DNA Double Helix: Videos & Practice Problems

The structure of DNA, famously described by researchers Franklin, Watson, and Crick, is characterized as a double helix composed of two antiparallel strands of nucleotides. The term "antiparallel" indicates that while the strands run parallel to each other, they do so in opposite directions. This double helix can be visualized as a twisted ladder, where the phosphate-sugar backbone forms the sides and the nitrogenous bases are situated in the center.

A nucleotide, the fundamental building block of DNA, consists of three components: a phosphate group, a pentose sugar (deoxyribose in DNA), and a nitrogenous base. The phosphate group is attached to the 5' carbon of the sugar, which is in turn connected to the 4' carbon of the pentose ring. The primary structure of DNA is established by linking nucleotides together through phosphodiester bonds, creating a strand that runs from the 5' end to the 3' end.

For the two strands of DNA to be complementary, the 3' end of one strand aligns with the 5' end of the other strand. This complementary pairing is facilitated by hydrogen bonds between the nitrogenous bases. The sugar-phosphate backbones, represented as blue parts in diagrams, support the structure, while the bases form the rungs of the twisted ladder.

To visualize the transition from antiparallel strands to the double helix, imagine rotating the top part of the ladder to the left and the bottom part to the right. This twisting motion results in the characteristic shape of the DNA double helix.

In summary, understanding DNA involves grasping the concepts of nucleotides, phosphodiester linkages, primary structure, and the complementary nature of base pairing, all of which contribute to the formation of the twisted ladder structure that defines the DNA double helix.

In DNA, the structure consists of two strands that run in opposite directions, a concept known as antiparallel orientation. Each strand has a 5' (five prime) end and a 3' (three prime) end, which are crucial for understanding how the strands interact. The 5' end of one strand aligns with the 3' end of the complementary strand, allowing for proper base pairing.

Base pairing in DNA follows specific rules: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This means that if a DNA strand has a sequence of bases, the complementary strand can be predicted by applying these pairings. For example, if the original strand has a base sequence of C, G, A, T, C, T, A, the complementary strand would be G, C, T, A, G, A, T, respectively.

Additionally, it is important to note the number of hydrogen bonds formed between these base pairs. A and T form two hydrogen bonds, while G and C form three hydrogen bonds, which contributes to the stability of the DNA structure. Understanding these base pairings and the antiparallel nature of DNA is essential for grasping the fundamental principles of genetics and molecular biology.