Deoxyribonucleic acid is made of nucleotides which are combined with covalent bonds and form a chain. Nucleotides consist of deoxyribose, the phosphate group, and a nitrogenous base (adenine, cytosine, thymine, and guanine).

Erwin Chargaff first noticed that the percentage of adenine and thymine, as well as guanine and cytosine, is almost the same in every DNA molecule. The first picture of this molecule was made by Rosalind Franklin. She isolated DNA molecules, which was then stretched until most of them were parallel. She successfully made an X-ray image, that has revealed the following:
1. The nitrogenous bases are near the center of the DNA molecule
2. DNA molecule is composed out of two strands
3. DNA molecule has the shape of a helix
Watson and Crick made the first model of the DNA molecule based on that very picture. In DNA, the strands are antiparallel, they are composed of nucleotides that are connected with covalent bonds. Two strands are connected with weak, hydrogen bonds. According to the Chargaff’s rule, adenine always binds with thymine of the other strand, while guanine binds with cytosine that is a part of the antiparallel strand.

Watson and Crick made the first model of the DNA molecule based on its X-ray picture, made by Rosalind Franklin. The model explained that the DNA molecule has the shape of helix. It is composed out of two antiparallel strands, interconnected with weak, hydrogen bonds. Each of these strands is composed of nucleotides, while nitrogenous bases are near the center of the DNA molecule. Nucleotides that make up one strand are interconnected with covalent bonds. According to the Chargaff’s rule, adenine always binds with thymine of the other strand, while guanine binds with cytosine that is a part of the antiparallel strand.

The DNA molecule has the shape of helix. It is composed out of two antiparallel strands, interconnected with hydrogen bonds, while nucleotides that compose one strand are interconnected with covalent bonds. A hydrogen bond is weak. That is an important characteristic which facilitates cell division, regarding the duplication of DNA molecule.

Erwin Chargaff first noticed that the percentage of adenine and thymine, as well as guanine and cytosine, is almost the same in every DNA molecule. This is known as the Chargaff’s rule.
If A = 25%, we can conclude that T = 25%.
The remaining 50% (100-25-25) of the DNA is composed out of guanine and cytosine (G+C=50%).
Since G = C, therefore G=25% as well as C=25%.

Rosalind Franklin made an X-ray image of the DNA molecule, that has revealed the following:
1. The nitrogenous bases are near the center of the DNA molecule
2. DNA molecule is composed out of two strands
3. DNA molecule has the shape of a helix
Watson and Crick made the first model of the DNA molecule based on that very picture. They concluded that the strands are antiparallel and that they are composed of nucleotides. Nucleotides of one strand are interconnected with covalent bonds, while two strands are connected with weak, hydrogen bonds. According to the Chargaff’s rule, adenine always binds with thymine of the other strand, while guanine binds with cytosine that is a part of the antiparallel strand.