How was it determined that the structure of DNA is a double helix with the two strands held together by hydrogen bonds formed between complementary nitrogenous bases?

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Understand that the determination of DNA's double helix structure involved multiple experimental approaches, including X-ray crystallography, chemical analysis, and base pairing rules.

Recognize that Rosalind Franklin's X-ray diffraction images of DNA fibers provided critical evidence, showing a helical pattern and specific dimensions consistent with a double helix.

Learn that Chargaff's rules established that the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C), suggesting specific base pairing.

Combine the X-ray data with Chargaff's findings to hypothesize that DNA consists of two strands forming a helix, with complementary bases pairing via hydrogen bonds (A with T, G with C).

Understand that Watson and Crick built a physical model incorporating these data, demonstrating that hydrogen bonds between complementary bases stabilize the double helix structure.

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X-ray Crystallography

X-ray crystallography is a technique used to determine the three-dimensional structure of molecules by analyzing the pattern of X-ray diffraction through crystallized samples. Rosalind Franklin's X-ray images of DNA provided critical evidence of a helical structure, showing a consistent repeating pattern indicative of a double helix.

Complementary Base Pairing

Complementary base pairing refers to the specific hydrogen bonding between nitrogenous bases in DNA: adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three. This specificity explains how the two DNA strands align and are held together, ensuring accurate replication and stable structure.

Watson and Crick’s Model of DNA

Watson and Crick proposed the double helix model of DNA based on experimental data, including Chargaff’s rules and Franklin’s X-ray images. Their model showed two antiparallel strands twisted into a helix, with bases paired in the interior via hydrogen bonds, explaining DNA’s structure and function in heredity.