Lecture 33

Introduction

  • Discussion about attendance on a Friday; the speaker expresses appreciation for the large crowd attending.

  • Humorous commentary on the dynamic relationship between invention and human experience, alluding to how creativity is often driven by necessity and innovation.

Recent Scientific News

Fruit Fly Study:

  • Scientists have discovered that they can reverse brain aging in fruit flies by preventing the buildup of filamentous actin (F-actin), a protein that plays a crucial role in the cytoskeleton of cells.

  • F-actin is essential for maintaining cell shape, organelle trafficking, and cytokinesis, which is the process where a cell divides its cytoplasm to form two daughter cells.

  • Research indicates that the accumulation of F-actin is linked to aging and impairs the cellular process known as autophagy, which is vital for removing waste within cells. This build-up contributes to cognitive decline observed in aging fruit flies, which mirrors similar patterns found in humans.

  • The study utilized RNA interference (RNAi) which effectively reduced the expression of genes associated with F-actin, resulting in a 30% increase in lifespan along with noticeable improvements in cognitive function.

  • This research highlights a growing interest in the scientific community concerning methods to extend both life expectancy and mental capacity.

Review of DNA

Importance of DNA in Biology:

  • DNA's role as the hereditary material was underscored in Fred Griffith's pivotal experiment with the bacterium streptococcus pneumonia in mice, demonstrating how genetic transformation can occur when non-virulent strains take up genetic material to become virulent.

  • This experiment was fundamental in supporting the hypothesis that DNA serves as a hereditary molecule crucial for passing traits from one generation to the next.

Types of Enzymatic Treatments in Griffith's Experiment:

  • Hypothetical Treatments of Proteases, RNases, and DNases were analyzed to investigate their effects on genetic transformation outcomes.

    • Only DNase proved effective in preventing transformation by digesting the DNA, thereby reinforcing the concept that DNA is the molecule of heredity.

Historical Context of DNA Identification:

Avery, McLeod, and McCarty's Contribution (1944):

  • These scientists conducted follow-up experiments that built on Griffith's findings, providing robust evidence confirming DNA as the transformative molecule.

  • Their work laid the groundwork for understanding the molecular basis of genetics.

Calvin Bridges’ Work:

  • He made significant strides in predicting visible traits such as eye color through karyotyping in fruit flies, emphasizing the critical role of DNA as a hereditary carrier

Chargaff's Rules:

Erwin Chargaff (1950s):

  • Discovered fundamental ratios in DNA composition, finding that the amount of Adenine (A) is approximately equal to Thymine (T) and the amount of Cytosine (C) is approximately equal to Guanine (G). These base pairing rules were pivotal in elucidating the structure and function of DNA.

Hershey-Chase Experiment (1952):

  • Conducted to ascertain whether protein or DNA was the carrier of genetic information in bacteriophages, particularly the T2 phage.

  • Utilized isotopes of sulfur to mark proteins and phosphorus to label DNA, leading to definitive results that demonstrated DNA’s entry into E. coli, confirming it as the hereditary material and dismissing proteins as the genetic transmitter.

Key Figures in DNA Structure Discovery:

Rosalind Franklin:

  • An accomplished X-ray crystallographer, her meticulous work was critical in understanding DNA structure.

  • She conducted pioneering studies on crystallized DNA and generated essential data illustrating DNA's helical structure (Illustrated as Picture 51).

Watson and Crick:

  • They built upon Franklin’s crucial data to develop models of DNA's double helix structure, despite not generating the original data themselves.

  • Their modeling work was essential yet raised ethical concerns regarding the proper acknowledgment of Franklin and Wilkins’ contributions.

Nobel Prize Controversy (1962):

  • The awarding of the Nobel Prize to Watson, Crick, and Wilkins while overlooking Franklin sparked widespread discussions about equity and acknowledgment in scientific achievements, highlighting the need for ethical practices in research.

Molecular Structure of DNA:

Components of DNA:

  • DNA is composed of nucleotides, which consist of three components: a sugar (deoxyribose), a phosphate group, and nitrogenous bases.

  • The four nitrogenous bases categorize nucleotides into two classifications: purines (Adenine (A), Guanine (G)) and pyrimidines (Cytosine (C), Thymine (T), and Uracil (U) in RNA).

  • In the context of DNA, Thymine is present, while Uracil is found only in RNA.

Phosphodiester Bonds:

  • These strong covalent bonds form the backbone of DNA, allowing nucleotides to connect and establish a stable structural framework for genetic information storage.

Base Pairing and DNA Structure:

  • DNA strands exhibit antiparallel orientation, with hydrogen bonds between bases (A-T bonded by two hydrogen bonds and C-G bonded by three hydrogen bonds) contributing to specificity and structural stability.

  • The overall structure resembles a twisted ladder, with the bases forming the rungs and the sugar-phosphate backbone forming the sides.

DNA Replication and Energy Sources:

Repair Mechanisms in DNA:

  • The weak hydrogen bonds facilitate partial unzipping of DNA, which is essential for DNA replication, allowing enzymes to access and replicate the genetic material.


Nucleotide Activation:

  • Nucleotides involved in the process are triphosphorylated, supplying energy for cleavage of phosphate bonds during the complex process of DNA synthesis, highlighting the intricate nature of cellular replication.

Quiz Questions from the Lecture:

  • What was the result of Griffith's experiments in demonstrating genetic transformation?

  • What conclusion did the Hershey-Chase experiment reach regarding DNA versus protein as the genetic material?

  • Discuss the significance of Rosalind Franklin's contributions to the discovery of DNA structure utilizing X-ray crystallography.


Study Questions from Lecture 33 (11/8/2024)

1. In 1944, how did Avery, McCleod, and McCarty show that the famous “transforming substance” from Fred Griffith’s experiment was DNA?

  • Key Finding: Demonstrated that the “transforming substance” in Griffith's experiment was DNA by isolating and purifying the DNA from pneumonia bacteria and showing it could cause transformation in non-virulent strains, confirming DNA as the hereditary material.

2. What did Erwin Chargaff show in the late 1940s (i.e. what were his “rules”)? How did these rules provide a major clue to Watson and Crick?

  • Finding: Chargaff discovered that in DNA, the amount of Adenine (A) equals Thymine (T) and the amount of Cytosine (C) equals Guanine (G). This finding indicated base pairing rules that suggested A pairs with T and C pairs with G, providing a major clue to Watson and Crick’s model of DNA structure.

3. Hershey Chase Experiment

  • b. What 2 organisms did they use in this experiment?

    • Organisms Used: E. coli (bacterium) and T2 phage (a type of bacteriophage).

  • c. What did they know about the bacteriophage's structure?

    • Molecules: Protein and DNA were the two types of molecules known to be present in the phages.

  • d. What did they use the radioisotopes 35S and 32P for?

    • Radioisotopes Usage: 35S was used to label proteins, and 32P was used to label DNA.

  • e. Did they put the two isotopes in the same flask or in separate flasks?

    • Flask Separation: They used separate flasks for each isotope, one for 35S (protein) and one for 32P (DNA).

  • f. In which flask was the supernatant radioactive?

    • Supernatant Radioactivity: The supernatant was radioactive for the flask with the 32P (DNA) labeled bacteriophages.

  • g. In which tube was the pellet radioactive?

    • Pellet Radioactivity: The pellet was radioactive in the flask with the 32P labeled bacteriophages.

  • h. Which plate did the plaques show radioactivity?

    • Radioactivity and Plaques: The plaques showed radioactivity on the plate corresponding to those infected with the 32P labeled phages, corroborating that DNA is the genetic material.

  • i. How did this resolve the question of whether the genetic material in bacteriophage is DNA and not protein?

    • Resolution of Genetic Material: This result confirmed that the genetic material in bacteriophages is DNA, not protein, as only the DNA entered the bacterial cells and caused infection.

  • j. What if, when they plated infected cells, both plates showed radioactivity?

    • Hypothetical Different Outcome: If both plates had shown radioactivity, it might have suggested that both DNA and protein were involved in the process of infection or transformation, complicating interpretations significantly.

4. Franklin, Wilkins, Watson, and Crick

  • b. What technique was Rosalind Franklin a leading expert at using?

    • Technique: Rosalind Franklin was a leading expert in X-ray crystallography.

  • c. At a simplistic level, how does this technique work?

    • Technique Function: This technique works by directing X-rays at a crystallized sample, which scatters the rays to produce patterns, revealing structural details about the molecules.

  • d. Where was she working (what college)?

    • Work Location: She worked at King’s College London.

  • e. Who was Maurice Wilkins?

    • Maurice Wilkins: Wilkins was a physicist and colleague of Franklin’s in the same lab, working on DNA structure.

  • f. Did she have a good relationship with Wilkins?

    • Relationship with Wilkins: Their relationship was marked by tension and miscommunication; they did not have a good working relationship.

  • g. What were Watson and Crick trying to figure out?

    • Watson and Crick’s Endeavor: They were trying to decipher the structure of DNA.

  • h. Were they generating data toward this endeavor?

    • Data Generation: No, they were not generating experimental data directly toward their model; rather, they were hypothesizing based on available data.

  • i. Who actually took the famous photo 51 and under whose direction was he/she working?

    • Photo 51: The famous photo 51 was taken by Rosalind Franklin herself, working under the direction of Wilkins.

  • j. Who did Wilkins show photo 51 to?

    • Photo Sharing: Wilkins showed photo 51 to Watson and Crick.

  • k. Did Wilkins have permission to share it?

    • Permission to Share: Wilkins did not have explicit permission from Franklin to share photo 51.

  • l. Why was that such a big deal? What did the photograph reveal to the trained eye?

    • Significance of the Photograph: The photograph revealed critical features of DNA, indicating it had a helical structure, which was crucial for deducing its molecular configuration.

  • m. How did they obtain the detailed numbers?

    • Measurement Acquisition: Watson and Crick obtained detailed measurements through Franklin's data and by consulting with other scientists, clarifying dimensions needed for their model of DNA.

  • n. Who solved the structure of DNA first?

    • Structure Resolution: Watson and Crick solved the structure of DNA first.

  • o. Was Franklin anywhere close to figuring it out?

    • Franklin's Proximity: Franklin was close to figuring out the structure, having significant insights from her data.

  • p. How did all the players in this drama get some measure of credit for their work in 1953?

    • Credit Distribution: All contributors received some measure of credit; Watson, Crick, and Wilkins received the Nobel Prize for their model, though Franklin's contributions were acknowledged posthumously.

  • q. Was Franklin eligible for the prize?

    • Franklin's Prize Eligibility: Franklin was not eligible for the Nobel Prize as she had passed away in 1958, and the prize cannot be awarded posthumously under the rules.

Aside

  • The ethical implications of the events surrounding the DNA discovery highlight the complex interplay between scientific achievement and recognition, particularly concerning Franklin's underappreciated contributions despite her crucial