8.1: Introduction

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Here we look at classic experiments that led to our understanding that genes are composed of DNA. We already knew that genes were on chromosomes (chromo – colored; soma-body). Early 20th century gene mapping even showed the relative location (locus) of genes on chromosomes. Compared to eukaryotes, bacteria contain a very small amount of DNA per cell. Subsequent bacterial gene mapping and electron microscopy revealed that the E. coli “chromosome is little more than a small closed, circular DNA double helix. In contrast, linear eukaryotic chromosomes are highly condensed structures composed of DNA and protein, visible only during mitosis or meiosis. During the much longer interphase portion of the eukaryotic cell cycle, chromosomes de-condense to chromatin, a less organized form of protein-associated DNA in the nucleus. Chromatin is the gatekeeper of gene activity in eukaryotic cells, a situation quite different from bacterial cells. Since we know that all cells of an organism contain the same DNA, and all cells must alter patterns of gene expression over time, understanding the structure and organization of DNA in cells is essential to an understanding of how and when cells turn genes on and off.

Learning Objectives

When you have mastered the information in this chapter, you should be able to:

1. summarize the evidence that led to acceptance that genes are made of DNA.

2. discuss how Chargaff’’s DNA base ratios support DNA as the “stuff of genes”.

3. interpret the results of Griffith, Avery et al. and Hershey & Chase, in historical context.

4. outline and explain how Watson and Crick built their model of a DNA double helix.

5. distinguish between conservative, semiconservative and dispersive replication.

6. describe and/or draw the progress of a viral infection.

7. trace the fate of 35SO4 (sulfate) into proteins synthesized in cultured bacteria.

8. distinguish between the organization of DNA in chromatin and chromosomes and speculate on how this organization impacts replication.

9. list some different uses of karyotypes.

10. compare and contrast euchromatin and heterochromatin structure and function.

11. outline an experiment to purify histone H1 from chromatin.

12. formulate an hypothesis to explain why chromatin is found only in eukaryotes.

13. describe the roles of different histones in nucleosome structure.

14. explain the role of Hfr strains in mapping genes in E. coli.

15. explain the chemical rationale of using different salt concentrations to extract 10 nm nucleosome fibers vs. 30nm solenoid structures from chromatin.