43. The amount of agarose required to prepare 250 mL of 0.8% agarose gel is __________ grams (rounded off to the nearest integer).
How Much Agarose Is Required to Prepare 250 mL of 0.8% Agarose Gel?
Detailed Explanation
The calculation of agarose required for gel preparation is a fundamental laboratory calculation used in molecular biology, biotechnology, genetics, and biochemistry. Agarose gel electrophoresis is routinely used to separate DNA fragments according to their size, and the concentration of agarose in the gel is selected according to the size range of DNA fragments that need to be resolved.
In this question, we need to determine how many grams of agarose are required to prepare 250 mL of a 0.8% agarose gel. The key to solving the problem is understanding what percentage concentration means when agarose gels are prepared.
For agarose gel preparation, the percentage concentration is generally expressed as weight per volume (% w/v). Therefore, a 0.8% agarose gel means that 0.8 grams of agarose are present in every 100 mL of final gel solution.
Once this definition is understood, the calculation becomes straightforward.
What Does a 0.8% Agarose Gel Mean?
Understanding Percentage Weight per Volume
The concentration of an agarose gel is generally written as a percentage weight per volume, represented as % w/v.
The general relationship is:
Percentage (w/v) = [Mass of solute (g) ÷ Volume of solution (mL)] × 100
For a 0.8% agarose gel:
0.8% agarose = 0.8 g agarose per 100 mL of solution
This means that if we were preparing exactly 100 mL of a 0.8% agarose gel, we would need 0.8 g of agarose.
However, the required volume in the question is 250 mL. Therefore, the amount of agarose must be increased proportionally.
Step-by-Step Calculation of Agarose Required
Step 1: Write the Meaning of 0.8% Agarose
A 0.8% agarose solution contains:
0.8 g agarose in 100 mL solution
Therefore:
100 mL solution requires 0.8 g agarose
We need to calculate the amount required for 250 mL.
Step 2: Set Up the Calculation
The required amount of agarose can be calculated using:
Amount of agarose = (Percentage concentration × Required volume) ÷ 100
Substituting the given values:
Amount of agarose = (0.8 × 250) ÷ 100
Step 3: Perform the Multiplication
0.8 × 250 = 200
Therefore:
Amount of agarose = 200 ÷ 100
Step 4: Calculate the Final Value
Amount of agarose = 2 g
The calculated value is already a whole number. Therefore, rounding it to the nearest integer does not change the answer.
Final Answer = 2 grams
Alternative Method Using Direct Proportion
The same question can also be solved using a direct proportion.
We know that:
0.8 g agarose is required for 100 mL solution
Therefore:
x g agarose is required for 250 mL solution
The proportion can be written as:
0.8 ÷ 100 = x ÷ 250
Cross-multiplying:
100x = 0.8 × 250
Therefore:
100x = 200
Dividing both sides by 100:
x = 2 g
Thus, the amount of agarose required is again calculated as 2 grams.
Why Is Agarose Concentration Expressed as % w/v?
Agarose is a solid substance that is weighed in grams, whereas the gel solution is prepared to a specific volume measured in milliliters. Therefore, agarose concentration is conventionally expressed as weight per volume.
A concentration of 1% w/v means 1 g of agarose in a final volume of 100 mL. Similarly, a concentration of 0.8% w/v means 0.8 g of agarose in a final volume of 100 mL.
This relationship can be used for any required volume. The amount of agarose changes proportionally with the final volume of the gel solution.
For example, preparing a larger volume requires more agarose, whereas preparing a smaller volume requires less agarose. However, the ratio of agarose mass to final solution volume must remain constant to maintain the desired percentage concentration.
General Formula for Agarose Gel Preparation
Formula for Calculating Agarose Mass
For routine agarose gel calculations, the following formula can be used:
Agarose required (g) = [Agarose concentration (%) × Final volume (mL)] ÷ 100
For the present question:
Agarose required (g) = (0.8 × 250) ÷ 100
Agarose required = 2 g
This formula is useful because it can be applied directly to almost any agarose gel preparation question.
For example, if a laboratory requires a 1% agarose gel with a final volume of 100 mL, the amount of agarose required would be 1 g. If a 2% agarose gel with a final volume of 50 mL is required, the amount would again be 1 g.
The concentration and volume must always be considered together.
How Is a 0.8% Agarose Gel Prepared in the Laboratory?
To prepare 250 mL of a 0.8% agarose gel, 2 g of agarose is weighed accurately. The agarose is added to an appropriate electrophoresis buffer, such as TAE or TBE buffer, with the final solution volume adjusted according to the experimental requirement.
The mixture is then heated until the agarose dissolves completely and the solution becomes clear. After cooling to a suitable temperature, the molten agarose is poured into a gel-casting tray containing a comb. As the solution cools, the agarose forms a porous gel matrix.
The comb creates wells into which DNA samples can later be loaded. When an electric field is applied, negatively charged DNA molecules migrate through the agarose matrix toward the positive electrode. Smaller DNA fragments generally move through the pores more rapidly than larger DNA fragments.
This is the basic principle of agarose gel electrophoresis.
Why Is a 0.8% Agarose Gel Used?
The concentration of agarose affects the pore size of the gel. A lower agarose concentration produces relatively larger pores, whereas a higher agarose concentration produces smaller pores.
A 0.8% agarose gel is considered a relatively low-concentration gel and is commonly useful for separating comparatively larger DNA fragments. Because the gel matrix contains larger pores than a high-percentage agarose gel, larger DNA molecules can migrate more effectively through it.
The appropriate agarose concentration depends on the expected size of the DNA fragments. Therefore, researchers select the gel concentration according to the specific requirements of the experiment.
Relationship Between Agarose Concentration and DNA Separation
Agarose concentration plays an important role in determining the resolving power of a gel. Low-percentage agarose gels contain larger pores and are generally better suited for resolving larger DNA fragments. High-percentage agarose gels contain smaller pores and are generally more suitable for separating smaller DNA fragments.
Therefore, changing the agarose concentration changes the physical structure of the gel matrix and influences DNA migration.
In this question, however, the main concept being tested is not DNA fragment separation but the calculation of percentage weight per volume. Recognizing that 0.8% means 0.8 g per 100 mL is sufficient to solve the problem accurately.
Why the Answer Must Be Rounded to the Nearest Integer
The question specifically asks for the answer in grams rounded off to the nearest integer. The calculated amount is:
2.0 g
Since 2.0 is already an integer, no further rounding is required.
Therefore:
2.0 g ≈ 2 g
Final Calculation
Given:
Agarose concentration = 0.8%
Final volume = 250 mL
Formula:
Agarose required (g) = [Concentration (%) × Volume (mL)] ÷ 100
Calculation:
Agarose required = (0.8 × 250) ÷ 100
Agarose required = 200 ÷ 100
Agarose required = 2 g
Final Answer
The amount of agarose required to prepare 250 mL of a 0.8% agarose gel is 2 grams.
Correct Answer: 2 grams
A 0.8% agarose gel contains 0.8 g of agarose per 100 mL of solution. Therefore, for 250 mL, the required amount is calculated as (0.8 × 250) ÷ 100 = 2 g. Since the answer is already a whole number, the value rounded to the nearest integer remains 2 grams.
