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DNA is defined as a genetic substance in the form of nucleotides that preserves genetic information about an individual. It is a right-handed double-helical structure made up of two polynucleotide chains coiled around each other. A nucleotide consists of a nitrogenous base, the pentose sugar deoxyribose, and phosphoric acid in DNA. In DNA, there are four nitrogenous bases. Adenine, Guanine, Cytosine, and Thymine are the four nucleotides that makeup DNA. Adenine forms hydrogen bonds with Thymine $\left(\mathrm{A}=\mathrm{T}\right)$, whereas Cytosine couples with Guanine data-custom-editor="chemistry" $\left(\mathrm{C}≡\mathrm{G}\right)$, according to Chargaff's rule.

According to the question,

Sea urchins have$32\%$ adenine.

According to Chargaff’s rule,

Adenine pairs with Thymine and Guanine pairs with Cytosine.

Percent Thymine in sea urchin DNA should be equal to Adenine$\%=32\%$

Percent Cytosine in sea urchin DNA should be equal to Guanine$\%=18\%$

Percent Guanine in sea urchin DNA should be equal to Cytosine$\%=18\%$