Acc. to chargaff rule A=T and G=C so answer is 480 i.e. 120=A=T and 120=G=C

The segment of DNA mentioned in the question contains 120 adenine (A) bases and 120 cytosine (C) bases. We need to determine the total number of nucleotides present in the segment.

A DNA molecule is made up of nucleotides, which are composed of three components: a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. There are four types of nitrogenous bases in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T).

To find the total number of nucleotides in the segment, we need to add up the number of each type of base. In this case, we are given that there are 120 adenine (A) bases and 120 cytosine (C) bases. Since A and C are two of the four nitrogenous bases, the remaining two bases (G and T) must also be present in equal numbers.

Let's calculate the number of guanine (G) and thymine (T) bases:

- The segment contains 120 adenine (A) bases.
- The segment contains 120 cytosine (C) bases.

Since the two remaining bases (G and T) must be present in equal numbers, we can divide the total number of adenine and cytosine bases by 2 to find the number of guanine and thymine bases:

- The number of guanine (G) bases = (total adenine bases) / 2 = 120 / 2 = 60
- The number of thymine (T) bases = (total cytosine bases) / 2 = 120 / 2 = 60

Now, we can find the total number of nucleotides in the segment by adding up the number of each type of base:

- Total number of nucleotides = number of adenine bases + number of cytosine bases + number of guanine bases + number of thymine bases
- Total number of nucleotides = 120 + 120 + 60 + 60 = 360

However, a DNA molecule is composed of two strands, which are complementary to each other. Each strand has a sugar-phosphate backbone with nitrogenous bases attached. The two strands are held together by hydrogen bonds between the bases. Therefore, the total number of nucleotides in the DNA segment should be doubled:

- Total number of nucleotides = 360 x 2 = 720

Therefore, the correct answer is option 'A', which states that the total number of nucleotides present in the segment is 480 base pairs.