Calculation of Total Number of Bases

To calculate the number of hydrogen bonds in a DNA molecule, we need to first calculate the total number of bases present in the molecule. We are given that the DNA molecule has 500 base pairs. Since each base pair consists of two bases, the total number of bases in the molecule can be calculated as follows:

Total number of bases = 500 base pairs x 2 bases per pair = 1000 bases

Calculation of Number of Guanosine Bases

Next, we need to calculate the number of Guanosine bases present in the molecule. We are given that 20% of the total bases are Guanosine. This can be calculated as follows:

Number of Guanosine bases = 20% x 1000 bases = 200 bases

Calculation of Number of Hydrogen Bonds

Finally, we can calculate the number of hydrogen bonds in the molecule using the following formula:

Number of hydrogen bonds = Total number of base pairs x Number of hydrogen bonds per base pair

In DNA, Adenine always pairs with Thymine, and Guanosine always pairs with Cytosine. Each base pair is held together by three hydrogen bonds between Adenine and Thymine, and two hydrogen bonds between Guanosine and Cytosine.

Since we know that 20% of the bases are Guanosine, we can calculate the number of base pairs that contain Guanosine as follows:

Number of base pairs with Guanosine = Number of Guanosine bases / 2 (since each base pair contains one Guanosine and one Cytosine)

Number of base pairs with Guanosine = 200 bases / 2 = 100 base pairs

Therefore, the number of base pairs that do not contain Guanosine is:

Number of base pairs without Guanosine = Total number of base pairs - Number of base pairs with Guanosine

Number of base pairs without Guanosine = 500 base pairs - 100 base pairs = 400 base pairs

Now, we can calculate the total number of hydrogen bonds in the molecule as follows:

Number of hydrogen bonds = (Number of base pairs with Guanosine x 2) + (Number of base pairs without Guanosine x 3)

Number of hydrogen bonds = (100 base pairs x 2 hydrogen bonds per pair) + (400 base pairs x 3 hydrogen bonds per pair)

Number of hydrogen bonds = 200 + 1200

Number of hydrogen bonds = 1400

Therefore, the correct answer is 1200, which is the total number of hydrogen bonds between all the base pairs in the DNA molecule that contains 500 base pairs and 20% of the total bases are Guanosine.