Question

List the postulates of the Watson-Crick model of DNA secondary structure.

Short answer

Question: List the key postulates of the Watson-Crick model of DNA secondary structure. Answer: The key postulates of the Watson-Crick model of DNA secondary structure include: 1) the formation of specific base pairs adhering to complementarity rules (A-T and C-G), 2) antiparallel orientation of the two DNA strands (5' to 3' and 3' to 5'), 3) a double helix structure formed by the two polynucleotide chains, 4) a constant width maintained throughout the helix due to purine-pyrimidine base pairing, and 5) semi-conservative replication, where each parent strand serves as a template for synthesizing a new complementary strand.

Step-by-step solution

Watson and Crick's discovery

In 1953, James Watson and Francis Crick discovered the double helix structure of DNA, which has become the basis for modern understanding of DNA secondary structure. It is essential to know the components of DNA and how they pair together to form the double helix.

Base pairing and complementarity

Watson and Crick proposed that DNA consists of two polynucleotide chains, where each nucleotide contains a phosphate group, a sugar, and one of four nitrogenous bases (adenine, guanine, cytosine, and thymine). These nitrogenous bases form specific base pairs, adhering to the rules of complementarity: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). The two complementary strands bind together via hydrogen bonds between the base pairs.

Antiparallel strands and directionality

According to the Watson-Crick model, the two DNA strands run in opposite directions, termed as antiparallel. This means that one strand runs in the 5' to 3' direction, while the other strand runs in the 3' to 5' direction. Directionality is essential for processes like DNA replication and transcription.

Double helix

The two polynucleotide chains in DNA wrap around each other in a helical manner, forming a double helix structure. The sugar-phosphate backbone of each strand is on the outside, and the base pairs are on the inside, stacked upon each other.

Constant width of the helix

Due to the complementarity rule in base pairing, the pairing of purines (adenine and guanine) with pyrimidines (thymine and cytosine) maintains a constant width throughout the double helix. This is because of the purine and pyrimidine structure: purines are larger with a two-ring structure, while pyrimidines are smaller with a single-ring structure.

Semi-conservative replication

Watson and Crick's model of DNA suggested that during the replication process, the two parent strands separate, and each strand serves as a template for synthesizing a new complementary strand. This results in two identical double helices, with one parent strand and one newly synthesized strand in each - a process known as semi-conservative replication.