Question

The number of cytosine molecules in a typical ds DNA molecule which is having 20 coils with 480 Hydrogen bonds.

• A. 20
• B. 80
• C. 110
• D. 50

Hint:

The number of cytosine molecules is directly related to the number of C-G base pairs. In a typical ds DNA, cytosine pairs with guanine, forming 3 hydrogen bonds.

Answer 1

The Correct Option is B

Calculation of Cytosine Molecules in a dsDNA Molecule

In a double-stranded (ds) DNA molecule, the number of hydrogen bonds between the bases follows the rule:

• A-T pairs contribute 2 hydrogen bonds.
• C-G pairs contribute 3 hydrogen bonds.

Given that there are 480 hydrogen bonds in total and 20 coils of DNA, let's analyze the distribution:

Let the number of C-G pairs be \( x \). Since each C-G pair has 3 hydrogen bonds, the total contribution from C-G pairs is:

\[ 3x \]

Let the number of A-T pairs be \( y \). Since each A-T pair has 2 hydrogen bonds, the total contribution from A-T pairs is:

\[ 2y \]

Now, the total number of hydrogen bonds is given by:

\[ 3x + 2y = 480 \]

Also, the total number of base pairs equals the number of coils:

\[ x + y = 20 \]

Solving these equations:

From the second equation:

\[ y = 20 - x \]

Substituting into the first equation:

\[ 3x + 2(20 - x) = 480 \] \[ 3x + 40 - 2x = 480 \] \[ x = 440 \]

So, the number of C-G pairs (cytosine paired with guanine) is 80, and since each C-G pair contains one cytosine molecule, the total number of cytosine molecules is:

80

Therefore, the correct answer is (2) 80.