Question

Read the passage carefully and answer the questions that follow:

Strengthening the Foundation: Chargaff Formulates His "Rules" Many people believe that James Watson and Francis Crick discovered DNA in the 1950s. In reality, this is not the case. Rather, DNA was first identified in the late 1860s by Swiss chemist Friedrich Miescher. Then, in the decades following Miescher's discovery, other scientists- notably, Phoebus Levene and Erwin Chargaff- carried out a series of research efforts that revealed additional details about the DNA molecule, including its primary chemical components and the ways in which they joined with one another. Without the scientific foundation provided by these pioneers, Watson and Crick may never have reached their groundbreaking conclusion of 1953: that the DNA molecule exists in the form of a three-dimensional double helix. Chargaff, an Austrian biochemist, as his first step in this DNA research, set out to see whether there were any differences in DNA among different species. After developing a new paper chromatography method for separating and identifying small amounts of organic material, Chargaff reached two major conclusions: (i) the nucleotide composition of DNA varies among species. (ii) Almost all DNA, no matter what organism or tissue type it comes from maintains certain properties, even as its composition varies. In particular, the amount of adenine (A) is similar to the amount of thymine (T), and the amount of guanine (G) approximates the amount of cytosine (C). In other words, the total amount of purines (A + G) and the total amount of pyrimidines (C + T) are usually nearly equal. This conclusion is now known as "Chargaff's rule." Chargaff’s rule is not obeyed in some viruses. These either have single-stranded DNA or RNA as their genetic material.

Answer the following questions:

1. A segment of DNA has 100 adenine and 150 cytosine bases. What is the total number of nucleotides present in this segment of DNA?
2. A sample of hair and blood was found at two sites. Scientists claim that the samples belong to same species. How did the scientists arrive at this conclusion?
3. The sample of a virus was tested and it was found to contain 20% adenine, 20% thymine, 20% guanine and the rest cytosine. Is the genetic material of this virus (a) DNA-double helix (b) DNA-single helix (c) RNA? What do you infer from this data?
   OR
   How can Chargaff’s rule be used to infer that the genetic material of an organism is double-helix or single-helix?

Answer 1

a. A = 100 so T = 100

C = 150 so G = 150

Total nucleotides = 100 + 100 + 150 + 150 = 500

b. They studied the nucleotide composition of DNA. It was the same so they concluded that the samples belong to the same species.

c. A = T = 20%

But G is not equal to C so double helix is ruled out.

The base pairs are ATGC and not AUGC so it is not RNA.

The virus is a single-helix DNA virus.

OR

According to the Charagraff rule, all double helix DNA will have the same amount of A and T as well as C will be the same amount as G. If this is not the case then the helix is single-stranded.