When running small DNA fragments, the agarose concentration should be:

AIIMS CRE, SNO-2024

Biochemistry & Nutrition

Medium

Appeared in: AIIMS CRE, SNO-2024

Explanation

Agarose gel electrophoresis separates DNA fragments by size.
The agarose gel forms a matrix that acts as a molecular sieve.
A higher concentration of agarose creates a denser matrix with smaller pores.
These smaller pores provide greater resistance to the movement of small DNA fragments, slowing them down and allowing them to be separated from each other with higher resolution.
Think of it as using a fine-mesh sieve to separate small particles; a coarse sieve would let them all fall through together.

Option A: A lower concentration of agarose creates larger pores in the gel matrix. Small DNA fragments would move too quickly through these large pores, resulting in poor separation and resolution.
Option C: The term 'neutral concentration' is scientifically meaningless in this context. Agarose concentration is measured as a percentage of weight to volume (w/v), not on a scale like pH which has a neutral point.
Option D: This is a false statement. High-concentration agarose gels (e.g., 1.5% to 3%) are a standard and effective method for separating small DNA fragments, such as those generated by PCR.

Visual 1: Diagram - A comparison of a low-concentration agarose gel (large pores, for large DNA) and a high-concentration gel (small pores, for small DNA), showing how the fragments separate differently.
Visual 2: Infographic - An image of a final stained agarose gel showing three lanes: a DNA ladder, a sample with large fragments run on a low % gel, and a sample with small fragments run on a high % gel, illustrating the concept of resolution.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Principles of Agarose Gel Electrophoresis as background academic context rather than a clinical decision trigger.
Agarose gel electrophoresis is a fundamental technique in molecular diagnostics, used to analyze PCR products, perform DNA fingerprinting, and identify genetic mutations.
For example, when testing for a specific viral infection using PCR, the amplified viral DNA is a small fragment. Using the correct (higher) agarose concentration is critical to visualize the band and confirm the diagnosis.
What if? If a lab technician uses a 0.8% gel (low concentration) to check for a 150 bp PCR product (small fragment), the band may appear as a faint, blurry smear at the bottom of the gel or run off completely, potentially leading to a false-negative result.

How to Approach the Question

Identify the core scientific principle being tested: the separation of molecules by size using gel electrophoresis.
Recall the function of the agarose gel: it acts as a sieve.
Use a physical analogy: to separate small items (like sand from pebbles), you need a sieve with small holes (a fine mesh). To separate large items (like rocks from pebbles), you need a sieve with larger holes.
Translate the analogy to the question: 'Small DNA fragments' are the 'sand'. A 'fine mesh' corresponds to a gel with small pores.
Connect pore size to concentration: A higher concentration of agarose material creates a denser gel with smaller pores.
Conclude that for small DNA fragments, a higher agarose concentration is required for effective separation.

Concept Tested & Keywords

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