The double-helix structure of DNA was discovered by James Watson and Francis Crick, as well as Rosalind Franklin. They published an article titled "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" that described the three-dimensional structure of DNA.

Linus Pauling was the first scientist to propose a helical structure for DNA; however, he proposed that DNA was a triple helix. His work regarding DNA structure heavily influenced Watson and Crick who won the Nobel Prize for their model of DNA.

The double-helix is the three dimensional structure of a DNA molecule. This structure arises due to the interaction (hydrogen bonding of base pairs) between the two strands of a DNA molecule. A double-helix can occur in any double-stranded molecule; therefore, a double-stranded RNA molecule can also form a double-helix if there is proper base pairing between the strands.

The base pairing in the double-helix involves hydrogen bonds, a type of noncovalent bond or intermolecular force. Since it is a noncovalent bond, hydrogen bonds between bases can be broken down by adding energy in the form of heat. Recall that the DNA backbone contains a series of pentose sugars that have a phosphate group attached to their 5' carbon. These pentose sugars in the DNA backbone are held together by covalent bonds called phosphodiester bonds; therefore, the DNA backbone is held together by covalent bonds.

One of the characteristics of double-helix is the presence of the major groove and the minor groove. The major groove is the region of the double-helix where the distance between the two DNA strands is largest. The minor groove, on the other hand, is the region of double-helix where the distance between the two strands is smallest.

DNA is a double-stranded helix. The two sugar-phosphate backbones are connected by hydrogen bonds that for between nitrogenous bases. The DNA bases are adenine (A), thymine (T), cytosine (C), and guanine (G). In DNA, adenine always pairs with thyine and cytosine always pairs with guanine. These pairings occur because of geometry of the base,s allow hydrogen bonds to form only between the "right" pairs. Adenine and thymine will form two hydrogen bonds, whereas cytosine and guanine will form three hydrogen bonds. This hydrogen bonding brings stability to the DNA. 

Nucleotides that bind together will always be present in equal amounts in a sample of DNA. This principle is known as Chargaff's rule. Essentially, the percentage of adenine will be equal to the percentage of thymine, while the percentage of guanine will be equal to the percentage of cytosine.

Together, all of the percentages must add up to 100%.

We know that the sample is 17% adenine, which means it is also 17% thymine.

We can use this equation to solve for the percentage of guanine.

Guanine and cytosine pair together, which means they will be present in equal amounts. To find the percentage of guanine, we can simply divide by 2.

You can check your answer if needed:

The four nitrogenous bases of DNA are thymine, adenine, guanine, and cytosine. Guanine and cytosine are bound together by three hydrogen bonds; whereas, adenine and thymine are bound together by two hydrogen bonds. This is known as complementary base pairing. In RNA, the nucleotide thymine is replaced by the nucleotide uracil.

DNA (Deoxyribonucleic acid) is a chain of two strands of nucleotide that are linked to one another through hydrogen bonding and arranged into a structure known as a double-helix. Nucleotides are molecular components of DNA that are composed of a deoxyribose sugar, a nitrogenous base, and a phosphate group. The backbone of DNA consists of phosphodiester bonds made up of interchanging phosphate groups and deoxyribose sugars. The four nitrogenous bases of DNA are thymine, adenine, guanine, and cytosine. Guanine and cytosine are bonded together by three hydrogen bonds; whereas, adenine and thymine are bonded together by two hydrogen bonds. This is known as complementary base pairing. One strand of the DNA double helix will be in a 5’  3’ direction in relation to the –OH group on the deoxyribose sugar. The other strand is oriented in the 3’  5’ direction.

DNA is a chain of two strands of nucleotides that are linked to one another through hydrogen bonds and arranged into a structure known as a double-helix. Nucleotides are molecular components of DNA that are composed of a deoxyribose sugar, a nitrogenous base, and a phosphate group. The backbone of DNA consists of phosphodiester bonds made up of interchanging phosphate groups and deoxyribose sugars. The four nitrogenous bases of DNA are thymine, adenine, guanine, and cytosine. Guanine and cytosine are bound together by three hydrogen bonds; whereas, adenine and thymine are bound together by two hydrogen bonds. This is known as complementary base pairing. One strand of the DNA double helix will be in a 5’  3’ direction in relation to the  group on the deoxyribose sugar. The other strand is oriented in the 3’  5’ direction. This is why DNA is considered antiparallel.

Nucleotides that bind together will always be present in equal amounts in a sample of DNA. This principle is known as Chargaff's rule. Essentially, the percentage of adenine will be equal to the percentage of thymine, while the percentage of guanine will be equal to the percentage of cytosine.

Together, all of the percentages must add up to 100%.

We know that the sample is 17% adenine, which means it is also 17% thymine.

We can use this equation to solve for the percentage of guanine.

Guanine and cytosine pair together, which means they will be present in equal amounts. To find the percentage of guanine, we can simply divide by 2.

You can check your answer if needed:

When determining the complementary strand, remember that it will be written in the opposite direction of the template strand. This means that the new strand's 5' end will begin at the 3' end of the template strand. The complementary strand will also be composed of the nucleotides that complete the base pairs found in DNA (A-T and C-G).

Template: 5'-GCCTCATGA-3'

Answer:     5'-TCATGAGGC-3'

To see these pairs match up, the 3' end of the answer must align with the 5' end of the template.

Template:       5'-GCCTCATGA-3'

Answer (3'-5'): 3'-CGGAGTACT-5'

There are two classes of bases in DNA and RNA: purines and pyrimidines. The difference between these classes is the structure of the base. Purines have two rings in their structure, while pyrimidines have only one. Purines will pair with pyrimidines.

The purines are adenine and guanine, and the pyrimidines are thymine, cytosine, and uracil. You can remember that the bases that contain a "y" are pyrimidines (thymine and cytosine).