Lecture 4

Lecture Overview

• Focus on the major experiments leading to the understanding of DNA structures.

• Emphasizes understanding key components rather than memorizing names and dates.

• Each concept presented as a "puzzle piece" that contributes to the overall story of DNA.

Puzzle Pieces of DNA

Nucleotide Components

• Identified by Phoebus Levene:

  • Sugar: deoxyribose sugar.

  • Phosphate Backbone

  • Nitrogenous Base: responsible for different nucleotides (A, T, C, G).

• Differentiated between DNA (deoxyribose) and RNA (ribose).

• Tetranucleotide Hypothesis:

  • Proposed that all DNA consists of equal amounts of A, T, C, G.

DNA as Hereditary Material

• Research by Frederick Griffith and Avery-McLeod-McCarty, Hershey-Chase:

  • Experiments indicated that DNA is the molecule of heredity, overcoming the belief that proteins were the main hereditary component.

X-ray Diffraction Insights

• Developed by William Asbury:

  • X-ray diffraction provided a way to visualize the structure of DNA.

  • Established regular repeating unit in DNA, approximately every 2.7 nanometers.

  • Illustrates how the bases are stacked like pancakes (0.34 nanometers apart).

Chargaff's Rules

• Erwin Chargaff's Discoveries: 1950s

  • Confirmed that:

    • A = T and G = C ratios in DNA.

  • Showed diversity in base percentages across different organisms, indicating molecular diversity.

The Race for DNA Structure: The 1950s

Initial Models

• Linus Pauling proposed a triple helical model for DNA based on protein helical structures but lacked support.

Exploration of X-ray Images

• Maurice Wilkins and Raymond Gosling worked with Rosalind Franklin on X-ray diffraction:

  • Noted the importance of sample condition (dry vs. wet) on image clarity and molecular understanding.

  • Franklin identified the water affinity in DNA and determined phosphates were on the outside of the helical structure, preventing water absorption inside.

Watson and Crick's Contributions

• James Watson and Francis Crick constructed DNA models based on available information and their own lab work.

  • B 51 Image: A pivotal X-ray diffraction image produced by Franklin and Gosling showing helical structure and base positioning.

  • Watson was exposed to Franklin's work during a talk but did not take notes, later seeking out Franklin's data.

Final Structuring of DNA

• Watson and Crick utilized new structural insights gathered from Franklin's work and others to propose the correct double helix model:

  • Recognized base pairing through hydrogen bonds between A-T and G-C.

  • Determined that the helix contained 10 base pairs per turn and consistent diameter measurements.

Recognition and Legacy

• Watson and Crick published their findings in 1953, receiving the Nobel Prize in 1962 for the structure of DNA.

• Notably, Rosalind Franklin was not included in the Nobel Prize, despite her substantial contributions to the discovery.

• Acknowledges the historical inequalities in recognition among scientists, particularly females in science during that time period.