Chapter 1: The Beginnings of Molecular Biology
I. Introduction
A. Introduction to Molecular Biology
1. Molecular Biology has found several important uses within modern day life a. Disease research b. Diagnosis and prevention of genetic disease c. Forensics
2. In the process, Molecular Biology has been given an almost larger than life status in modern society a. Inspired artists b. Inspired science fiction writers c. Inspired development of television shows and movies d. Challenges ethics and religious viewpoints
B. Introduction: Definition of Molecular Biology
1. The term Molecular Biology was coined by Dr. Warren Weaver in 1938
2. He defined molecular biology as the study of biological phenomena at the molecular level a. This definition covers a wide range of phenomena b. Doesn’t really explain what one is studying, when studying Molecular Biology
3. The modern definition of molecular biology is the study of the molecule of heredity (DNA) and the process of gene expression
II. Historical Perspectives
A. Historical Perspectives: The “Pre-History” of Molecular Biology
1. Molecular Biology is defined as the study of DNA and the process of gene expression
2. Modern Molecular Biology has a foundation based in genetics and is the most logical extension of the study of heredity

3. Heredity is defined as the transmission of characteristics from parent to offspring
4. The study of heredity started long ago in ancient Greece with
Aristotle (384-322 BC) a. Aristotle proposed the theory of pangenesis b. Pangenesis: Hereditary characteristics are transmitted by gemmules from individual body cells
5. Almost 2100 years passed from when Aristotle studied heredity to when the actual mechanisms of heredity were uncovered
6. The person responsible for determining the mechanism of heredity was the Austrian monk Gregor Mendel
7. Mendel bred pea plants with different varieties of pea plants at the monastery a. Seed shape b. Seed color c. Plant height d. Plant color
8. He was able to figure out how each trait was inherited from parent to offspring
9. From his studies, he was able to state two important laws pertaining to heredity a. Law of Segregation b. Law of independent assortment c. Concept of Dominance
10. However, his studies did not reveal the material responsible for housing the information of heredity
11. Historical Perspectives: The “Pre-History” of Molecular
Biology (Figure 1.3)
12. Mendel was outside the scientific community, and thus, his work although published was not exactly viewed favorably (36 years)

13. His work sat idle until 1900 when Hugo DeVries, Karl Correns and Erich Von Tschermak independently recreated his work
14. The rediscovery of Mendel’s work ushered in the age of
Mendelian Genetics
15. However, studies in Mendelian Genetics did not reveal the material responsible for housing the information of heredity
16. Others sought to discover what compound housed the genetic information
B. Historical Perspectives: The Beginnings of Molecular Biology
1. Many different scientists actually played a role in determining that DNA was the compound that housed the information of heredity a. Some were not studying DNA for this purpose b. Some were studying DNA for the purpose of determining whether it held this important information
2. Intense study into the nature of DNA occurred concurrent to the age of Mendelian Genetics
3. Many of the scientists who where studying DNA at the turn of the century were Biochemists and were not necessarily interested in its role in Biology, they more or less wanted to look at its structure
4. It was not until later on (~1930) that scientists really started to look at the relevance of DNA as a material of heredity
5. Historical Perspectives: The Beginnings of Molecular Biology
(Figure 1.2)
C. Historical Perspectives: Friedrich Meischer’s Contributions To
Molecular Biology
1. At about the same time that Mendel was working on his pea plants, Friedrich Meischer (1868) was embarking on studying the chemical makeup of cells
2. Meicher theorized that to determine the material of heredity one must understand the chemical nature of cells

3. Meicher, in order to determine the material of heredity studied the chemistry of pus
4. Pus includes bacteria, which cause an infection, as well as many white blood cells, which are called on to fight the infection
5. Miecher took the white blood cells and isolated their nuclei to study what was inside
6. Upon analysis, he expected to find protein inside the nucleus, however, he found the ratios of carbon and nitrogen to be inconsistent the presence of protein
7. As well, he found that the material was slightly acidic and importantly was high in phosphorus-he called this material nuclein
8. With further analysis of nuclein, he found that the three main components of nuclein were phosphate, sugar and a nitrogen containing base
D. Historical Perspectives: August Weisman’s Contributions To
Molecular Biology
1. August Weisman (1893) was one of the first scientists to link behavior of chromosomes to heredity
2. When studied segregation of chromosomes, he noticed that the number of chromosomes in the nucleii of germ cells is halved.
Therefore, he postulated that the material of heredity (genetic information) is located in the nucelus
3. He proposed that there was some component of germ-cells
(egg and sperm) that was important for heredity
4. From this study of the chromosomes, he developed his Germplasm Theory
5. The Germ-plasm theory states that cells in the reproductive organs carry a complete set of genetic information, and that this information is passed along to the egg and sperm
E. Historical Perspectives: The Material of Heredity DNA or Protein?

1. As Weismann’s germ-plasm theory was accepted, scientists wished to determine what specific material was responsible for heredity a. DNA b. Protein
2. In the early 1900’s many scientists thought that the material of heredity was protein a. Proteins were more complex b. 26 possible amino acids that could make up a protein c. DNA was less complex d. Only four nitrogenous bases could be found in DNA
F. Historical Perspectives: August Fredrick Griffith’s Contributions To
Molecular Biology
1. Fredrick Griffith (1928) made an important discovery to which would lead to determining whether DNA or protein was the material of heredity, although it was under-appreciated at the time
2. Griffith performed an elegant in vivo experiment using both pathogenic and non-pathogenic Streptococcus pneumoniae to infect mice
3. Griffith worked with two strains of S. pneumoniae a. Type R which was non-pathogenic and had a rough coat b. Type S which was pathogenic and had a smooth protective coat
4. When he injected type R bacteria into mice, he found that it had no effect on the mice, whereas when he injected type S bacteria, the mice became sick and died
5. Next Griffith treated Type S bacteria with heat, thus killing them
6. When he injected the heat killed type S bacteria, he found that the mice remained healthy
7. The last experiment he did was he mixed the heat killed type S bacteria with the live type R bacteria

8. He then injected this mixture, and found that the mice became sick and died
9. He concluded that there was a transfer of some component from the dead pathogenic (Type S) bacteria to the live nonpathogenic Type R bacteria to make it become pathogenic
10. He called this component the transforming principle, that when transmitted from the dead S to the live R bacteria allowed them to become pathogenic
11. Griffith was not able to determine the true chemical nature of the transforming principle
G. Historical Perspectives: In vitro Experiments Based on Griffith’s work
1. In 1931, Henry Dawson showed that the mouse was not needed for transformation a. He heat killed the pathogenic type S bacteria and then mixed it with the the live type R bacteria b. Instead of injecting the mixture into mice, he plated the mixture on agar plates c. He found some type R colonies and some type S colonies on his plates
2. In 1933, Lionel Alloway showed that a cell-free extract prepared from broken type S bacteria could also be used for transformation of live type R cells to type S cells
3. In 1941 Oswald Avery, Colin MacLeod and Maclyn McCarty took Griffith’s experiment further to determine the true chemical nature of the transforming principle a. Purified both protein and DNA from type S bacteria b. They then treated live type R cells with either the DNA,
RNA or the protein c. They found that the DNA led to transformation of the live type R to type S bacteria, whereas the other two did not
4. Supplemental Figure: Avery MacLeod and
H. Historical Perspectives: The One Gene, One Enzyme Hypothesis

1. In 1941, George Beadle and Edward L. Tatum were the first to demonstrate the link between a gene and a step in a metabolic pathway catalyzed by an enzyme
2. Beadle and Tatum worked backwards using specific mutants of the pink bread moled, Neurospora crassa in which specific chemical reactions were blocked
3. They induced mutations into the Neurospora by using Xirradiation and then plated them on minimal medium
4. As long as a metabolic step is not affected by the X-irradiation, the Neurospora should grow on the media
5. However, failure to grow on the media indicates that a growth defect has occurred
6. These mutants need media with specific supplemented compounds (usually amino acids) to grow-these mutant strains of Neurospora are known as auxotrophs
7. One can determine the specific nature of the auxotroph by placing the auxotrophic strain supplemented with various intermediates in the pathway of niacin synthesis
8. Beadle and Tatum observed a one-to-one correspondence between the genetic mutations and the lack of a specific enzyme required in a biochemical pathway, and they dubbed this the
“One Gene – one enzyme hypothesis”
9. Figure 1.5: Historical Perspectives: The One Gene, One Enzyme
Hypothesis
10. Eventually, the one gene-one enzyme hypothesis was modified to the one gene-one polypeptide hypothesis a. Not all genes encoding proteins encode enzymes b. Some genes encode structural proteins
11. This hypothesis has been amended further, because there are some genes that do not even encode polypeptides, they encode
RNAs
I. Historical Perspectives: The Hershey-Chase Experiment

1. In order to determine whether protein or DNA was being inserted into the host cell, Hershey and Chase needed to find a way to label each type of molecule
2. Hershey and Chase used the T2 bacteriophage, in their experiments
3. T2 bacteriophage is a virus that infect E. coli. Viruses are unable to reproduce on their own, they need to reproduce use a host cell
4. At the time, it was known that a virus had an outer protein coat, and inside this protein coat was DNA
5. When a T2 bacteriophage infects and E. coli, it attaches to the outside, and then injects its genetic material into the E. coli cell.
Once injected, the cell uses this genetic material to make new virus
6. What Hershey and Chase wanted to do was to figure out what got inserted into the host cell because that must be the genetic material
7. They knew protein contained sulfur, whereas DNA did not and
DNA contained phosphorus whereas proteins did not
8. They labeled proteins with a radioactive form of sulfur (S 35 )
9. They labeled DNA with a radioactive form of phosphorus (P 32 )
10. Next they created Bacteriophage that had either their DNA labeled with P 32 or their protein labeled with S 35
11. They then took their phage that either contained radioactive protein or radioactive DNA and infected E. coli with them
12. Upon infection, the viruses would bind to the outside of the E.
coli cell and insert their genetic material
13. Next, they took their mixture containing infected E. coli and used a blender to lightly shear off whatever was left that was bound to the outside of the cell
14. They then centrifuged their sample to pellet the bacteria.
This leaves any part of the phage that was not inserted into the cell left in solution (supernatant)

15. When they looked at sample in which the phage contained radioactive protein (S 35 ), they found that the radioactivity was found in the supernatant and not in the bacterial pellet
16. This suggests that protein is not inserted into the host cell, and thus protein would not be the genetic material
17. In contrast, when they looked at the sample in which the phage contained radioactive DNA (P 32 ), they found that the radioactivity was found in the bacterial pellet and not in the supernatant
18. This suggests that DNA is being inserted into the host cell, and thus, DNA would be the genetic material
J. Historical Perspectives: A Model For the Structure of DNA
1. Previously, it had been shown that DNA is composed of three different components a. Sugar b. Phosphate c. Nitrogenous bases
2. It was known that there were four nitrogenous bases a. Adenine b. Thymine c. Cytosine d. Guanine
3. Quantitative methods by Erwin Chargaff had shown that the the number of [A] = [T] and the amount of [G] =[C] (However,
[G+C] does not equal [A+T]
4. Based off of this work, and by X-ray diffraction analysis on DNA by Maurice Wilkins and Rosalind Franklin, James Watson and
Francis Crick were able to determine the 3-D structure of DNA a. Found that the shape of DNA is in the form of a helix of constant diameter b. Found that the nitrogenous bases were stacked towards the interior of the molecule, with the backbone containing sugar (deoxyribose) and phosphate

c. They were able to determine the distance between the stacked bases
K. History of Understanding the Molecular Basis of Heredity - The Three
Criteria Material of Heredity Must Meet
1. First, the genetic material must contain complex information
2. Second, the genetic material must be able to replicate faithfully-the genetic information must be copied accurately to ensure reproduction
3. Third, genetic material must encode phenotype-must be a mechanism by which a gene is expressed, such that a phenotype is obtained.