Which Methods Are Used to Detect a 150 Base Pair DNA Fragment?

Correct Answer: (A) and (C) — Agarose Gel Electrophoresis and Polyacrylamide Gel Electrophoresis

The correct answers are Option (A), agarose gel electrophoresis, and Option (C), polyacrylamide gel electrophoresis. A DNA fragment of 150 base pairs (150 bp) is a relatively small double-stranded DNA fragment, and it can be separated and detected using electrophoretic methods designed for nucleic acids.

Agarose gel electrophoresis can detect a 150 bp DNA fragment when an appropriate agarose concentration and suitable electrophoretic conditions are used. However, because 150 bp is relatively small, polyacrylamide gel electrophoresis (PAGE) generally provides superior resolution and is especially effective for separating small DNA fragments that differ only slightly in length.

The other two methods do not represent appropriate answers for this question. Northern blotting is primarily used to detect specific RNA molecules, whereas western blotting is used to detect specific proteins. Therefore, the correct methods for detecting the 150 bp DNA fragment among the given options are agarose gel electrophoresis and polyacrylamide gel electrophoresis.

Why Can a 150 Base Pair DNA Fragment Be Detected by Electrophoresis?

DNA molecules carry a net negative charge because of the phosphate groups present in their sugar-phosphate backbone. When DNA is placed in an electric field, it migrates toward the positively charged electrode.

In gel electrophoresis, the gel acts as a molecular sieve. Smaller DNA fragments generally move through the pores of the gel more easily and therefore migrate faster than larger DNA fragments. This allows a mixture of DNA molecules to be separated according to fragment size.

A DNA fragment containing 150 base pairs is small enough to migrate rapidly through an agarose gel, but it can still be visualized under properly optimized conditions. The same fragment can also be separated in a polyacrylamide gel, where the smaller pore size and high resolving power of the matrix provide particularly effective separation of short DNA molecules.

Therefore, both agarose gel electrophoresis and polyacrylamide gel electrophoresis can be used for the detection of a 150 bp DNA fragment.

What Does 150 Base Pairs Mean?

The abbreviation bp stands for base pairs. Double-stranded DNA is composed of two complementary polynucleotide strands in which bases pair according to specific rules. Adenine generally pairs with thymine, while guanine pairs with cytosine.

A DNA fragment of 150 base pairs therefore contains a double-stranded region extending for 150 paired nucleotide positions. In molecular biology, this is considered a relatively small DNA fragment compared with genomic DNA, large restriction fragments, plasmids and many PCR products.

The size of the DNA fragment is important when selecting a gel matrix. Agarose gels are widely used for routine DNA analysis over a broad size range, while polyacrylamide gels are particularly useful when very small fragments or small differences in DNA length must be resolved accurately.

Why Option (A) Agarose Gel Electrophoresis Is Correct

Option (A), agarose gel electrophoresis, is correct because agarose gel electrophoresis is one of the most widely used techniques for separating and visualizing DNA fragments.

Agarose is a polysaccharide that forms a porous gel matrix. When an electric field is applied, negatively charged DNA molecules migrate through the pores of the gel toward the positive electrode. Smaller DNA fragments move more rapidly than larger fragments.

A 150 bp DNA fragment can be detected using agarose gel electrophoresis, particularly when an appropriate concentration of agarose is selected. Higher-percentage agarose gels have smaller effective pore sizes and provide improved separation of relatively small DNA fragments compared with lower-percentage gels.

After electrophoresis, the DNA can be visualized using a suitable nucleic acid stain. The position of the 150 bp fragment can be estimated by comparing its migration with a DNA size marker or DNA ladder containing fragments of known length.

Therefore, Option (A) is a correct method for detecting a 150 bp DNA fragment.

Principle of Agarose Gel Electrophoresis

Agarose gel electrophoresis separates DNA primarily according to molecular size. DNA molecules possess negatively charged phosphate groups, so they move through the gel toward the positive electrode when voltage is applied.

The agarose network creates resistance to DNA movement. Larger DNA molecules experience greater difficulty moving through the pores and therefore migrate more slowly. Smaller DNA fragments pass through the matrix more easily and migrate more rapidly.

The general relationship is:

Smaller DNA fragment → Faster migration through the gel

Larger DNA fragment → Slower migration through the gel

A 150 bp DNA fragment is relatively small and therefore migrates rapidly. Appropriate gel concentration, voltage, run time and staining conditions are important to prevent the fragment from migrating too far and to ensure that it can be detected clearly.

Why Is Agarose Concentration Important for a 150 bp DNA Fragment?

The concentration of agarose influences the effective pore size of the gel. A lower agarose concentration produces relatively larger pores and is more suitable for separating larger DNA molecules. A higher agarose concentration produces smaller pores and improves the separation of shorter DNA fragments.

Because a 150 bp fragment is relatively small, a gel concentration suitable for small DNA fragments should be selected. If the pores are too large or the electrophoresis is continued for too long, the fragment may migrate very rapidly and resolution may be poor.

With appropriate optimization, agarose gel electrophoresis can successfully separate and detect DNA fragments around 150 bp, making Option (A) correct.

How Is a 150 bp DNA Band Visualized in an Agarose Gel?

DNA itself is not usually visible as a distinct band to the naked eye in an unstained gel. Therefore, a nucleic acid-binding stain is used to make DNA fragments detectable.

After staining, the DNA bands are visualized using an appropriate imaging system. The migration position of the unknown fragment is compared with a DNA ladder containing DNA fragments of known sizes.

If the sample band migrates to a position corresponding to the 150 bp marker in the DNA ladder, the approximate size of the fragment can be identified as 150 base pairs.

Why Option (B) Northern Blotting Is Incorrect

Option (B), northern blotting, is incorrect because northern blotting is primarily used for the detection and analysis of RNA, not for routine detection of a DNA fragment.

In a northern blotting experiment, RNA molecules are first separated according to size by gel electrophoresis. The separated RNA is then transferred to a membrane and detected using a complementary labeled nucleic acid probe.

Northern blotting is commonly used to investigate RNA transcripts, gene expression patterns, transcript size and the abundance of specific RNA molecules.

Because the molecule given in the question is specifically a 150 base pair DNA fragment, northern blotting is not the appropriate method among the listed choices. Therefore, Option (B) is incorrect.

What Does Northern Blotting Detect?

Northern blotting is designed primarily to detect specific RNA molecules in a complex sample. The technique can provide information about whether a particular transcript is present, the approximate size of the transcript and, under appropriate experimental conditions, its relative abundance.

The general sequence of a northern blotting experiment involves separation of RNA, transfer to a membrane, hybridization with a labeled probe and detection of the probe-target hybrid.

The defining target of northern blotting is therefore RNA. Since the question asks about detecting a DNA fragment, northern blotting should not be selected.

Why Option (C) Polyacrylamide Gel Electrophoresis Is Correct

Option (C), polyacrylamide gel electrophoresis, is correct and is particularly suitable for analyzing a relatively small DNA fragment such as a 150 bp fragment.

Polyacrylamide gels have smaller and more precisely controlled pore sizes than typical agarose gels. This gives PAGE a high resolving power and allows it to separate small nucleic acid fragments with excellent precision.

A 150 bp DNA fragment lies well within the size range that can be analyzed using polyacrylamide gel electrophoresis. PAGE is especially useful when researchers need to distinguish DNA fragments that differ by only a small number of nucleotides or base pairs.

Therefore, Option (C) is correct. For fine resolution of small DNA fragments, PAGE may provide better separation than conventional agarose gel electrophoresis.

Principle of Polyacrylamide Gel Electrophoresis for DNA

Polyacrylamide gel electrophoresis separates DNA molecules as they migrate through a cross-linked polyacrylamide matrix under the influence of an electric field.

As with agarose gel electrophoresis, negatively charged DNA moves toward the positive electrode. The polyacrylamide matrix acts as a molecular sieve, slowing larger molecules more strongly than smaller ones.

Because the pore size of the gel can be carefully controlled by changing the concentrations of acrylamide and cross-linking components, PAGE can achieve very high resolution for short DNA fragments.

This makes polyacrylamide gel electrophoresis highly appropriate for the detection and analysis of a 150 bp DNA fragment.

Why Is PAGE Particularly Suitable for Small DNA Fragments?

Small DNA fragments may migrate very rapidly through low-percentage agarose gels, and fragments of similar size may not always be separated clearly. Polyacrylamide gels provide a tighter molecular sieving network and therefore greater resolving power for small nucleic acid molecules.

PAGE can distinguish relatively small differences in DNA length under suitable conditions. This is why polyacrylamide gels have historically been important in applications requiring high-resolution analysis of short nucleic acids.

For a DNA fragment of only 150 bp, PAGE is therefore an excellent analytical method and clearly qualifies as a correct answer.

Why Option (D) Western Blotting Is Incorrect

Option (D), western blotting, is incorrect because western blotting is used for the detection of proteins, not DNA fragments.

In a typical western blot experiment, proteins are first separated by polyacrylamide gel electrophoresis, commonly under denaturing conditions. The proteins are then transferred to a membrane and detected using antibodies that specifically recognize the target protein.

The technique is widely used to determine whether a particular protein is present in a sample and to examine properties such as approximate molecular size and relative abundance.

A 150 bp DNA fragment cannot be identified by a standard western blot because the method relies on antibody-based detection of proteins. Therefore, Option (D) is incorrect.

What Does Western Blotting Detect?

Western blotting is fundamentally a protein detection technique. The target molecules are proteins, and the specificity of detection is usually provided by antibodies.

The general process includes protein separation, transfer to a membrane, blocking, incubation with a primary antibody, detection using an appropriate secondary system and visualization of the protein signal.

Since DNA fragments are nucleic acids rather than proteins, western blotting is not an appropriate method for detecting the 150 bp DNA fragment described in the question.

Agarose Gel Electrophoresis Versus Polyacrylamide Gel Electrophoresis

Both agarose gel electrophoresis and polyacrylamide gel electrophoresis can separate DNA molecules, but they differ in gel structure, resolution and common applications.

Agarose gels are widely used for routine analysis of DNA fragments because they are relatively simple to prepare and can separate DNA over a broad size range. A 150 bp fragment can be detected on an appropriately optimized agarose gel.

Polyacrylamide gels generally provide greater resolving power for small DNA fragments. Their smaller and more controlled pore sizes make them particularly effective when the DNA fragments are short or differ from one another by only a small number of base pairs.

Therefore, both methods are correct for a 150 bp DNA fragment, although PAGE is generally preferred when very high resolution is required.

Why Are Both Options (A) and (C) Correct?

The question asks which method or methods can be used to detect a DNA fragment of 150 base pairs. It does not ask exclusively for the method with the highest possible resolution.

Agarose gel electrophoresis can detect a 150 bp DNA fragment under suitable conditions. Polyacrylamide gel electrophoresis can also detect the same fragment and generally provides excellent resolution for small DNA molecules.

Therefore, the presence of a more highly resolving method does not automatically make the other valid electrophoretic method incorrect. Since both techniques can be used for this purpose, Options (A) and (C) are correct.

Why the Size of a DNA Fragment Matters When Choosing a Gel

The size of a DNA fragment strongly influences the choice of electrophoretic matrix. Very large DNA molecules require a gel with sufficiently large pores to allow migration, while small DNA fragments benefit from matrices that provide greater resistance and finer separation.

Agarose concentration can be adjusted to modify pore size. Higher agarose concentrations generally improve the separation of smaller DNA fragments. However, for very fine resolution of short fragments, polyacrylamide is often more effective.

A 150 bp fragment is small enough that gel selection and running conditions become particularly important. Both a suitable agarose gel and a polyacrylamide gel can be used, but PAGE generally offers superior resolution.

How Does a DNA Ladder Help Detect a 150 bp Fragment?

A DNA ladder is a mixture of DNA fragments with known lengths. It is loaded into a separate lane of the gel and electrophoresed alongside the experimental sample.

After electrophoresis and visualization, the migration position of the unknown DNA band is compared with the bands in the DNA ladder. If the sample band migrates to the same or a comparable position as a known 150 bp marker, the fragment can be identified as approximately 150 base pairs long.

For small fragments, a DNA ladder containing closely spaced low-molecular-size markers is particularly useful because it allows more accurate size estimation.

Difference Between DNA, RNA and Protein Detection Methods

Understanding the target biomolecule of each laboratory method is essential for answering this question correctly. Agarose gel electrophoresis and polyacrylamide gel electrophoresis can be used to separate nucleic acids, including DNA, under appropriate conditions.

Northern blotting is primarily associated with the detection of specific RNA molecules. Western blotting is associated with the detection of specific proteins using antibodies.

The relationship among the methods can therefore be summarized as:

Agarose Gel Electrophoresis → DNA separation and detection → Correct

Northern Blotting → RNA detection → Incorrect

Polyacrylamide Gel Electrophoresis → High-resolution separation of small DNA fragments → Correct

Western Blotting → Protein detection → Incorrect

What Is the Role of Southern Blotting in DNA Detection?

Although Southern blotting is not included among the answer choices, it is useful to understand its relationship with DNA analysis. Southern blotting is a classical technique used to detect a specific DNA sequence within a mixture of DNA fragments.

In Southern blotting, DNA fragments are separated by gel electrophoresis, transferred to a membrane and detected by hybridization with a complementary labeled probe.

However, this question asks which of the listed methods can be used to detect a 150 bp DNA fragment. Among the given options, agarose gel electrophoresis and polyacrylamide gel electrophoresis are the correct choices.

Importance of Polyacrylamide Gel Electrophoresis in Small DNA Fragment Analysis

Polyacrylamide gel electrophoresis is especially valuable when high-resolution analysis of small DNA fragments is required. Because the gel matrix has a finely controlled pore structure, short DNA molecules can be separated with greater precision than is typically achieved with routine agarose gels.

This property has made PAGE useful in many applications involving short nucleic acid fragments, including the analysis of small PCR products, oligonucleotides and other short DNA molecules.

For the 150 bp fragment given in this question, PAGE is therefore a particularly strong analytical choice.

Final Answer

Correct Options: (A) and (C)

Agarose gel electrophoresis and polyacrylamide gel electrophoresis can be used to detect a DNA fragment of 150 base pairs.

Agarose gel electrophoresis can separate and visualize a 150 bp DNA fragment when an appropriate gel concentration and electrophoretic conditions are used. Polyacrylamide gel electrophoresis is also suitable and generally provides superior resolution for relatively small DNA fragments.

Northern blotting is primarily used for RNA detection, while western blotting is used for protein detection. Therefore, these two methods are not appropriate answers for direct detection of the DNA fragment described in the question.

(A) Agarose gel electrophoresis → Correct

(B) Northern blotting → Incorrect — Primarily used for RNA detection

(C) Polyacrylamide gel electrophoresis → Correct

(D) Western blotting → Incorrect — Used for protein detection

Therefore, the methods used to detect a 150 base pair DNA fragment are agarose gel electrophoresis and polyacrylamide gel electrophoresis, making Options (A) and (C) the correct answers.