8.1: Introduction

In 1868, seeking a source for the hereditary variation to support evolution by natural selection, Charles Darwin came up with pangenesis (from the Greek pan, “all”; genetikos, “origins”) in which hereditary particles he called gemmules (from Greek, meaning “small buds”) are gathered from body cells into germ cells and redistributed to body cells during embryogenesis. In the 1890s, Hugo de Vries, at first unaware of Mendel’s work independently rediscovered the Mendelian laws of heredity in plants, adopting Darwin’s term pangenes for Mendel’s heritable factors. One of three scientists credited with the rediscovery of Mendel’s work, he also coined the term mutation for novel traits that suddenly appeared in his plants. In 1905, William Bateson first defined genetics as the study of inheritance and in 1909, Wilhelm Johannsen, a Danish botanist, shortened pangene to gene, also coining the terms genotype and phenotype to distinguish genes from the visible traits they determine. Finally, we knew by the start of the twentieth century that chromosome shapes were species-specific, that each species had a characteristic karyotype (from the Greek karyo, “nuclear”). Feel free to search for more historical details. This brief etymological history brings us to the topic of this chapter – the stuff of genes!

Here we look at classic experiments that led to our understanding that genes are composed of DNA. We already knew that genes are on chromosomes (chromo: “colored”; soma: “body”). Early twentieth-century gene mapping had shown the relative location (locus) of genes on the linear chromosomes of eukaryotes. We’ll see that 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 decondense to chromatin, a less-organized form of protein-associated DNA in the nucleus. In this chapter, we’ll learn that eukaryotic chromatin is the gatekeeper of gene activity, a situation quite different from the case in bacterial cells. Unlike eukaryotic cells, bacteria contain a tiny amount of more simply organized DNA. We’ll learn that bacterial gene (mutation) mapping revealed the bacterial “chromosome” to be a closed, naked, and circular DNA double helix. Such knowledge of DNA structure and organization is essential to our understanding of how and when cells turn genes on and off. When we look at the gene regulation in an upcoming chapter, remember this and that the genetic content of eukaryotic as well as of prokaryotic organisms is species-specific.

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.