Answer: A - It is a double helix.
The double helix is the three-dimensional structure of DNA, which was first described by James Watson and Francis Crick in 1953. The double helix consists of two complementary strands of nucleotides that twist around each other in a right-handed spiral.
Each nucleotide is composed of a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. There are four different nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). The nitrogenous bases form the "rungs" of the DNA ladder, while the sugar and phosphate molecules make up the "backbone" of the ladder.
The two DNA strands are held together by hydrogen bonds between the nitrogenous bases. Specifically, A always pairs with T, and G always pairs with C. These base pairs are referred to as complementary base pairs.
The two DNA strands run in opposite directions, which is referred to as anti-parallel. One strand runs in the 5' to 3' direction (five prime to three prime), while the other runs in the 3' to 5' direction. The sugar and phosphate molecules in the backbone of each strand are joined by phosphodiester bonds.
The double helix has a uniform diameter of about 2 nanometers, and each complete turn of the helix is about 3.4 nanometers long. The two strands are twisted around each other in a right-handed manner, with one complete turn of the helix every ten base pairs.
The double helix structure of DNA is critical to its function as the genetic material of cells. The sequence of the nitrogenous bases in DNA determines the genetic code, which is used to direct the synthesis of proteins and other cellular components. The double helix structure also allows DNA to be replicated accurately during cell division, ensuring that each daughter cell receives a complete and identical set of genetic information.
