Bio 112 Cell and Molecular Experiments
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Questions and Answers

What primarily caused Rosalind Franklin to leave King's College?

  • Disagreements with colleagues
  • Personal health issues
  • Pressure from the administration to stop her research (correct)
  • Lack of funding for her projects

What type of research did Franklin conduct at Birkbeck College?

  • Development of X-ray crystallography techniques
  • Investigation of bacterial infections
  • Studies on gene therapy
  • Research into the tobacco mosaic virus (correct)

Which of the following best describes the method Franklin used in her DNA research?

  • Fluorescence microscopy
  • Nuclear magnetic resonance spectroscopy
  • Cryo-electron microscopy
  • X-ray diffraction (correct)

How did Franklin's group contribute to science?

<p>By laying the foundation for structural virology (B)</p> Signup and view all the answers

What was the cause of Franklin's health issues later in her life?

<p>Ovarian cancer (B)</p> Signup and view all the answers

Which DNA replication model suggests that both parental strands remain intact after replication?

<p>Conservative model (C)</p> Signup and view all the answers

What did the first replication in Meselson and Stahl's experiment produce?

<p>Hybrid DNA (A)</p> Signup and view all the answers

What is essential for DNA synthesis to occur in vitro?

<p>All necessary components must be present (D)</p> Signup and view all the answers

Which model of DNA replication is supported by Meselson and Stahl's findings?

<p>Semiconservative model (A)</p> Signup and view all the answers

Why do eukaryotes require multiple origins of replication?

<p>To ensure rapid DNA replication of large linear chromosomes (A)</p> Signup and view all the answers

What type of nitrogen isotope was used to label the parental DNA strands in the experiment?

<p>15N (D)</p> Signup and view all the answers

What conclusion was drawn after the second replication in Meselson and Stahl's experiment?

<p>Both light and hybrid DNA were produced (D)</p> Signup and view all the answers

Which statement accurately reflects a difference between eukaryotic and bacterial DNA replication?

<p>Eukaryotic replication involves multiple origins compared to bacterial replication. (A)</p> Signup and view all the answers

What occurs following the addition of acid in the DNA replication process described?

<p>The precipitation of DNA strands (C)</p> Signup and view all the answers

In eukaryotic DNA replication, what is the result of bidirectional synthesis from multiple origins?

<p>It allows faster replication of larger DNA strands. (B)</p> Signup and view all the answers

What significant contribution did Rosalind Franklin make to the study of DNA?

<p>Advancements in X-ray diffraction techniques (A)</p> Signup and view all the answers

What is Chargaff's rule based on?

<p>The equality of adenine and thymine percentages, and cytosine and guanine percentages (A)</p> Signup and view all the answers

What was the early hypothesis regarding the interaction of DNA strands proposed by Watson and Crick?

<p>The strands interact through phosphate-Mg++ crosslinks (C)</p> Signup and view all the answers

What did the diffraction pattern obtained by Rosalind Franklin suggest about DNA?

<p>It has a helical structure with multiple strands (A)</p> Signup and view all the answers

What prompted Watson and Crick's investigation into the structure of DNA?

<p>Key observations from previous experiments by others (C)</p> Signup and view all the answers

What was used to trace the radioactivity in the experiment involving phages?

<p>Radioactive DNA (C)</p> Signup and view all the answers

Which radioactive isotope was used to label the DNA in batch 2 of the phage experiment?

<p>Phosphorus-32 (C)</p> Signup and view all the answers

What does Erwin Chargaff's findings regarding DNA suggest about its role as genetic material?

<p>DNA varies between species, suggesting diversity. (A)</p> Signup and view all the answers

What was a key outcome of the centrifugation process in the phage experiments?

<p>Radioactivity was concentrated in the pellet. (C)</p> Signup and view all the answers

Which of the following correctly states a component of a nucleotide in DNA?

<p>A nitrogenous base, a deoxyribose sugar, and a phosphate group (A)</p> Signup and view all the answers

What was the significance of using both sulfur and phosphorus in the experiments?

<p>To differentiate between DNA and protein components. (A)</p> Signup and view all the answers

What role does RNA play in some viruses, according to the information provided?

<p>RNA serves as the genetic material in certain viruses. (C)</p> Signup and view all the answers

What does the presence of radioactive proteins in the phage shell demonstrate in the experiment?

<p>The protein coat can remain intact while injecting genetic material. (D)</p> Signup and view all the answers

What was the main purpose of the experiment using E. coli cultured in different nitrogen mediums?

<p>To determine the mode of DNA replication (B)</p> Signup and view all the answers

After the first replication in the E. coli experiment, what type of DNA was primarily observed?

<p>Hybrid DNA (A)</p> Signup and view all the answers

What finding after two generations of DNA replication contradicted the dispersive model?

<p>The presence of both light and hybrid DNA (C)</p> Signup and view all the answers

What did Arthur Kornberg hypothesize regarding DNA synthesis precursors?

<p>Deoxynucleoside triphosphates are the precursors of DNA synthesis (B)</p> Signup and view all the answers

Which model of DNA replication was supported after analyzing the results of the E. coli experiment?

<p>Semiconservative model (A)</p> Signup and view all the answers

In Kornberg's experiments, why were long DNA strands not soluble in acidic solutions?

<p>Long DNA strands precipitate at low pH (B)</p> Signup and view all the answers

What was the significance of using a CsCl gradient in the DNA density analysis?

<p>It separated different forms of DNA based on density (B)</p> Signup and view all the answers

What did the second replication produce in the E. coli experiment that was crucial for drawing conclusions?

<p>Light and half-heavy DNA (D)</p> Signup and view all the answers

Flashcards

Rosalind Franklin's work

Rosalind Franklin studied DNA using X-ray diffraction, aiming to understand its structure.

X-ray diffraction

A technique used to analyze the structure of molecules, like DNA, by shooting X-rays at them and studying resulting patterns.

DNA structure

The arrangement of the atoms that make up a DNA molecule.

Franklin's research on viruses

Rosalind Franklin conducted research on the tobacco mosaic virus, laying groundwork for structural virology.

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Franklin's premature death

Rosalind Franklin died in 1958 at age 37 from ovarian cancer.

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Hershey-Chase Experiment

A famous experiment that proved that DNA, not protein, is the genetic material.

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Radioactive Phage

Bacteriophage containing radioactive protein or radioactive DNA to track which material is taken into the bacterial cell, in the Hershey-Chase experiments

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Bacterial Cell

A cell that the virus infects.

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35S

Radioactive isotope of sulfur, used to label protein in the Hershey-Chase experiment.

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32P

Radioactive isotope of phosphorus, used to label DNA in the Hershey-Chase experiment.

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Genetic Material

The material responsible for carrying and transmitting hereditary information.

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DNA

Deoxyribonucleic acid; a molecule that carries the genetic information in most organisms.

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Virus

A tiny infectious agent that requires a host cell to reproduce.

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Conservative model of DNA replication

During DNA replication, both original parental strands remain together as one complete molecule, while the new strands form a separate double helix.

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Semiconservative model of DNA replication

Each newly formed DNA molecule contains one original strand from the parent molecule and one newly synthesized strand.

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Dispersive model of DNA replication

Both parent DNA strands are broken down into small fragments that are scattered into both newly synthesized strands, resulting in a mix of old and new DNA in each molecule.

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Meselson and Stahl's experiment

A scientific experiment that used heavy isotopes of nitrogen to distinguish between parent and daughter DNA strands during replication, ultimately supporting the semiconservative model of DNA replication.

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How did Meselson and Stahl's experiment rule out the conservative model?

The experiment produced a hybrid band of DNA after the first replication, which was not possible if the parental strands stayed together in the conservative model.

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What did Rosalind Franklin's X-ray diffraction studies reveal about DNA?

Her X-ray diffraction patterns suggested DNA is a helical structure with multiple strands, and approximately 10 base pairs per complete turn.

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Chargaff's Rule

Chargaff's rule states that in DNA, the amount of adenine (A) always equals the amount of thymine (T), and the amount of guanine (G) always equals the amount of cytosine (C).

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What was the initial incorrect hypothesis about how DNA strands interact?

Scientists initially proposed that DNA strands interacted through phosphate-Mg++ crosslinks, but this was proven wrong.

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Watson and Crick's model building

Watson and Crick used ball-and-stick models to represent DNA and tried to build a structure that incorporated all known experimental data, including Chargaff's rule and Franklin's X-ray diffraction data.

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How did Chargaff's data help Watson and Crick?

Chargaff's rule, showing equal amounts of A and T, and G and C, provided crucial evidence for Watson and Crick to understand the base pairing within DNA.

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In vitro DNA Synthesis

The process of creating new DNA strands in a test tube, outside of a living organism, using essential components such as template DNA, nucleotides, and enzymes.

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Radiolabeled Nucleotides

Nucleotides (building blocks of DNA) that have been tagged with a radioactive isotope, allowing scientists to track their incorporation into newly synthesized DNA.

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Eukaryotic DNA Replication

The process of copying DNA in eukaryotes, which is more complex than in bacteria due to larger linear chromosomes, tight packaging within nucleosomes, and complicated cell cycle regulation.

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Multiple Origins of Replication

The presence of multiple starting points for DNA replication along a eukaryotic chromosome, enabling faster and more efficient replication of the large DNA molecule.

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Bidirectional DNA Replication

DNA replication that occurs in both directions from the origin of replication, resulting in a replication fork that expands on both sides.

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Meselson-Stahl Experiment

An experiment that demonstrated the semi-conservative nature of DNA replication. Using isotopes of nitrogen (14N and 15N), researchers tracked the distribution of DNA strands during replication.

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Conservative DNA Replication Model

A model suggesting that during DNA replication, the original, intact DNA molecule remains and a completely new, replicated molecule is created.

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Semiconservative DNA Replication Model

The currently accepted model for DNA replication, where each new DNA molecule is composed of one original strand and one newly synthesized strand.

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Dispersive DNA Replication Model

A model suggesting that during replication, the original DNA molecule is fragmented, and the new molecule is composed of a mixture of original and newly synthesized fragments.

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Density Gradient Centrifugation

A technique used to separate molecules based on their density. A solution with a density gradient is created, and when molecules are centrifuged, they settle at positions corresponding to their densities.

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In Vitro DNA Replication

The study of DNA replication in a laboratory setting, outside of a living organism.

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Arthur Kornberg

A Nobel laureate who pioneered the in vitro study of DNA replication, demonstrating the process using purified enzymes and deoxynucleoside triphosphates.

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Deoxynucleoside Triphosphates

The building blocks of DNA, consisting of a deoxyribose sugar, a nitrogenous base, and three phosphate groups.

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Study Notes

Bio 112 Cell and Molecular Experiments

  • Experiments were used to elucidate the macromolecule that carries genetic information
  • Experiments were also used to discover DNA structure and how DNA is duplicated

Experiments Proving DNA is Genetic Material

  • Identifying the molecules of inheritance was a challenge in the early 20th century
  • The knowledge of genes being on chromosomes and their components DNA/protein meant they were considered potential candidates for genetic material
  • The key to determining the genetic material was choosing appropriate experimental organisms.
  • The role of DNA in heredity was first discovered through studying bacteria and viruses that infect them.

Criteria for Identifying Genetic Material

  • The genetic material must contain the information to make an entire organism.
  • It must be able to be transmitted from parent to offspring.
  • The genetic material must be able to replicate itself.
  • It must be capable of changes to account for known phenotypic variation in species.

Identification of DNA as Genetic Material: Experiments

  • Griffith's experiments with Streptococcus pneumoniae (1928) showed transformation. Living R bacteria were transformed after exposure to heat-killed S bacteria, and contained the genes from the latter bacteria.
  • The transforming principle was the substance causing the transformation and was determined through further experiments
  • Avery, Macleod, and McCarty's experiments (1940s) concluded that DNA is the transforming principle; DNA extracted from type IIIS bacteria could convert type IIR bacteria to type IIIS. They focused only on DNA, and not other relevant macromolecules from the bacteria
  • Hershey and Chase's experiment (1952) showed that DNA, not protein, is the genetic material of bacteriophages; 32P was found inside the bacterial cells after infection, while 35S was found outside.

Evidence That Viral DNA Can Program Cells

  • More evidence for DNA as the genetic material came from studies of bacteriophages (viruses that infect bacteria)
  • Bacteriophages are used extensively in molecular genetics research

The Structure of DNA

  • After identifying DNA as the genetic material, determining its structure that accounts for its role became the next challenge
  • In 1953, James Watson and Francis Crick introduced a double-helix model for DNA structure.
  • Their scientific breakthrough was supported by scientists like Linus Pauling, Rosalind Franklin, Maurice Wilkins, and Erwin Chargaff
  • An early hypothesis for DNA strand interaction was the phosphate-Mg++ crosslinking, which was deemed incorrect. Watson and Crick further refined the model to conform to X-ray data and determined base pairings
  • Adenine always pairs with Thymine and Guanine pairs with Cytosine; This was named Chargaff's rule
  • The Watson-Crick model successfully explains Chargaff’s rules

DNA Replication Experiments

  • Watson and Crick's semiconservative model predicts the replication of the double helix based on pairing and one old strand being conserved per new strand
  • Alternative models (conservative and dispersive) were also proposed but were later disproved by Meselson–Stahl experiment (1958). This experiment supported the semiconservative model using 15N and 14N isotopes.

In Vitro Bacterial Replication and Eukaryotic DNA Replication

  • In vitro study of DNA replication was spearheaded by Arthur Kornberg in the 1950s (Nobel Prize 1959)
  • Kornberg hypothesized that deoxynucleoside triphosphates are precursors for DNA synthesis.
  • Eukaryotic DNA replication has similarities to bacterial replication but is more complex because of linear chromosomes and tight packaging within nucleosomes.
  • Eukaryotic DNA replication originates from multiple origins of replication. This is crucial to ensure timely and complete replication. Multiple origins of replication (ARS elements) are found in eukaryotes and are similar to bacterial DNA boxes.
  • A six-subunit complex called Origin Recognition Complex (ORC) plays an essential role as the initiator of eukaryotic DNA replication
  • Eukaryotes contain over a dozen DNA polymerases with alpha, delta, epsilon, and gamma having the primary function

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Description

This quiz covers essential experiments that elucidate the role of DNA as the genetic material. You will explore how scientists identified DNA structure, its replication, and the criteria that classify it as genetic material. Test your understanding of the historical context and key experiments in molecular biology.

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