Chapter 9 Section 2 The Structure of DNA

A Winding Staircase James Watson and Francis Crick determined that a DNA molecule is a double helix – two strands twisted around each other, like a spiral staircase.

A winding staircase Nucleotides are the subunits that make up DNA. Each nucleotide is made of three parts: 5 Carbon sugar molecule Phosphate group Nitrogen containing base

Nucleotide Draw This Nucleotide

Nucleotide The five carbon sugar in DNA nucleotides is called deoxyribose. The nitrogen base in a nucleotide can be either a bulky, double-ring purine, or a smaller, single-ring pyrimidine.

Chargaff’s Observations In 1949, Erwin Chargaff observed that for each organism he studied, the amount of adenine always equaled the amount of thymine. A = T

Chargaff’s Observations Likewise, the amount of guanine always equaled the amount of cytosine. G=C

Chargaff’s Observations However, the amount of adenine + thymine and of guanine + cytosine varied between different organisms. C=42% G=42% C=51% G=51% A=58% T=58% A=49% T=49%

Wilkins and Franklin’s Photographs By analyzing the complex patterns on X-ray diffraction photo, scientists can determine the structure of the molecule. In 1952, Maurice Wilkins and Rosalind Franklin developed high quality x-ray diffraction photographs of strands of DNA

Wilkins and Franklin’s Photographs These photographs suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of nucleotides.

Watson and Crick’s experiment Franklin’s image was such a key clue for Watson and Crick that it only took them a few weeks to figure out the structure of DNA after they saw it.

Watson and Crick’s experiment In 1953, Watson and Crick built a model of DNA with the configuration of a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axis.

Watson and Crick’s experiment The double helical model of DNA takes into account Chargoff’s observations and the patterns on Franklin’s X- ray diffraction. In 1962, Watson, Crick and Wilkins received the Nobel Peace Prize – Rosalind Franklin did not share in the award because she died in 1958.

Pairing between Bases An adenine on one strand always pairs with a thymine on the opposite strand, and a cytosine on one strand always pairs with a guanine on the opposite strand.

Pairing between Bases These base pairing rules are supported by Chargaff’s observations. The strictness of base pairing results in two strands that contain complimentary base pairs.