Biology 205 DNA Basics: Chemistry of DNA

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I.  Discovery of DNA and its role in genetics

A. Identify the discover of DNA and explain his procedure for isolating DNA- Frederick Miescher

• Isolated cell nuclei using pig pepsin
• Purified a substance with C,N,O,P "nuclein"
• Later on staining showed that "nuclein" present in chromosomes
• Historical note: Miescher's work marked the first isolation of an organelle

B. Discuss the evidence that DNA is generally the genetic material.

• Explain why people thought proteins might be the genetic material
• Describe Griffith's 1928 transformation experiment. Fig 8.2
  • Describe his conclusion.
  • Explain the procedure used by Avery, MacLeod and McCarty to show that DNA was the transforming principle not RNA Fig 8.3
• Describe Alfred Hershey and Margaret Chase's experiment using bacteriophages 1953. Fig 8.4

C. Compare and contrast the basic chemistry of DNA and RNA

• Describe the basic chemical bonds ionic, covalent, polar covalent and "hydrogen" bond on your own
• Discuss the basic chemistry of nucleotides and nucleic acids
• Diagram the basic structure of a nucleotide
• Distinguish between Ribose vs Deoxyribose-make sure you can label the carbons properly in the sugars! See figure 8.5
• Recognize the main types of nitrogenous bases: Purines Vs Pyrimidines Fig 8.6
• List which nitrogenous bases are found in DNA Vs RNA
• Diagram the basic linkage (phosphodiester) between nucleotides in a polynucleotide Fig 8.7 and explain the meaning of the 5' and 3' ends of a polynucleotide

II. Discovery of DNA Structure

A. Discuss the Information available to Watson and Crick

• State Chargaff's rule and explain the molecular basis of this rule
• Explain the basic idea of X ray diffraction and the importance of data from Maurice Wilkins and Rosalind Franklind
• Note that Franklind concluded DNA has a helical structure with periods of .34nm and 3.4 nm
• Briefly identify the role of the following people in the discovery of DNA: See Figs 8.8 -8.9
  • Maurice Wilkins and Rosland Franklin
  • Erwin Chargaff
  • James Watson and Francis Crick

B. Review the main components of Watson and Crick's model for "B" DNA

• DNA molecule two polynucleotide strands wound clock wise(right handed)around each other. See Fig 8.10
• Strands are antiparallel
• Sugar - phosphate bonds between nucleotides form the backbone of each strand
• Bases are flat and perpendicular to the axis of the helix. Distance between bases = 0.34nm
• Explain the role of hydrogen bonds in DNA structure.
• Define the Principle of complementarity Fig 8.11
• Note that other DNA structures are found-
  • A DNA right handed - Dehydrated DNA
  • B DNA right handed - most common form in cells
  • Z DNA left handed DNA may exist in some cells

C. Explain how Watson and Crick's model provided a ready mechanism for DNA replication.

• Distinguish between possible replication models: Semi conservative, conservative, dispersive
• Describe the Meselson and Stahl experiment p 186 - 187 and Figure 9.2

III. DNA in Eukaryote Chromosomes

• Define the term genome and contrast chromosome structure in viruses, prokaryotes and eukaryotes
• Viral Chromosomes: Note many viruses have RNA rather than DNA as genetic material.
  • Comment: viral genomes are highly variable depending on the virus. See p 171. This suggests that different types of viruses may have arisen independently of each other.
• Describe the basic features of most Prokaryotic chromosomes Figs 8.14 - 8.15
  • Usually circular loop and associated proteins in the nucleoid
  • E-coli DNA 4.6 Mb pairs and is 1.1mm long. Be able to verify this
  • Explain the difference between positive and negative supercoiling and the function of topoisomerases.
  • Note that supercoiling and looped domains(Fig 8.16) appear to to be the main ways prokaryote chromsomes are compacted.
• Eukaryotic chromosomes
  • Define C value Table 8.3
    • Do more complex organisms always have higher C values than less complex ones?
  • Define chromatin
  • Distinguish between histone and non histone proteins and their functions
    • Comment: Histones are basic proteins involved mainly in the formation of the nucleosome. See figure 8.17. The four histone proteins that make up the nucleosome are very similar in most eukaryotes, suggesting that they have the same function in all eukaryotes. Possibly the nucleosome proteins have a common evolutionary origin. Histones often bind directly to DNA.
    • Non histones are acidic and often bind to histones. Involved in higher order organization of the chromosome. See for instance figure 8.21
  • Define nucleosome and describe or diagram the main levels of chromatin packing. See figure 8.22
  • Distinguish between euchromatin and heterochromatin: note the Barr body is heterochromatin
  • Distinguish between Constitutive heterochromatin Facultative heterochromatin.
  • Comment: constitutive Heterochromatin is usually repetitive DNA such as the centromere and telomeres; facultative maybe non repetitive DNA coding for proteins that is not being expressed as in the Barr body.

Be able to do the questions at the end of the chapter.

pgd 09/19/02 revised 10/01/04