**Question:** The helical structure model of two polynucleotide chains of DNA was proposed by

A. James Watson and Francis Crick
B. Erwin Chargaff
C. Rosalind Franklin
D. Linus Pauling

**Correct Answer:** A. James Watson and Francis Crick

**Core Concept:** The DNA (Deoxyribonucleic Acid) molecule is a crucial component of hereditary information storage. Its structure plays a vital role in understanding DNA replication, transcription, and translation. DNA is a double-stranded molecule consisting of two polynucleotide chains.

**Why the Correct Answer is Right:** James Watson and Francis Crick are well-known scientists who proposed the famous double helix structure of DNA in 1953. Their model demonstrates two polynucleotide chains forming a double helix, with each base pairing to its complementary base. This model accurately represents the molecular structure of DNA and has been widely accepted in the scientific community.

**Why Each Wrong Option is Incorrect:**

B. Erwin Chargaff (1904-1988) is a chemist who made significant contributions to the understanding of DNA, particularly regarding base pairing. However, he is not credited with proposing the DNA structure but with the discovery of base pair stoichiometry, which states that the amount of adenine (A) is equal to thymine (T), cytosine (C), and guanine (G) in DNA.

C. Rosalind Franklin (1920-1958) was a British crystallographer who played a crucial role in determining the structure of DNA through X-ray crystallography. However, the structure was independently discovered by Watson and Crick.

D. Linus Pauling (1901-1994) is a renowned chemist, not a biologist. While he contributed to the understanding of molecular structure, specifically the concept of covalent bonds, he is not associated with the discovery of DNA structure.

**Clinical Pearl:** The accurate understanding of DNA structure, including the double helix model proposed by Watson and Crick, is essential for medical professionals to comprehend DNA replication, transcription, and gene expression processes.

**Explanation of the Core Concept:** The structure of DNA, as proposed by Watson and Crick, consists of two polynucleotide chains that run anti-parallel and are held together by hydrogen bonds between the purine (A and G) and pyrimidine (T and C) bases. The model helps explain the genetic information storage, replication, and transcription processes in cells.