7.1: Discovering how nucleic acids store genetic information

To follow the historical pathway that led to our understanding of how heredity works, we have to start back at the cell. As it became more firmly established that all organisms are composed of cells, and that all cells were derived from pre-existing cells, it became more and more likely that inheritance had to be a cellular phenomena. As part of their studies, cytologists (students of the cell) began to catalog the common components of cells; because of resolution limits associated with available microscopes, these studies were restricted to larger eukaryotic cells. One such component of eukaryotic cells was the nucleus. At this point it is worth remembering that most cells do not contain pigments. Under these early microscopes, they appear clear, after all they are ~70% water. To be able to discern structural details cytologists had to stabilize the cell and to visualize its various components. As you might suspect, stabilizing the cell means killing it. To be observable, the cell had to be killed (known technically as “fixed”) in such a way as to insure that its structure was preserved as close to the living state as possible. Originally, this process involved the use of chemicals, such as formaldehyde, that could cross-link various molecules together. Cross-linking stops these molecules from moving with respect to one another. Alternatively, the cell could be treated with organic solvents such as alcohols; this leads to the local precipitation of the water soluble components. As long as the methods used to visualize the fixed tissue were of low magnification and resolution, the results were generally acceptable. In more modern studies, using various optical methods¹⁹⁹ and electron microscopes, such crude fixation methods became unacceptable, and have been replaced by various alternatives, including rapid freezing. Even so it was hard to resolve the different subcomponents of the cell. To do this the fixed cells were treated with various dyes. Some dyes bind preferentially to molecules located within particular parts of the cell. The most dramatic of these cellular sub-regions was the nucleus, which could be readily identified because it was stained very differently from the surrounding cytoplasm. One standard stain involves a mixture of hematoxylin (actually oxidized hematoxylin and aluminum ions) and eosin, which leaves the cytoplasm pink and the nucleus dark blue²⁰⁰. The nucleus was first described by Robert Brown (1773-1858), the person after which Brownian motion was named. The presence of a nucleus was characteristic of eukaryotic (true nucleus) organisms²⁰¹. Prokaryotic cells (before a nucleus) are typically much smaller and originally it was impossible to determine whether they had a nucleus or not (they do not).

The careful examination of fixed and living cells revealed that the nucleus underwent a dramatic reorganization as a cell divides, losing its (typically) roughly spherical shape which was replaced by discrete stained strands, known as chromosomes (colored bodies). In 1887 Edouard van Beneden reported that the number of chromosomes in a somatic (diploid) cell was constant for each species and that different species had different numbers of chromosomes. Within a particular species the individual chromosomes can be recognized based on their distinctive sizes and shapes. For example, in the somatic cells of the fruit fly Drosophila melanogaster there are two copies of each of 4 chromosomes. In 1902, Walter Sutton published his observation that chromosomes obey Mendel's rules of inheritance, that is that during the formation of the cells that fuse during sexual reproduction (gametes: sperm and eggs), each cell received one and only one copy of each chromosome. This strongly suggested that Mendel's genetic factors were associated with chromosomes²⁰². Of course by this time, it was recognized that there were many more Mendelian factors than chromosomes, which means that many factors must be present on a particular chromosome. These observations provided a physical explanation for the observation that many traits did not behave independently but acted as if they were linked together. The behavior of the nucleus, and the chromosomes that appeared to exist within it, mimicked the type of behavior that a genetic material would be expected to display.

These cellular anatomy studies were followed by studies on the composition of the nucleus. As with many scientific studies, progress is often made when one has the right “model system” to work with. It turns out that some of the best systems for the isolation and analysis of the components of the nucleus were sperm and pus (isolated from discarded bandages from infected wounds (yuck)). It was therefore assumed, quite reasonably, that components enriched in this material would likely be enriched in nuclear components. Using sperm and pus as a starting material Friedrich Miescher (1844 – 1895) was the first to isolate a phosphorus-rich compound, called nuclein²⁰³. At the time of its original isolation there was no evidence linking nuclein to genetic inheritance. Later nuclein was resolved into an acidic component, deoxyribonucleic acid (DNA), and a basic component, primarily proteins known as histones. Because they have different properties (acidic DNA, basic histones), chemical “stains” that bind or react with specific types of molecules and absorb visible light, could be used to visualize the location of these molecules within cells using a light microscope. The nucleus stained for both highly acidic and basic components - which suggested that both nucleic acids and histones were localized to the nucleus, although what they were doing there was unclear.

Contributors and Attributions

• Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.