variable Number Tandem Repeat	Short Tandem Repeat
Repeat arrangement	Tandem	Tandem
Classical NCERT DNA fingerprinting	Yes	Not the primary NCERT term
Repeat unit	Relatively longer	Shorter
Important for modern forensic profiling	Less commonly used than STRs	Very important

30. DNA Fingerprinting- Complete Flowchart

Biological sample

↓

Isolation of DNA

↓

Restriction digestion

↓

DNA fragments

↓

Gel electrophoresis

↓

Southern blotting

↓

VNTR probe hybridisation

↓

Autoradiography

↓

DNA banding pattern

↓

Comparison of patterns

↓

Identification / relationship analysis

Ultra-short sequence

DNA → Restriction enzyme → Gel → Southern blot → VNTR probe → Autoradiography → Fingerprint

31. What Exactly Makes the Pattern Different?

This is an important conceptual question. The difference is primarily due to:

Variation in the number of tandem repeats.

Suppose a VNTR repeat unit has length r. If one individual has n repeats, the repeated region has an approximate length:

n × r

If another individual has m repeats:

m × r

When n ≠ m:

→ fragment lengths differ

→ electrophoretic migration differs

→ band positions differ.

Thus:

Variable repeat number is converted into a variable DNA fragment length, which becomes a variable band position.

32. Alleles and VNTRs

At a particular VNTR locus, different numbers of repeats correspond to different alleles.

For example:

VNTR allele 1 → 5 repeats

VNTR allele 2 → 8 repeats

The two homologous chromosomes may carry different numbers of repeats. This creates genetic variation.

33. Maternal and Paternal Contribution

A person receives:

• One homologous chromosome from the mother
• One homologous chromosome from the father

Therefore, at a particular VNTR locus, the individual may have two different alleles.

For example:

Maternal chromosome → 5 repeats

Paternal chromosome → 8 repeats

This contributes to the individual's DNA profile.

Paternal and maternal alleles can contain different copy numbers of VNTRs.

34. Why is the DNA Fingerprint Highly Individual-Specific?

Consider multiple VNTR loci. At one locus: A and B may coincidentally have the same allele. At another locus: They may differ.

At several loci together, the probability that unrelated individuals will have exactly the same pattern becomes very low. Therefore:

Analysis of multiple polymorphic regions provides a highly individual-specific DNA profile. This is the fundamental logic behind DNA-based identification.

35. Important Terms-  One-Line Definitions

DNA Fingerprinting

Technique of identifying individuals by analysing polymorphic DNA patterns.

DNA Polymorphism

Variation in DNA sequence among individuals in a population.

Repetitive DNA

DNA containing sequences repeated multiple times.

Satellite DNA

Highly repetitive DNA sequences.

Minisatellite

A class of satellite DNA containing tandemly repeated sequences.

VNTR

Variable Number Tandem Repeat; a repetitive DNA sequence in which the number of tandem repeats varies.

Probe

A labelled nucleic acid sequence that hybridizes with a complementary target sequence.

Hybridization

Pairing of complementary nucleic acid sequences.

Autoradiography

Technique used to detect radioactive signals on a medium, producing detectable bands/images.

Southern Blotting

Technique for transferring DNA fragments from a gel onto a membrane for subsequent detection.

PCR

Polymerase Chain Reaction; technique used to amplify a specific DNA sequence.

36. High-Yield Facts

1. DNA fingerprinting was developed by Alec Jeffreys.
2. DNA fingerprinting is based on DNA polymorphism.
3. It analyses specific variable regions of DNA.
4. Important regions are repetitive DNA sequences.
5. The major NCERT marker is VNTR.
6. VNTR = Variable Number Tandem Repeat.
7. VNTR belongs to minisatellite DNA.
8. Minisatellite is a type of satellite DNA.
9. VNTR shows a high degree of polymorphism.
10. NCERT gives VNTR size as approximately 0.1–20 kb.
11. VNTR differences generate different DNA fragment sizes.
12. DNA fragments can be separated by gel electrophoresis.
13. DNA fragments are transferred using Southern blotting.
14. Detection involves hybridisation with a VNTR probe and, classically, autoradiography.
15. DNA fingerprinting can be used in forensics, relationship testing, identification, and population/genetic diversity studies.

37. DNA Fingerprinting vs Fingerprint

Conventional fingerprint	DNA fingerprint
Physical ridge pattern	DNA pattern
Obtained from fingers	Obtained from biological samples
Skin ridge characteristics	DNA polymorphism
Not based on VNTR	Based classically on VNTR
Anatomical/physical identification	Genetic identification

38. DNA Fingerprinting vs DNA Sequencing

DNA Fingerprinting	DNA Sequencing
Analyses selected polymorphic regions	Determines nucleotide sequence
Classical method uses VNTRs	Determines exact base order
Produces a characteristic pattern	Produces sequence information
Useful for identification	Useful for detailed genetic analysis
Does not require sequencing the whole genome	Can determine sequence of selected regions or entire genomes

39. DNA Fingerprinting vs PCR

DNA Fingerprinting	PCR
Identification/profile generation technique	DNA amplification technique
Uses polymorphic DNA markers	Amplifies selected DNA regions
Classical method involves VNTR analysis	Uses primers and DNA polymerase
Produces DNA profile/banding pattern	Produces amplified DNA
PCR can improve sensitivity	PCR can support DNA fingerprinting

40. DNA Fingerprinting vs Southern Blotting

These are also not identical.

DNA fingerprinting

The overall identification procedure/application.

Southern blotting

One laboratory step/technique used in the classical procedure.

Therefore: Southern blotting is a technique involved in classical DNA fingerprinting; it is not synonymous with DNA fingerprinting.

41. Most Important Exam Associations

Question	Answer
DNA fingerprinting developed by	Alec Jeffreys
Basis	DNA polymorphism
Important DNA sequences	Repetitive DNA
Major marker	VNTR
VNTR full form	Variable Number Tandem Repeat
VNTR belongs to	Minisatellite DNA
Minisatellite belongs to	Satellite DNA
Separation of DNA fragments	Gel electrophoresis
Blotting technique	Southern blotting
Detection probe	VNTR probe
Classical detection	Autoradiography
Amplification technique	PCR
Important application	Forensic science
Exception to individual uniqueness	Monozygotic twins
DNA charge	Negative
DNA migration	Towards positive electrode
VNTR size in NCERT	0.1–20 kb

42. Diagram Logic

When interpreting the DNA fingerprinting figure, understand it like this:

CHROMOSOME

     │

     ├── VNTR region

     │      │

     │      ├── repeat × 4

     │      │

     │      └── repeat × 8

     │

     ↓

Different fragment lengths

     ↓

Gel electrophoresis

     ↓

Different migration distances

     ↓

Different bands

     ↓

DNA fingerprint

The critical idea is:

Copy number of VNTR varies → DNA fragment size varies → band position varies.

43. Why is a Probe Needed?

After restriction digestion and electrophoresis, the DNA contains an enormous number of fragments. A probe helps identify the fragments containing the particular repetitive sequence of interest. Thus:

DNA fragments

↓

VNTR-specific labelled probe

↓

Probe binds to complementary VNTR sequences

↓

Only relevant fragments are detected

This makes the pattern interpretable.

44. Why Does PCR Increase Sensitivity?

Imagine that the sample contains only a very small quantity of DNA. Direct analysis may be difficult. PCR can amplify the DNA:

1 copy

↓

2

↓

4

↓

8

↓

16

↓

...

After repeated cycles, the target DNA can be amplified enormously. Therefore, a very small starting sample may provide enough DNA for analysis. DNA from a single cell can be sufficient.

45. One-Minute Revision

DNA Fingerprinting

→ Developed by Alec Jeffreys

→ Based on DNA polymorphism

→ Analyses repetitive DNA

→ Important sequences = VNTR

→ VNTR = Variable Number Tandem Repeat

→ VNTR is a minisatellite

→ Minisatellite = satellite DNA

→ Number of repeats varies among individuals

→ Different repeats → different fragment sizes

→ Restriction digestion

→ Gel electrophoresis

→ Southern blotting

→ VNTR probe hybridization

→ Autoradiography

→ Characteristic banding pattern

→ Used in forensics / relationship testing / identification / genetic diversity

→ PCR increases sensitivity

→ Monozygotic twins are the important exception to individual-specific patterns.

46. Super-High-Yield Memory Chain

J → P → R → V → M → S → P → A

J = Jeffreys
↓
P = Polymorphism
↓
R = Repetitive DNA
↓
V = VNTR
↓
M = Minisatellite
↓
S = Southern blotting
↓
P = Probe
↓
A = Autoradiography

And remember: VNTR → Variable Number Tandem Repeat. This is probably the most important single phrase in the topic.

51. Final Exam Checklist

Before moving on from this topic, make sure you can answer all of these without looking at your notes:

• Who developed DNA fingerprinting?
• What is the basis of DNA fingerprinting?
• What is DNA polymorphism?
• What is repetitive DNA?
• What is satellite DNA?
• What is a minisatellite?
• What does VNTR stand for?
• Why are VNTRs highly polymorphic?
• Why does repeat-number variation produce different bands?
• What is the role of restriction enzymes?
• What is gel electrophoresis?
• Why does DNA move towards the positive electrode?
• What is Southern blotting?
• What is a VNTR probe?
• What is hybridization?
• What is autoradiography?
• Why is PCR useful?
• What are the major applications?
• Why are monozygotic twins an exception?

For this chapter, the highest-yield chain is:

Alec Jeffreys → DNA polymorphism → repetitive DNA → VNTR → minisatellite → restriction digestion → electrophoresis → Southern blotting → VNTR probe → hybridization → autoradiography → characteristic DNA fingerprint → forensic/relationship/population applications.