MODERN BOTANY - P. RAVEN - 1990

SECTION III. GENETICS

CHAPTER 8. CHEMISTRY AND HEREDITY

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Ever since humans first began to observe the world around them, The phenomenon of heredity has puzzled and amazed them. Why is it that the offspring of all living creatures—whether a dandelion, a dog, an aardvark, or an oak tree—always resemble their parents and never show a resemblance to other species? Why does a child have its mother's eyes, its father's chin, or, even more remarkably, its grandfather's Nose?

These questions were already being asked in the times of the ancient Greeks, and were probably not new even then. Such problems have always been significant. Throughout history, biological inheritance remained a primary factor in the distribution of wealth, power, lands, and privileges. From a biological perspective, heredity has always been an essential component of any DEFINITION OF LIFE.

Some say that the twentieth century will be remembered by mankind for the time when a living creature first reached the Moon; others believe it will go down in history as the era of the Discovery of the Nature of DNA and, consequently, as the beginning of unraveling the mysteries of heredity.

The Chemistry of the Gene: DNA or Protein?

Biologists had long known that heredity is associated with the Cell Nucleus and, specifically, with Chromosomes (Fig. 8-1). Eukaryotic chromosomes are complexes of DNA and protein that appear as thin threads when appropriately stained and viewed under a Light Microscope. Because researchers established that chromosomes are the carriers of Genetic information, the problem for a long time boiled down to this: do Proteins or DNA play the primary role in heredity?

By the early 1950s, a substantial body of evidence had accumulated favoring DNA as the carrier of genetic information, namely: (1) specific staining Methods showed that DNA is present in the chromosomes of all Cells and that the bulk of it is located precisely within the chromosomes; (2) PLANT AND ANIMAL body cells contain twice as much DNA as their Gametes; (3) as shown in Table 8-1, The ratio of Purines to Pyrimidines varies from species to species (such variations are essential for molecules associated with the "language of life"). Even more importantly, The amount of guanine in DNA is always equal to the amount of cytosine, and the amount of adenine is equal to that of thymine. These ratios, known as Chargaff's rules, played a tremendous role in decoding The process of heredity; (4) DNA isolated from certain bacterial cells can alter the genetic traits of other Bacteria (Fig. 8-2); (5) upon infection of bacterial cells by Bacteriophages, only the DNA enters The Cell; it directs The formation of new Viral Particles (Fig. 8-3).

Table 8-1. Nucleotide Composition of DNA from various animal species and humans (in percentages)

Fig. 8-2. The identification of the "transforming factor" can be considered a decisive experiment in establishing the Biological Role of DNA. "Smooth" pneumococci are pathogenic (named so because when grown on Agar they form polysaccharide capsules that give the colonies a glossy, smooth appearance). "Rough" pneumococci are non-pathogenic. Both traits are hereditary: the progeny of "smooth" cells form "smooth" colonies, whereas the progeny of "rough" cells form dull, rough colonies. If "smooth" cells are killed and their cellular debris is added to a culture of "rough" cells, some of the "rough" cells acquire the traits of the "smooth" ones and form corresponding colonies. This phenomenon, known as transformation, was first observed in 1928. Sixteen years later, in 1944, it was proven that the "transforming factor"—DNA—actually alters the GENETIC APPARATUS OF "rough" cells

Fig. 8-3. A brief outline of the Hershey-Chase experiments, which proved that DNA is what encodes genetic information in Viruses. By growing virus-infected bacteria in various radioactive media, the scientists isolated two viral samples: one containing DNA labeled with radioactive ³²P, and the other containing coat protein labeled with ³⁵S (DNA contains no sulfur, and the proteins of this virus contain no phosphorus). Bacteria grown on isotope-free medium were then infected with the labeled phages. As a result, one bacterial culture was infected with the ³²P-labeled phage, and the other with the ³⁵S-labeled phage. Shortly after THE START OF the infection cycle, the cells were blended to shear off viral fragments, and the mixture was then centrifuged to separate the cells from the viral material. The scientists found that the ³⁵S label remained outside the cells, whereas ³²P entered the cells and was subsequently incorporated into the progeny viral particles. These experiments demonstrated that DNA is indeed the carrier of the hereditary information required for the formation of new phage particles

Despite the facts outlined above, the genetic role of DNA remained unclear until its Structure was elucidated.