Central Dogma

The Search for the Genetic Material

The Requirements of Genetic Material

Once it had been accepted that there was genetic transmission of traits, the search began for the factor that carried the information. It was established that the following characteristics were required of genetic material:

It must be able to replicate, in order to be in each cell of a growing organism.
It must be able to control expression of traits. Since we know that traits are determined by the enzymes and proteins that act within us, and that these proteins are determined by their sequences, the genetic material must be able to encode the sequence of proteins.
It must be able to change in a controlled way, in order to ensure survival of a species in a changing environment.

We will show in this chapter how DNA fulfills all of these requirements, and how it was discovered that DNA was, in fact, the genetic material.

Hereditary Material is Bound on Chromosomes

The identity of Mendel's "factors" remained unsubstantiated until the turn of the century, some forty years after Mendel's painstaking experiments. At that time, two exciting scientific developments came together, allowing scientists to actually see the material found inside the cell's nucleus . These two developments were the construction of increasingly powerful microscopes and the discovery of dyes or stains that selectively colored the various components of the cell. As scientists examined cellular nuclei, they observed long, thin, rod-like structures, which tended to become colored when the cell was treated with certain stains. They called these nuclear structures chromosomes . Many more microscopic observations confirmed the role of chromosomes:

A variety of chromosome types, as defined by relative size and shape, were found to be present in the nucleus of each cell. Furthermore, there usually were two copies of each type of chromosome. This cell is called a diploid cell.
All of the cells of an organism, excluding sperm cells, egg cells, and red blood cells, and all organisms of the same species, were observed to have the same number of chromosomes.
The number of chromosomes in any cell appeared to double immediately prior to the cell division processes of mitosis and cytokinesis, in which a single cell splits to form two identical offspring cells.
The sex or germ cells (i.e., sperm and egg) appeared to have exactly half of the number of chromosomes as were found in the non-germ or somatic cells of any organism. Furthermore, the germ cells were shown to have just one copy of each chromosome type. Such cells are called haploid cells.
The fertilization of an egg with a sperm cell produces a diploid cell called a zygote, which has the same number of chromosomes as the somatic cells of that organism.

Suddenly, the implications of Mendel's work became obvious: chromosomes behaved like the particles or factors that Mendel described. Mendel's hereditary factors were located on the newly discovered chromosomes or were the chromosomes themselves.

Proof that the chromosomes were Mendel's hereditary factors did not come until 1905, when the first physical trait was shown to be the result of the presence of specific chromosomal material and, conversely, that the absence of that specific chromosome meant the absence of the particular physical trait. Microscopic observations had discovered the presence of what have come to be called the sex chromosomes. These chromosomes, distinguished from other chromosomes and from each other by their size, were named "X" and "Y."

Here is a representation of the 23 paired chromosomes of the human male, dissected away from a cell and laid out in order:

Human Chromosomes Diagram

Researchers in 1905 were surprised to observe that somatic cells taken from female donors always contained two copies of the X chromosome, while somatic cells taken from male donors always contained one copy of the X chromosome and one copy of the Y chromosome. All of the other chromosomes in the nucleated cells of both male and female donors appeared identical. Although scientists were not sure of the mechanism, it seemed quite clear that the sex of an organism was directly related to the identity of the chromosomes in that organism's cells. Thus, sex was shown to be the direct result of a specific combination of chromosomal material, and sex became the first phenotype (physical characteristic) to be assigned a chromosomal location - specifically the X and Y chromosomes.

Which Chromosomal Subunity Carries Hereditary Information?

Quantitative analysis of chromosomes shows a composition of about forty percent DNA and sixty percent protein. At first, it seemed that protein must be responsible for carrying hereditary information, since not only is protein present in larger quantities than DNA, but protein molecules are composed of twenty different subunits while DNA molecules are composed of only four. It seemed clear that a protein molecule could encode not only more information, but a greater variety of information, because it possessed a substantially larger collection of ingredients with which to work.

Pamela Peters, from "Biotechnology: A Guide To Genetic Engineering." Wm. C. Brown Publishers, Inc., 1993.
Taken from Genentech's Access Excellence

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