The March of Heredity
Pamela Peters, from "Biotechnology: A Guide To Genetic Engineering." Wm. C. Brown Publishers, Inc., 1993.
The Storage and Transmission of Hereditary Information
In the mid-1800s, a monk named
working in Brunn, Austria, carried out an amazing piece of scientific
detective work. Mendel observed that the offspring of certain plants
had physical characteristics similar to the physical characteristics
of the plants' parents or ancestors. Gregor Mendel
wondered why related organisms, both plant and animal, tended to
resemble one another and how familial resemblances might be explained.
Gregor Mendel reasoned that close observation of inheritance
might provide him with the answer for which he searched. He therefore
set out to examine and quantify the physical traits in pea plants
(because of their speedy reproductive cycles) in an attempt to predict
the traits that would occur in future generations.
During years of painstaking work, Mendel counted many thousands of instances of seven different traits, including plant height, flower color and position, seed color and shape, and pod color and shape. Mendel concluded that certain particles or "factors" were being transmitted from parent to offspring and so on, thus providing a connection from one generation to the next. Mendel suggested that these factors were directly responsible for physical traits. His interpretation of the experimental data further suggested that each individual had not one, but two factors for each trait, and that these factors interacted to produce the final physical characteristics of the individual. Both the location and the identity of Mendel's factors remained unknown for years.
The Cell Nucleus - Storehouse for Hereditary Information
In 1943, a Danish biologist named Joachim Hammerling carried out an important experiment in which he searched for the part of a cell that directs its physical appearance or phenotype. Hammerling used large unicellular green algae called Acetabularia. Each individual of the Acetabularia species is composed of one single, large cell about 6 cm long. Each cell has three main body parts: a foot or base containing the nucleus and anchoring the cell to a rock or other support, a stalk resembling a plant stem, and a cap that carries out the process of photosynthesis.
Hammerling used two different species of Acetabularia. One of the species, Acetabularia mediterranea, has a disk-shaped cap, while the other species, Acetabularia crenulata, has a branched cap, more like a flower. Hammerling cut the stalk and cap off of an A. mediterranea cell and grafted a stalk from A. crenulata in its place. He than asked the question: What type of cap - disk or branched - will grow on the A. mediterranea base?
The answer was that a modified branched cap, with marked similarity to A. crenulata caps, grew. Hammerling then removed the new cap from the A. mediterranea base to see what type of cap would now grow on the grafted stalk. This time a disk-shaped cap, just like the A. mediterranea caps, grew.
This experiment suggested to Hammerling that the factor directing the growth of the algal cap was located in the base of the cell. He felt that the growth of the modified branched cap resulted from the presence of a message of some sort in the grafted stalk. That message was used up in directing the production of the modified A. mediterranea cap, so that when a second cap was regenerated, it grew according to instructions derived from the nucleus contained in the base of the cell.
Hammerling concluded from these experiments that the nucleus of the cell was both directing its development and somehow specifying its hereditary characteristics. Later experiments by other scientists confirmed this conclusion and broadened its implications to suggest that not only did the nucleus direct the synthesis of new parts of an individual cell, but also that the nucleus directed the growth and development of entire multicellular organisms. These experiments reinforced and expanded data gathered by the microscopic analysis of cellular nuclei which had begun in the 20th century.
Go to Graphics Gallery: Mendel: Experiment 1,
Go to: MendelWeb at Brown University
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