This question is designed to catch a) students who are not reading the question carefully, and b) students unsure of which nucleotides pair with which.

The correct answer is 29%, because cytosine pairs with guanine in a 1:1 ratio. If you answered 21%, then you likely thought the question was more complex than it was.

The correct answer is hydrogen bonding, and each nucleotide attracts its pairing mate because they have corresponding number of hydrogen bonds. Adenine is attracted to thymine to create two hydrogen bonds, and cytosine is attracted to guanine to form three hydrogen bonds. While phosphodiester bonds are very important in creating the strand of DNA, they are not the bond that keeps the two strands in the double helix structure.

When nucleotides bond together and form DNA strands, the first and last nucleotides in the strand have slightly different structures than the rest of the nucleotides between them. On one end of the strand, the nucleotide has an exposed hydroxyl group bound to the third carbon in the carbon ring: this end of the strand is thus called 3'. On the opposite end of the strand, the nucleotide has a phosphate group attached to the 5' carbon in the carbon ring, and is thus called the 5' end. These two groups are exposed because they are used in the bonding of nucleotides to one another to form the strand, but each strand ends with one nucleotide that only is bound on one side: thus, leaving either the hydroxyl or phosphate group exposed (depending on which end you are observing).

These terms are useful because they allow us to discuss the directionality of DNA-related events- if we didn't have terms for directionality the concept would be much more confusing. Example: "DNA polymerase synthesizes the new DNA strand in the 5'-3' direction." Without 3'/5' how would we determine which way the reaction occurs?

According to Chargaff's rule, DNA nucleotides pair in a 1:1 ratio. Therefore, if we know how much of the particular gene is made up of one nucleotide, we can extrapolate that known variable to find the other three unknown variables.

To do so, you must remember that adenine pairs with thymine, and cytosine pairs with guanine (A-T, C-G), and that since the ratio between each pair is 1:1 then a gene with 33% adenine must also have 33% thymine. Combine these numbers and subtract from 100: the number leftover is the % of total cytosine and guanine in the gene.

100% - 66% = 34%

Finally, since we know that 34% of the DNA is both C and G, and that the ratio between C-G is 1:1, C and G must both be 17%.

In DNA, guanine pairs with cytosine and adenine pairs with thymine. In RNA, which does not have thymine, adenine pairs with uracil. Thus, if a sample contains  guanine, it also contains  cytosine. Together, the two make up  of the  total. The remaining  is divided evenly between the paired adenine and thymine molecules, so the DNA sample contains  percent each of adenine and thymine.  is the correct answer.

DNA and RNA are both made of sugar-phosphate backbones. Ribose is the sugar found in RNA; deoxyribose is the sugar found in DNA. DNA also contains the nucleic acid bases adenine, guanine, cytosine, and thymine. Both DNA and RNA contain phosphate groups as part of the backbone.