7.1: Discovering how nucleic acids store genetic information

To follow the historical pathway that led to our understanding of how heredity works, we start back at the cell, the basic living unit. As it was established that all organisms are composed of one or more types of cells, and that all cells are derived from pre-existing cells, it became more and more likely that inheritance was a cellular phenomenon. As part of their studies, cytologists (students of cells) began to catalog the common components of cells. Because of resolution limits associated with available microscopes, these studies were initially limited to larger eukaryotic cells. One such visible component of eukaryotic cells is the nucleus. Using early (bright-field) microscopes, cells appear clear and transparent, after all they are ~70% water. To discern structural details cytologists had to stabilize the cell; stabilizing the cell means killing it. Biological samples were “fixed” to ensure that structures were preserved as close to the living state as possible. Originally, this process involved the use of chemicals, such as formaldehyde or organic solvents, that cross-link or precipitated various molecules together. Fixation stops molecular movement; it is not unlike boiling an egg. In more modern studies, using higher resolution optical methods^[274] and electron microscopes, such crude fixation methods have been replaced by alternatives that include very rapid freezing and cryoelectron microscopy.

At the level of light microscopy, it can be hard to resolve cellular subcomponents. One approach is to treat fixed cells with various dyes, dyes that bind preferentially to specific types of molecules. These molecules were often found located in particular cellular regions. The most dramatic of these cellular sub-regions is the nucleus, which due to its bulk chemical composition stains differently from the surrounding cytoplasm. One common stain consists of a mixture of hematoxylin and eosin; it leaves the cytoplasm pink and the nucleus dark blue.^[275] 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) organisms.^[276] Prokaryotic cells (before a nucleus) are typically much smaller which made it technically difficult to determine whether they had a nucleus or not – they do not.

The careful examination of fixed and living eukaryotic cells revealed that the nucleus undergoes a dramatic reorganization during the process of cell division; it loses its roughly spherical shape, which is replaced by discrete stained strands known as chromosomes (colored bodies). In 1887 Edouard van Beneden (1846-1910) 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 could 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 four chromosomes (→). In 1902, Walter Sutton (1877-1916) published his observation that chromosomes obey Mendel's rules of inheritance, that is that during the formation of the gametes that fuse during sexual reproduction, each cell received one and only one copy of each chromosome. This strongly suggested that Mendel's genetic factors were associated with chromosomes.^[277] It was recognized that there were many more Mendelian factors than chromosomes, which implied that many factors must be present on each chromosome. These observations provided a physical explanation for the observation that many genetic traits do not behave independently but acted as if they were somehow 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.

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 assumed, quite reasonably, that components enriched in this material would likely be enriched in nuclear, genetic information containing components. Using sperm and pus as starting materials Friedrich Miescher (1844-1895) isolated a phosphorus-rich compound called nuclein.^[278] Nuclein was later 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 initially unclear.

---