DNA and Genetics
Loretta F. Kasper, Ph.D.
Gregor Mendel, the Father of Genetics
Gregor Mendel (1823-1884) discovered the rules of heredity. The experiments that led Mendel to this important discovery were conducted in a monastery at Brunn during the years 1856 to 1864. Mendel developed his theory of heredity based on his work with pea plants. His work was so brilliant and unprecedented at the time it appeared that it took thirty-four years for the rest of the scientific community to catch up to it. The short monograph, "Experiments with Plant Hybrids," in which Mendel described how traits were inherited, has become one of the most enduring and influential publications in the history of science.
Mendel was not a scientist by profession, rather he was an Augustinian monk who taught natural science to high school students. Mendel's attraction to research was based on his love of nature. He was not only interested in plants, but also in meteorology and theories of evolution. Mendel often wondered how plants obtained atypical characteristics. On one of his frequent walks around the monastery, he found an atypical variety of an ornamental plant. He took it and planted it next to the typical variety. He grew their progeny--offspring--side by side to see if any of the parent plants' traits would be passed on to the next generation. He found that the plants' respective offspring retained the essential traits of the parents. This simple test gave birth to the idea of heredity.
Once Mendel crossed peas and mice of different varieties and saw that traits were inherited in certain numerical ratios. He then came up with the idea of dominant and recessive genes and decided to test this idea in peas. From his studies, Mendel derived certain basic laws of heredity. His first basic law stated that hereditary factors do not combine, but are passed complete to the next generation. The second law stated that each member of the parental generation transmits only half of its hereditary factors to each offspring (with certain factors dominant over others). Finally, the third basic law stated that different offspring of the same parents receive different sets of hereditary factors. Mendel's work became the foundation for modern genetics.
DNA--The Carrier of Genetic Information
The true identity of Mendel's hereditary factors remained a mystery for more than forty years. At that time, two exciting scientific developments came together. These developments finally allowed scientists to 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. Suddenly, it became obvious that Mendel's hereditary factors were closely linked to the newly discovered chromosomes.
We now know that chromosomes are made up of about forty percent DNA, or deoxyribonucleic acid, and sixty percent protein. Because chromosomes contain larger amounts of protein than of DNA, scientists initially thought that protein was responsible for carrying hereditary information. However, in 1952, two American scientists named Martha Chase and Alfred Hershey discovered that the genetic information of the cell was carried in DNA molecules. For almost all organisms, these important DNA molecules are located in the nucleus of the cell. The cell nucleus contains dioxyribonucleic acid (DNA), as well as ribonucleic acid (RNA). Both DNA and RNA are necessary for heredity.
Despite proof that DNA carries genetic information from one generation to the next, the structure of DNA and the means by which genetic information is passed on to the next generation remained the single greatest unanswered question in biology until 1953. It was in that year that James Watson, an American geneticist, and Francis Crick, an English physicist, proposed that the structure of DNA was a double helix. This means that the DNA molecule consists of two strands which are twisted around each other.
According to Watson and Crick, heredity is transmitted through information chemically coded in DNA. The two-stranded DNA molecule is made up of four chemical bases of two different types. The first type, the purines, consist of adenine (A) and guanine (G). The second type, the pyrimidines, consist of cytosine (C) and thymine (T). Each chemical base is bound, or connected, to another partner base. A purine must bind with a pyramidine, and so adenine is bound to thymine and guanine is bound to cytosine.
In the late 1940's, Erwin Chargaff and his colleagues at Columbia University discovered that the four chemical bases may occur in a variety of proportions in the DNA's of different organisms, but that he number of A bases is always equal to the number of T bases and the number of G bases is always equal to the number of C bases. In the double helix the two strands of DNA run in opposite directions and consist of complementary (matched) pairs of A - T and G - C base pairs. Both strands of the double helix contain the same genetic information, and this information is passed from one generation to the next by a process called DNA replication.
DNA replication is a process by which the essential chemical components of each DNA molecule are duplicated, or reproduced. The complementary pairing of the chemical bases ensures that, when DNA replicates, an exact duplicate of the parent molecule's genetic information is produced. In the process of replication, the strands of the double helix separate and unwind, and each separated strand becomes a template, or a pattern, for a new partner strand. The result is that in both offspring of a divided cell, each DNA molecule has one original strand and one newly synthesized strand. This process ensures precise copying of the nucleotide base sequences in DNA.
Actually, the DNA's instructions are not transmitted directly; a copy consisting of ribonucleic acid (RNA) serves as an intermediate step in the process. The original DNA remains safely in the nucleus, while the RNA copy is produced by duplicating just one strand of DNA, which carries the instructions for manufacturing protein.
Making Multiple Copies of DNA: The Discovery of PCR
For their landmark discovery of the structure of DNA, Watson and Crick shared the 1962 Nobel Prize for Physiology and Medicine. Yet, while the discovery of the structure and the characteristics of DNA was critical to the advancement of modern biology and genetics, the story does not end there. Because DNA replication is so essential to both present and future life, scientists after Crick and Watson continued to study this process. In 1983, a California scientist named Kary Mullis came up with an idea that revolutionized the science of modern biology. While cruising the Pacific Coast Highway, Mullis discovered a technique called the Polymerase Chain Reaction. The polymerase chain reaction, better known as PCR, is a technique that amplifies DNA. This amplification technique enables scientists to make millions - or even billions - of copies of a DNA molecule in a very short time.
Mullis won a Nobel Prize in Chemistry in 1993 for discovering the Polymerase Chain Reaction. PCR has many uses. It has been used to diagnose genetic diseases, to determine paternity, to identify a criminal, to detect bacteria or viruses (particularly the AIDS virus), and to research human evolution.
Directions: Use the context of the reading passage to write a definition for the following words.
Directions: Using the information provided in the reading, write a definition for each of the following biological terms.
1. double helix
4. DNA replication
5. Polymerase Chain Reaction
Important Names to Know
Directions: Match the scientists with their achievements.
1. Mendel A. Complementary pairs in DNA
2. Chase and Hershey B. Polymerase Chain Reaction
3. Watson and Crick C. Hereditary factors
4. Chargaff and colleagues D. Genetic information in cell
5. Mullis E. Double-helix
Directions: Using the information provided in the reading and your own words, write an answer to each of the following questions.
1. Why is Gregor Mendel called, "the father of genetics?"
2. How did Mendel make his important discoveries?
3. What is meant by "dominant" and "recessive" genes?
4. How did scientists discover that Mendel's hereditary factors were contained within the chromosomes of the cell?
5. What important piece of information was provided by Watson and Crick's discovery that DNA was a double helix?
6. What important information did Chargaff's research add to Watson and Crick's?
7. How are the nucleotide base sequences in the DNA molecule reproduced?
8. Are the instructions for reproducing a DNA molecule transmitted directly from the DNA itself? Why not?
9. Why was the discovery of PCR considered to be so revolutionary?
10. What has PCR been used for?
Research in genetics has produced many important discoveries such as the structure and function of DNA. In the past several years, a large project has been undertaken to try to determine the molecular details of the complete set of human genes. This project, known as the Human Genome Project, may be able to provide us with information about our inherent similarities and differences, as well as with some clues that will help us to discover cures for many dangerous diseases.
Use the library to find at least two different sources of information on the human genome project. These sources should include one print (books) and one electronic (computer) source. After you have obtained the information, put it together in a well-organized essay. Be sure to cite all sources in a bibliography at the end of your essay.