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
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.
Vocabulary
Directions:
Use the context of the reading passage to write a definition for the
following words.
1. conducted
2. unprecedented
3. atypical
4. progeny
5. retained
6. derived
7. transmits
8. foundation
9. geneticist
10. bound
11. complementary
12. duplicate
13. template
14. offspring
15. precise
16. landmark
17. amplifies
18. paternity
Biological Terms
Directions:
Using the information provided in the reading, write a definition for
each of the following biological terms.
1. double helix
2. purines
3. pyrimidines
4. DNA replication
5. Polymerase Chain Reaction
Important Names to Know
Directions:
Match the scientists with their achievements.
Scientist Achievement
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
Comprehension Questions
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?
Essay Question
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.