Into the Looking Glass

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:

1. 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.
   
   
2. 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.
   
   
3. 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.
   
   
4. 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.
   
   
5. 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."

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.

This question was finally answered in the early 1950s by using a type of virus called bacteriophage T2 or phage T2, to infect bacterial cells. Scientists Alfred Hershey and Martha Chase carried out experiments in which they prepared one group of phage particles which had incorporated radioactive phosphorus into their DNA molecules, and another group which had incorporated radioactive sulfur into the protein molecules of the virus coat. This radioactive labeling allowed phage protein to be distinguished from phage DNA by the different radioactive energies associated with each component.

Hershey and Chase knew that virus particles, including phage T2, function by binding to a target cell and injecting chromosomal material into that cell. The injected viral material contains hereditary information which directs the synthesis of new virus particles using the machinery of the infected cell. Eventually, the cell bursts and releases new virus particles, which are able to infect more cells.

Hershey and Chase asked a most important question: What component of the virus - protein or DNA - directed the synthesis of new virus particles in the infected cell? In other words, does the virus store hereditary information in the viral DNA or in the viral protein? To answer this question, these researchers allowed their radioactively labeled virus particles to infect target cells. After the viral chromosome had been injected into the cell, Hershey and Chase then determined the location of the viral DNA and of the viral protein by looking for radioactive phosphorus and radioactive sulfur, respectively. Their experiments detected the presence of only radioactive phosphorus in the infected cells. These data suggested that because only viral DNA, and not viral protein, could be found in the infected cells, viral DNA must be carrying the hereditary information.

Go to Graphics Gallery: Human Chromosomes

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