L7: The Structure of DNA

• What was the main reason scientists stopped believing that DNA is the genetic material?

  They were influenced by earlier theories and models that did not support DNA's role.
• What key events in the history of genetics contributed to the understanding of DNA?

  • 1865: Mendel's work on inheritance
  • 1902: Sutton's chromosome theory
  • 1928: Griffith's transformation experiment
  • 1944: Avery's identification of DNA as the transforming principle
  • 1952: Hershey and Chase's experiments confirming DNA's role
• What are the components of DNA?

  DNA is composed of a sugar, phosphate, and nitrogenous bases.
• What did Phoebus Levene demonstrate about DNA in the 1930s?

  He showed that each building block of DNA is a nucleotide consisting of a phosphate group, deoxyribose sugar, and a nitrogenous base.
• What type of sugar is used in DNA?

  The sugar used in DNA is deoxyribose.
• How are the carbon atoms in deoxyribose sugar numbered?

  The carbon atoms in deoxyribose are numbered from 1' to 5'.
• What are the four nitrogenous bases found in DNA?

  The four nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
• How are nucleosides formed?

  Nucleosides are formed from a nitrogenous base and a sugar.
• What are nucleotides?

  Nucleotides are phosphorylated nucleosides.
• What are the different forms of nucleotides based on their nitrogenous bases?

  Nucleotides can be classified as dAMP, dADP, dATP, dGMP, dGDP, dGTP, dTMP, dTDP, and dCTP.
• What is the significance of polarity in DNA?

  DNA has polarity due to the directionality of the phosphodiester bonds linking nucleotides.
• Why is DNA polarity important?

  DNA polarity is crucial for understanding the directionality of nucleic acid processes.
• What was Phoebus Levene's tetranucleotide model?

  Levene proposed that the four nucleotides occurred in tetranucleotide blocks with bases pointing outwards.
• What did Chargaff conclude from his experiments regarding nucleotide proportions?

  Chargaff concluded that the proportions of adenine and thymine are equal, as well as guanine and cytosine.
• What are Chargaff's rules?

  Chargaff's rules state that %A = %T and %G = %C in DNA.
• Who was Linus Pauling and what did he contribute to DNA structure research?

  Linus Pauling was a leading structural chemist who described the alpha helix and used X-ray crystallography.
• How does X-ray crystallography help in determining molecular structures?

  X-ray crystallography diffracts X-rays through crystallized molecules to produce a unique diffraction pattern.
• What happens when light passes through a small opening in terms of diffraction?

  A wave front is propagated, resulting in a single spot on a screen.
• What occurs when light passes through two slits in terms of diffraction?

  The diffraction patterns interfere, creating a complex double slit interference pattern on the screen.
• How does a diffraction grating improve the diffraction pattern?

  A diffraction grating produces a sharper image and reduces intermediate peaks to invisibility.
• What did Augustin Fresnel demonstrate about diffraction around solid objects?

  Fresnel showed that diffraction occurs around solid objects, producing familiar diffraction patterns.
• What is the significance of the Fresnel/Arago/Poisson spot in diffraction studies?

  • It demonstrates the phenomenon of diffraction around solid objects.
  • It illustrates how light behaves when encountering obstacles.
  • It is a key concept in understanding wave behavior in optics.
• How does spatial separation of points affect interference fringes?

  Features that are close produce widely separated reflections, while distant features produce closely separated reflections.
• What is the effect of grids on diffraction patterns?

  Grids produce a cross pattern in the diffraction image.
• Why do reflections from a helix form a cross pattern in diffraction studies?

  Aligned helices effectively form a diffraction grid, resulting in a cross pattern.
• What was the race to describe the structure of DNA about?

  • It involved multiple scientists contributing to the understanding of DNA.
  • Key discoveries included Chargaff's rules and X-ray crystallography.
  • It culminated in the identification of the double helix structure by Watson and Crick.
• What is the distance between the slits mentioned in the study material?

  150 μm
• Who showed that diffraction occurs around solid objects with the same width as a slit?

  Augustin Fresnel
• How does the diffraction pattern change when there are two solid objects?

  The diffraction patterns interfere, resembling a double slit interference pattern
• What is the name of the spot associated with Fresnel's work?

  The Fresnel/Arago/Poisson spot
• How does spatial separation of points affect interference fringes?

  Close features produce widely separated reflections, while distant features produce closely separated reflections
• What pattern do horizontal gratings produce?

  Vertical spots
• What do angled grids produce?

  An angled cross
• Why do reflections from a helix form a cross pattern?

  Aligned helices effectively form a diffraction grid
• Who stretched DNA for X-ray fibre diffraction?

  Maurice Wilkins
• What was the condition of DNA when Raymond Gosling stretched it for X-ray diffraction?

  It was left hydrated
• What significant finding did Photo 51 reveal about DNA?

  DNA must be helical
• What form of DNA did Maurice Wilkins produce in 1950?

  The 'A' form
• Who proposed a triple helix model of DNA?

  Linus Pauling
• What model did Watson and Crick propose for DNA?

  A double helix model