Microbiology, 2021

10.1 • Using Microbiology to Discover the Secrets of Life 371
nuclein. In the last two decades of the 19th century, German biochemist Albrecht Kossel (1853–1927) isolated
and characterized the five different nucleotide bases composing nucleic acid. These are adenine, guanine,
cytosine, thymine (in DNA), and uracil (in RNA). Kossell received the Nobel Prize in Physiology or Medicine in
1910 for his work on nucleic acids and for his considerable work on proteins, including the discovery of
histidine.
Foundations of Genetics
Despite the discovery of DNA in the late 1800s, scientists did not make the association with heredity for many
more decades. To make this connection, scientists, including a number of microbiologists, performed many
experiments on plants, animals, and bacteria.
Mendel’s Pea Plants
While Miescher was isolating and discovering DNA in the 1860s, Austrian monk and botanist Johann Gregor
Mendel (1822–1884) was experimenting with garden peas, demonstrating and documenting basic patterns of
inheritance, now known as Mendel’s laws.
In 1856, Mendel began his decade-long research into inheritance patterns. He used the diploid garden pea,
Pisum sativum, as his primary model system because it naturally self-fertilizes and is highly inbred, producing
“true-breeding” pea plant lines—plants that always produce offspring that look like the parent. By
experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring
that might occur if he used plants that were not true-breeding. Mendel performed hybridizations, which
involve mating two true-breeding individuals (P generation) that have different traits, and examined the
characteristics of their offspring (first filial generation, F 1 ) as well as the offspring of self-fertilization of the F 1
generation (second filial generation, F 2 ) (Figure 10.2).
Figure 10.2
In one of his experiments on inheritance patterns, Mendel crossed plants that were true- breeding for violet flower color with
plants true-breeding for white flower color (the P generation). The resulting hybrids in the F 1 generation all had violet flowers. In the F 2
generation, approximately three-quarters of the plants had violet flowers, and one-quarter had white flowers.
In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural
history society. He demonstrated that traits are transmitted faithfully from parents to offspring independently
of other traits. In 1866, he published his work, “Experiments in Plant Hybridization,” 1 in the Proceedings of
the Natural History Society of Brünn. Mendel’s work went virtually unnoticed by the scientific community,
which believed, incorrectly, in the theory of blending of traits in continuous variation.
He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until
1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the
chromosomal basis of heredity.
1 J.G. Mendel. “Versuche über Pflanzenhybriden.” Verhandlungen des naturforschenden Vereines in Brünn, Bd. Abhandlungen 4
(1865):3–7. (For English translation, see http://www.mendelweb.org/Mendel.plain.html)