Describe the various characteristics of the Watson–Crick double-helix model for DNA.

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1. Start by explaining that the Watson–Crick model describes DNA as a double helix, which means it consists of two strands twisted around each other in a spiral shape.
2. Describe the backbone of each DNA strand, which is made up of alternating sugar (deoxyribose) and phosphate groups, connected by phosphodiester bonds.
3. Explain that the two strands run in opposite directions, meaning they are antiparallel; one strand runs 5' to 3' and the other runs 3' to 5'.
4. Discuss the base pairing rules: adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds, which stabilizes the double helix.
5. Mention that the bases are located on the inside of the helix, stacked like steps of a spiral staircase, while the sugar-phosphate backbones form the outside, and that the helix has a major and minor groove important for protein binding.

Key Concepts

Structure of the Double Helix

The Watson–Crick model describes DNA as two antiparallel strands twisted into a right-handed double helix. Each strand consists of a sugar-phosphate backbone with nitrogenous bases projecting inward, forming the helical shape essential for DNA's stability and function.

Base Pairing Rules

In the double helix, nitrogenous bases pair specifically: adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds. This complementary base pairing ensures accurate DNA replication and genetic information storage.

Antiparallel Orientation

The two DNA strands run in opposite directions, one 5' to 3' and the other 3' to 5'. This antiparallel arrangement is crucial for enzymatic processes like replication and transcription, allowing enzymes to read and synthesize DNA efficiently.