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DNA FOOTPRINTING


DNA FOOTPRINTING


Introduction


DNA footprinting is a molecular biology technique used to identify the specific site(s) on DNA where proteins (such as transcription factors) bind. It reveals the exact nucleotide sequences protected by bound proteins against cleavage by nucleases or chemical agents.
It is widely used to study DNA-protein interactions, transcription regulation, and gene expression control.


Definition


DNA footprinting:

A technique used to locate the binding site of DNA-binding proteins on DNA by detecting protected regions that are resistant to enzymatic or chemical cleavage.

Principle


DNA-binding proteins protect the DNA segment they occupy.
DNA exposed to nucleases (DNase I) or chemical cleavage agents is cut at accessible regions.
Regions bound by protein remain unaffected, leaving a “footprint”.
When fragments are separated on a denaturing polyacrylamide gel, the missing bands correspond to protein-binding sites.
Key idea:
Cleavage occurs everywhere except where the protein is bound.
The protected region is visualized as a gap or footprint on the gel.


Requirements / Materials
DNA fragment (radioactively or fluorescently labeled at one end)
DNA-binding protein of interest
DNase I or chemical cleavage reagent (e.g., hydroxyl radicals, DMS)
Buffer solutions for binding
Gel electrophoresis setup (denaturing PAGE for single-base resolution)
Autoradiography or fluorescence detection system


Methodology


1. Preparation of DNA


Select the DNA fragment containing the suspected binding site.
Label one end of the DNA strand with 32P, fluorescent tag, or biotin.
Purify DNA to remove unincorporated labels.

2. Protein-DNA Binding

Incubate labeled DNA with DNA-binding protein under physiological buffer conditions.
Incubation ensures specific binding to target sequences.

3. DNA Cleavage


Two approaches:
A. Enzymatic cleavage (DNase I method)
DNase I randomly cleaves DNA at phosphodiester bonds.
Protein-bound regions are protected.

B. Chemical cleavage

Agents like hydroxyl radicals or DMS cleave DNA chemically.
Protein-bound DNA remains resistant.

4. Removal of Protein

Treat the mixture to remove bound protein without disturbing DNA.
Only DNA fragments remain for analysis.

5. Gel Electrophoresis

Denaturing polyacrylamide gel separates DNA fragments by size.
Single-nucleotide resolution allows precise mapping of the protected site.

6. Detection

Autoradiography (for radioactive labels) or fluorescence scanning (for fluorescent labels).
Lanes:
DNA + protein + DNase I → footprint shows missing bands
DNA + DNase I only (no protein) → control shows all fragments
Compare lanes to locate protein-protected region.

Interpretation
The footprint is the gap in the cleavage pattern.
It indicates:
Exact nucleotides protected
Protein-binding site length and position
Can help calculate binding affinity and sequence specificity.

Example
Lambda repressor protein binds DNA.
DNase I digestion produces a continuous ladder of DNA fragments in control.
When repressor is bound, a region of missing bands appears—this is the DNA footprint.


Applications


Identification of transcription factor binding sites
Study of gene regulation mechanisms
Mapping of protein-DNA interactions in vitro
Understanding promoter activity
Comparison of binding sites across species
Drug design targeting DNA-protein interactions.


Advantages
High-resolution identification of binding sites
Can determine exact nucleotides involved
Applicable to different DNA-binding proteins
Provides information about binding affinity


Limitations
Requires labeled DNA
Labor-intensive and time-consuming
Only detects strong, specific protein-DNA interactions
Not suitable for very large DNA fragments
Requires careful control experiments
Diagrammatic Representation (for exams)


1. DNA ladder (control)
Shows complete cleavage of DNA by DNase I
2. DNA + Protein
Missing bands (footprint) indicate protein binding site
3. Analysis
Position of footprint corresponds to the protein-binding nucleotides
(You can draw a simple gel with numbered fragments and a “gap” in protein lane.)


Key Points to Remember


Footprinting identifies where on DNA a protein binds.
DNase I footprinting is most common.
Labeling one DNA end is essential for detecting fragments.
Footprint = protected region on DNA.
Critical for gene regulation studies.



50 MCQs – DNA Footprinting


Basic Concepts


DNA footprinting is used to study:
A) DNA replication
B) RNA synthesis
C) DNA–protein interaction ✅
D) Protein–protein interaction


The main purpose of DNA footprinting is to identify:
A) DNA sequence
B) Protein structure
C) Protein-binding site on DNA ✅
D) RNA binding site


A “footprint” refers to:
A) DNA mutation
B) Protected DNA region ✅
C) DNA cleavage site
D) Protein degradation


DNA footprinting was first used to study:
A) DNA methylation
B) Transcription factor binding ✅
C) DNA sequencing
D) RNA splicing


DNA footprinting provides information at:
A) Chromosome level
B) Gene level
C) Nucleotide level ✅
D) Protein level
Principle & Enzymes


DNA footprinting is based on the principle that:
A) Proteins degrade DNA
B) Bound proteins protect DNA from cleavage ✅
C) DNA cannot bind proteins
D) Enzymes cut DNA at fixed sites


Most commonly used enzyme in footprinting is:
A) RNase
B) DNase I ✅
C) Restriction endonuclease
D) DNA ligase
DNase I cleaves DNA:
A) At specific sequences
B) Randomly at accessible sites ✅
C) Only at AT regions
D) Only at GC regions
Which chemical can be used in chemical footprinting?
A) SDS
B) DMS ✅
C) EDTA
D) Urea
Hydroxyl radicals cleave DNA by attacking:
A) Bases
B) Sugar-phosphate backbone ✅
C) Hydrogen bonds
D) Peptide bonds
DNA Labeling & Preparation
DNA used in footprinting is usually labeled at:
A) Both ends
B) One end only ✅
C) Middle
D) Random sites
Radiolabeling commonly uses:
A) ³H
B) ¹⁴C
C) ³²P ✅
D) ¹²C
Labeling helps in:
A) DNA digestion
B) Protein binding
C) Detection of DNA fragments ✅
D) DNA methylation
The DNA fragment selected should contain:
A) Entire genome
B) Suspected protein-binding region ✅
C) Only coding region
D) Only introns
Unlabeled DNA cannot be used because:
A) It cannot bind proteins
B) It cannot be detected on gel ✅
C) It cannot be cleaved
D) It degrades easily
Methodology
Protein binding is carried out:
A) After DNA digestion
B) Before DNA digestion ✅
C) During electrophoresis
D) After autoradiography
Protein-DNA complex formation requires:
A) High temperature
B) Denaturing conditions
C) Physiological buffer conditions ✅
D) Acidic pH
Cleavage in footprinting is performed under:
A) Complete digestion
B) Partial digestion conditions ✅
C) No digestion
D) Over-digestion
Partial digestion ensures:
A) No cleavage
B) Cleavage at every site
C) Single cut per DNA molecule on average ✅
D) DNA degradation
After digestion, proteins are removed by:
A) Heating only
B) Protease treatment or denaturation ✅
C) Restriction enzymes
D) PCR
Gel Electrophoresis & Detection
DNA fragments are separated by:
A) Agarose gel
B) Denaturing polyacrylamide gel electrophoresis ✅
C) SDS-PAGE
D) Native PAGE
Denaturing PAGE provides:
A) Protein separation
B) Single-nucleotide resolution ✅
C) Circular DNA separation
D) RNA degradation
Autoradiography is used to:
A) Digest DNA
B) Visualize radiolabeled DNA fragments ✅
C) Bind proteins
D) Stain proteins
In control lane (no protein), the gel shows:
A) No bands
B) Missing bands
C) Complete ladder of fragments ✅
D) Only one band
In protein-bound lane, the footprint appears as:
A) Extra bands
B) Dark band
C) Missing bands region ✅
D) Smear
Interpretation
The footprint corresponds to:
A) Cleaved region
B) Mutated region
C) Protected DNA sequence ✅
D) Unlabeled DNA
Length of the footprint indicates:
A) Protein size
B) Binding site length ✅
C) DNA length
D) Enzyme activity
Position of footprint indicates:
A) Protein molecular weight
B) Exact DNA binding site ✅
C) DNA sequence
D) Transcription start site
Strong protein binding results in:
A) Faint footprint
B) Clear and distinct footprint ✅
C) No footprint
D) DNA degradation
Weak protein-DNA interaction produces:
A) Strong footprint
B) No digestion
C) Partial or faint footprint ✅
D) Complete protection
Applications
DNA footprinting is widely used to study:
A) DNA replication
B) Gene regulation ✅
C) Translation
D) Protein synthesis
It helps identify:
A) Promoter-binding proteins ✅
B) Ribosomes
C) tRNA
D) Lipids
DNA footprinting is important in:
A) Mutation breeding
B) Transcription factor analysis ✅
C) DNA cloning only
D) Protein purification
It is mainly an:
A) In vivo technique
B) In vitro technique ✅
C) Clinical technique
D) Imaging technique
Footprinting can compare:
A) DNA size
B) Protein sequences
C) Binding affinity of proteins to DNA ✅
D) RNA expression
Advantages & Limitations
Major advantage of DNA footprinting:
A) Low cost
B) High resolution binding site detection ✅
C) Genome-wide analysis
D) No labeling required
A limitation of DNA footprinting is:
A) Low specificity
B) Requires labeled DNA ✅
C) Cannot detect binding sites
D) Cannot use enzymes
DNA footprinting is not suitable for:
A) Short DNA fragments
B) Large genomic DNA directly ✅
C) Transcription factor studies
D) Promoter analysis
Compared to ChIP, footprinting is:
A) In vivo
B) In vitro and highly specific ✅
C) Genome-wide
D) RNA-based
DNA footprinting cannot identify:
A) Binding site
B) Binding strength
C) Protein identity without prior knowledge ✅
D) Protected DNA region
Advanced & Conceptual
Chemical footprinting differs from DNase I footprinting because it:
A) Uses enzymes
B) Uses chemical cleavage agents ✅
C) Uses restriction enzymes
D) Uses PCR
Hydroxyl radical footprinting gives information about:
A) DNA sequence only
B) DNA backbone accessibility ✅
C) Protein sequence
D) RNA structure
DNA footprinting requires single-end labeling to:
A) Increase cleavage
B) Avoid overlapping signals ✅
C) Denature DNA
D) Bind proteins
A transcription factor protects DNA by:
A) Breaking hydrogen bonds
B) Binding major/minor groove ✅
C) Cutting DNA
D) Denaturing DNA
The footprint pattern depends on:
A) Protein concentration ✅
B) Agarose concentration
C) DNA ladder
D) Gel voltage
Comparison & Miscellaneous
DNA footprinting is different from EMSA because EMSA shows:
A) Exact binding site
B) DNA-protein complex mobility shift ✅
C) DNA cleavage
D) Nucleotide resolution
EMSA cannot identify:
A) DNA-protein binding
B) Exact binding nucleotides ✅
C) Binding affinity
D) Complex formation
DNase I footprinting is sensitive to:
A) Over-digestion conditions ✅
B) Gel staining
C) DNA ladder
D) RNA contamination
Footprinting experiments must include:
A) Only protein sample
B) Only DNA sample
C) Control without protein ✅
D) Only labeled DNA
DNA footprinting is best described as:
A) DNA sequencing method
B) DNA-protein interaction mapping technique ✅
C) RNA analysis method
D) Protein purification technique.

✅ Answer Key (Quick View)

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