A nucleotide consists of three different sub-units. A section of double-stranded DNA consists of 600 of these sub-units in total. Of these sub-units, 28 are the base adenine. What percentage of the sub-units of this section of DNA is guanine? (Assume no mutations.)

Answer:

21) C

The piece of DNA has 600 sub-units and 28 are adenines. Therefore, nucleotides containing adenine
28
3 subunits make up one nucleotide. Nucleotides containing adenine will have a total of = 28 x 384 sub-units
Remaining sub-units 600 – 84 = 516
Adenine binds with thymine, so there will be equal amounts of both. There are 28 Adenine nucleotides, so there will be 28 Thymine nucleotides. Therefore, nucleotides containing thymine = 28. Nucleotides containing thymine will have a total of 28 x 3 = 84 sub-units Remaining sub-units 516-84 = 432
There are an equal number of nucleotides containing C and G. Each nucleotide contains 3 sub-units. Total number of subunits in nucleotides containing C and G = 3C + 3G Since CG, we get the following: 3C + 3G 3G + 3G = 6G
432 = 6G
G = 72
Proportion 72/600 = 0.12 = 12%