**30 CALCULATION QUESTIONS AND ANSWERS RELATED TO NUCLEIC ACIDS**

Question 1:

What is the molecular weight of a single nucleotide with the sequence “A” (adenine)?

Answer 1:

The molecular weight of a single nucleotide with the sequence “A” is approximately 329.2 g/mol.

Question 2:

How many hydrogen bond interactions are there between the base pairs adenine (A) and thymine (T) in a DNA double helix?

Answer 2:

There are two hydrogen bond interactions between adenine (A) and thymine (T) in a DNA double helix.

Question 3:

Calculate the molar concentration (in mol/L) of a DNA solution with an absorbance of 1.5 at 260 nm and a path length of 1 cm, given the molar absorptivity is 13,000 M^-1cm^-1.

Answer 3:

Molar concentration = Absorbance / (Molar absorptivity × Path length)

Molar concentration = 1.5 / (13,000 M^-1cm^-1 × 1 cm) ≈ 0.000115 mol/L

Question 4:

Given a DNA strand with the sequence 5′-ATCGGTA-3′, what is the complementary RNA sequence?

Answer 4:

The complementary RNA sequence to 5′-ATCGGTA-3′ is 5′-UACCGAU-3′.

Question 5:

If a DNA double helix contains 30% adenine (A), what percentage of the bases will be guanine (G)?

Answer 5:

Since adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C), if adenine is 30%, then guanine will also be 30%.

Question 6:

Calculate the melting temperature (Tm) of a DNA duplex with the sequence 5′-GCTAATCG-3′, using the formula: Tm = 2°C × (A + T) + 4°C × (G + C), where A, T, G, and C are the percentages of the respective bases in the sequence.

Answer 6:

Given the sequence is 5′-GCTAATCG-3′:

A = 25% (adenine), T = 25% (thymine), G = 25% (guanine), C = 25% (cytosine)

Tm = 2°C × (25% + 25%) + 4°C × (25% + 25%) = 50°C + 100°C = 150°C

Question 7:

What is the charge of a DNA strand with 30 cytosine (C) bases and 20 guanine (G) bases at pH 7?

Answer 7:

The charge of a DNA strand can be calculated using the formula: Charge = (# of positively charged groups) – (# of negatively charged groups)

In DNA, cytosine (C) and guanine (G) have an extra amino group, making them positively charged. At pH 7, both groups are fully protonated.

Number of positively charged groups = 30 (cytosine) + 20 (guanine) = 50

Number of negatively charged groups = 50 (phosphate backbone)

Charge = 50 – 50 = 0

So, the DNA strand has a net charge of 0 at pH 7.

Question 8:

If a DNA molecule is 120 base pairs long, how many complete turns does the double helix make?

Answer 8:

In B-DNA (common DNA conformation), one complete turn of the DNA helix occurs every 10.4 base pairs.

Number of complete turns = 120 base pairs / 10.4 base pairs/turn ≈ 11.54 turns

Question 9:

Calculate the length of a DNA molecule in nm if it contains 250 base pairs.

Answer 9:

In B-DNA, each base pair contributes approximately 0.34 nm to the length of the DNA molecule.

Length of DNA = 250 base pairs × 0.34 nm/base pair ≈ 85 nm

Question 10:

What is the concentration of a 250 bp DNA solution in ng/μL if its molar concentration is 0.1 mol/L?

Answer 10:

To convert molar concentration (mol/L) to mass concentration (ng/μL), we need to use the molecular weight of the DNA molecule.

Given: Molecular weight of a double-stranded DNA base pair = 650 g/mol

250 base pairs = 250 bp × 650 g/mol ≈ 162,500 g/mol

Mass concentration (ng/μL) = Molar concentration (mol/L) × Molecular weight (g/mol)

Mass concentration = 0.1 mol/L × 162,500 g/mol ≈ 16,250 ng/μL

Question 11:

A DNA sample absorbs light at 280 nm with an absorbance of 0.8. Calculate the concentration of the DNA sample in mg/mL, given its molar absorptivity at 280 nm is 30,000 M^-1cm^-1.

Answer 11:

Molar concentration (mol/L) = Absorbance / (Molar absorptivity × Path length)

Path length = 1 cm (standard cuvette)

Molar concentration = 0.8 / (30,000 M^-1cm^-1 × 1 cm) ≈ 0.00002667 mol/L

To convert molar concentration to mg/mL, we need the molecular weight of the DNA. Let’s assume it is 660 g/mol (average molecular weight for a DNA base pair).

Mass concentration (mg/mL) = Molar concentration (mol/L) × Molecular weight (g/mol)

Mass concentration ≈ 0.00002667 mol/L × 660 g/mol ≈ 0.0176 mg/mL

Question 12:

If a DNA molecule has a total of 500 nucleotides, how many phosphate groups are present in the backbone?

Answer 12:

In DNA, each nucleotide contains one phosphate group in the backbone.

Number of phosphate groups = 500 nucleotides

Question 13:

Calculate the percentage of cytosine (C) bases in a DNA sequence with the following composition: 30% adenine (A), 25% thymine (T), 20% cytosine (C), and 25% guanine (G).

Answer 13:

Percentage of cytosine (C) = 20%

Question 14:

What is the total number of hydrogen bonds present in a DNA double helix with 500 base pairs?

Answer 14:

In a DNA double helix, there are two hydrogen bonds between adenine (A) and thymine (T), and three hydrogen bonds between guanine (G) and cytosine (C).

Total number of hydrogen bonds = (Number of A-T pairs × 2) + (Number of G-C pairs × 3)

Assuming equal number of A-T and G-C pairs in a random sequence:

Number of A-T pairs = 500 base pairs / 2 = 250 pairs

Number of G-C pairs = 500 base pairs / 2 = 250 pairs

Total number of hydrogen bonds

= (250 × 2) + (250 × 3) = 500 + 750 = 1250 hydrogen bonds

Question 15:

If a DNA strand contains 40% guanine (G) bases, what percentage of the bases will be adenine (A)?

Answer 15:

Since guanine (G) pairs with cytosine (C) and adenine (A) pairs with thymine (T), the percentages of G and C must add up to the percentages of A and T. Therefore, if G is 40%, then A will also be 40%.

Question 16:

Calculate the absorbance of a DNA solution with a molar concentration of 0.05 mol/L and a path length of 1 cm, given that its molar absorptivity at 260 nm is 12,000 M^-1cm^-1.

Answer 16:

Absorbance = Molar concentration × Molar absorptivity × Path length

Absorbance = 0.05 mol/L × 12,000 M^-1cm^-1 × 1 cm = 0.6

Question 17:

What is the complementary DNA sequence to 5′-TACGTA-3′?

Answer 17:

The complementary DNA sequence to 5′-TACGTA-3′ is 5′-TACGTA-3′.

Question 18:

If a DNA molecule contains 20% adenine (A), what percentage of the bases will be thymine (T)?

Answer 18:

Since adenine (A) pairs with thymine (T) in DNA, if A is 20%, then T will also be 20%.

Question 19:

Calculate the GC content (percentage of guanine and cytosine bases) in a DNA sequence with the following composition: 30% adenine (A), 25% thymine (T), 20% cytosine (C), and 25% guanine (G).

Answer 19:

GC content = Percentage of guanine (G) + Percentage of cytosine (C) = 20% + 25% = 45%

Question 20:

If a DNA double helix is 50 nm long, how many base pairs does it contain?

Answer 20:

In B-DNA, each base pair contributes approximately 0.34 nm to the length of the DNA molecule.

Number of base pairs = Length of DNA / Length per base pair

Number of base pairs = 50 nm / 0.34 nm/base pair ≈ 147 base pairs

Question 21:

What is the concentration of a 150 bp DNA solution in μg/mL if its molar concentration is 0.02 mol/L?

Answer 21:

To convert molar concentration (mol/L) to mass concentration (μg/mL), we need to use the molecular weight of the DNA molecule.

Given: Molecular weight of a double-stranded DNA base pair = 650 g/mol

150 base pairs = 150 bp × 650 g/mol ≈ 97,500 g/mol

Mass concentration (μg/mL) = Molar concentration (mol/L) × Molecular weight (μg/mol)

Mass concentration = 0.02 mol/L × 97,500 μg/mol ≈ 1950 μg/mL

Question 22:

If a DNA sample has an A260/A280 ratio of 1.8, is it pure, or does it contain protein contamination?

Answer 22:

An A260/A280 ratio of 1.8 indicates that the DNA sample is relatively pure and does not contain significant protein contamination. DNA typically has an A260/A280 ratio between 1.7 and 1.9.

Question 23:

Calculate the length of an RNA molecule in nm if it contains 200 nucleotides.

Answer 23:

In RNA, each nucleotide contributes approximately 0.33 nm to the length of the RNA molecule.

Length of RNA = 200 nucleotides × 0.33 nm/nucleotide ≈ 66 nm

Question 24:

What is the percentage of adenine (A) bases in a DNA sequence with the following composition: 30% adenine (A), 25% thymine (T), 20% cytosine (C), and 25% guanine (G)?

Answer 24:

Percentage of adenine (A) = 30%

Question 25:

How many hydrogen bond interactions are there between the base pairs guanine (G) and cytosine (C) in a DNA double helix?

Answer 25:

There are three hydrogen bond interactions between guanine (G) and cytosine (C) in a DNA double helix.

Question 26:

Calculate the molar concentration (in mol/L) of a DNA solution with an absorbance of 0.6 at 260 nm and a path length of 2 cm, given the molar absorptivity is 15,000 M^-1cm^-1.

Answer 26:

Molar concentration = Absorbance / (Molar absorptivity × Path length)

Molar concentration = 0.6 / (15,000 M^-1cm^-1 × 2 cm) ≈ 0.02 mol/L

Question 27:

Given a DNA strand with the sequence 5′-CGATCGTA-3′, what is the complementary RNA sequence?

Answer 27:

The complementary RNA sequence to 5′-CGATCGTA-3′ is 5′-GUAGCAUC-3′.

Question 28:

If a DNA double helix contains 40% thymine (T), what percentage of the bases will be adenine (A)?

Answer 28:

Since adenine (A) pairs with thymine (T) in DNA, if T is 40%, then A will also be 40%.

Question 29:

Calculate the melting temperature (Tm) of a DNA duplex with the sequence 5′-ATTAGCCG-3′, using the formula: Tm = 2°C × (A + T) + 4°C × (G + C), where A, T, G, and C are the percentages of the respective bases in the sequence.

Answer 29:

Given the sequence is 5′-ATTAGCCG-3′:

A = 25% (adenine), T = 25% (thymine), G = 25% (guanine), C = 25% (cytosine)

Tm = 2°C × (25% + 25%) + 4°C × (25% + 25%) = 100°C

Question 30:

What is the charge of a DNA strand with 20 cytosine (C) bases and 30 guanine (G) bases at pH 7?

Answer 30:

The charge of a DNA strand can be calculated using the formula: Charge = (# of positively charged groups) – (# of negatively charged groups)

In DNA, cytosine (C) and guanine (G) have an extra amino group, making them positively charged. At pH 7, both groups are fully protonated.

Number of positively charged groups = 20 (cytosine) + 30 (guanine) = 50

Number of negatively charged groups = 50 (phosphate backbone)

Charge = 50 – 50 = 0

So, the DNA strand has a net charge of 0 at pH 7.