Meselson–Stahl experiment

The first problem in understanding DNA replication was to figure out whether the mechanism of replication was semiconservative, conservative, or dispersive. In 1958, two young scientists, Matthew Meselson and Franklin Stahl, set out to discover which of these possibilities correctly …

DNA Replication

The copying mechanism to which Watson and Crick referred is called semiconservative. The sugar-phosphate backbones are represented by thick ribbons, and the sequence of base pairs is random. Let’s imagine that the double helix is like a zipper that unzips, …

DNA: the genetic material

Before we see how Watson and Crick solved the structure of DNA, let’s review what was known about genes and DNA at the time that they began their historic collaboration: 1. Genes— the hereditary “factors” described by Mendel—were known to …

Dormancy

During seed maturation , the embryo enters a quiescent phase in response to desiccation. Seed germination can be defined as the resumption of growth of the embryo of the mature seed; it depends on the same environmental conditions as vegetative …

Abscisic Acid

Unlike auxins, gibberellins, and cytokinins, the hormone abscisic acid(ABA) is represented by a single 15-carbon sesquiterpene. ABA also appears to have a more limited range of specific effects than auxins, gibberellins, and cytokinins. THE DISCOVERY OF ABSCISIC ACID As more …

Ethylene

During the 19th century, when coal gas was used for street illumination, it was observed that trees in the vicinity of streetlamps defoliated more extensively than other trees. Eventually it became apparent that coal gas and air pollutants affect plant …

Cytokinins

The cytokinins were discovered in the search for factors that stimulate plant cells to divide (i.e., undergo cytokinesis). Since their discovery, cytokinins have been shown to have effects on many other physiological and developmental processes, including leaf senescence, nutrient mobilization, …

Gibberellins

Gibberellins are members of a large and varied family of plant constituents known as the terpenoids. Terpenes are normally recognized on the basis of their chemical structure, which may be dissected into one or more 5-carbon isoprene units. Gibberellins are …

Auxins

The first plant hormone we will consider is auxin. Auxin deserves pride of place in any discussion of plant hormones because it was the first growth hormone to be discovered in plants, and much of the early physiological work on …