Question

In sea urchin DNA, which is double-stranded, 17% of the bases were shown to be cytosine. The percentages of the other three bases expected to be present in this DNA are
(a)G-17%, A-33%, T-33%
(b)G-8.5%, A-50%, T-24%
(c)G-34%, A-24.5%, T-24.5%
(d)T-17%, A-16.5%, T-32.5%

Answer 1

Hint: Chargaff's rules state that the stoichiometric ratio (base pair rule) of pyrimidine and purine bases should be 1:1 for DNA from any cell of any organism, and, more precisely, the quantity of guanine should be equal to the quantity of cytosine and thymine should be equal to the quantity of adenine.

Complete answer:
Rule by Chargaff:
-First rule
The first rule states that there is a percentage base-pair equality for a double-stranded DNA molecule worldwide: percent A is equal to percent T and percent G is equal to percent C. In the DNA double helix model, the robust validation of the law constitutes the basis of Watson-Crick pairs.
-Second rule
For each of the two DNA strands, the second rule holds that both percent A is equal to percent T and percent G is equal to percent C. In a single DNA strand, this describes only a global function of the base composition.
Both cellular DNAs, irrespective of the organisms, have a number of adenosine residues equal to the number of thymidine residues according to Chargaff's law, which means that A is equal to T; and the number of guanosine residues is equal to the number of cytidine residues; G is equal to C. There is 17 percent cytosine in the given question, meaning the percentage of guanine is also 17 percent.
Therefore,
$(A+T)=(G+C)$
$\Longrightarrow \frac { (A+T) }{ (G+C) } =1$
We know that,
$(A+T+G+C)=100$ percent
$A+T+17+17=100$
$A+T=100-34$
$A+T=66$
As $A=T$,
$2A=66$
$A=33$
And $T=33$
Thus, if cytosine is 17%, G is 17%, A is 33%, and T is 33%
So, the correct answer is, ‘G-17%, A-33%, T-33%’.

Note: Typically, spiny, globular species are sea urchins, echinoderms in the class Echinoidea. Around 950 species live on the seabed, inhabiting from the intertidal to 5,000 meters (16,000 ft; 2,700 fathoms) in all oceans and depth zones. Their experiments (hard shells) are round and spiny, usually 1 to 4 inches (3 to 10 cm) across. Sea urchins move slowly, creeping with their feet in the tube, and sometimes with their spines pulling themselves. They feed mostly on algae, but slow-moving or sessile animals are also eaten.