8: DNA, Chromosomes and Chromatin
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- 8.1: Introduction
- 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 do
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- 8.2: The Stuff of Genes
- That all eukaryotic cells contain a nucleus was understood by the late 19th century. By then, histological studies had shown that nuclei contained largely proteins and DNA. At around the same time, the notion that the nucleus contains genetic information was gaining traction. In 1910, Albrecht Kossel received the 1910 Nobel Prize in Physiology or Medicine for his discovery of the adenine, thymine, cytosine and guanine (the four DNA bases), as well as of uracil in RNA.
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- 8.3: DNA Structure
- By 1878, a substance in the pus of wounded soldiers derived from cell nuclei (called nuclein) was shown to be composed of 5 bases (the familiar ones of DNA and RNA). The four bases known to make up DNA (as part of nucleotides) were thought to be connected through the phosphate groups in short repeating chains of four nucleotides. By the 1940s, we knew that DNA was a long polymer. Nevertheless, it was still considered too simple to account for genes.
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- 8.4: Genes and Chromatin in Eukaryotes
- Chromosomes and chromatin are a uniquely eukaryotic association of DNA with more or less protein. Bacterial DNA (and prokaryotic DNA generally) is relatively ‘naked’ – not visibly associated with protein. The electron micrograph of an interphase cell (below) reveals that the chromatin can itself exist in various states of condensation.
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- 8.5: Structure and Organization of DNA in Bacteria
- Sexual reproduction allows compatible genders (think male and female) to share genes, a strategy that increases species diversity. It turns out that bacteria and other single celled organisms can also share genes… and spread diversity. We will close this chapter with a look at sex (E. coli style!), and gene-mapping experiments showing linearly arranged genes on a circular bacterial DNA molecule (the bacterial ‘chromosome’).
Thumbnail: DNA double helix. (public domain; NIH - Genome Research Institute).