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

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1. The Watson-Crick model describes DNA as a double helix, where two strands of nucleotides are wound around each other.
2. Each strand is composed of a sugar-phosphate backbone with nitrogenous bases attached to the sugars.
3. The two strands are held together by hydrogen bonds between complementary bases: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
4. The strands are antiparallel, meaning they run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'.
5. The double helix has a uniform diameter because purines (A and G) pair with pyrimidines (T and C), ensuring consistent spacing between the strands.

Key Concepts

Double Helix Structure

The Watson-Crick model describes DNA as a double helix, resembling a twisted ladder. The two strands of the helix are composed of nucleotides, which include a sugar, a phosphate group, and a nitrogenous base. The helical structure allows for the compact storage of genetic information and provides stability through base pairing.

Base Pairing Rules

In the Watson-Crick model, specific nitrogenous bases pair together: adenine (A) with thymine (T) and cytosine (C) with guanine (G). This complementary base pairing is crucial for DNA replication and transcription, ensuring that genetic information is accurately copied and expressed. The hydrogen bonds between the bases hold the two strands together.

Antiparallel Orientation

The two strands of the DNA double helix run in opposite directions, a feature known as antiparallel orientation. One strand runs from the 5' to 3' direction, while the other runs from 3' to 5'. This orientation is essential for the mechanisms of DNA replication and transcription, as enzymes involved in these processes can only add nucleotides in a specific direction.