Title: Unit 3 Learning Module
Course: SAT 170 Introductory Biotechnology > Section: SAT 170 Introductory Biotechnology

Unit 3 Learning Module

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DNA-The Molecule of Heredity

Discovery of DNA as the Genetic Material

DNA carries our genetic information, and is passed from one generation to the next. This information is coded for in a sequence of nucleotides which has all of the information that is needed to control life’s activities. As we will examine in the next unit, these DNA nucleotides are first copied into RNA nucleotides. This RNA is then used to build proteins. It is these proteins that give us our particular characteristics. Although it is now considered to be common knowledge that DNA is our genetic information, this discovery was attributed to the work of several researchers over a many years. We will discuss the most significant of those experiments in more detail.

- The first of these experiments was conducted in 1928 by Frederick Griffith. Griffith was studying Streptococcus pneumoniae, a pathogenic bacterium which causes pneumonia. There were two strains of this bacterium: an R strain which was non-pathogenic and an S strain which was pathogenic. When mice were injected with the R strain bacteria, they did not get sick, but when they were injected with the S strain bacteria they developed pneumonia and died. Griffith next decided to first heat kill the S strain bacteria before injecting them into mice and the mice did not get sick. However, when combining the heat-killed S cells with live R cells, he noticed that the R cells were transformed into pathogenic cells. This led Griffith to the question of what was being transferred from the heat-killed S cells to the live R cells causing their transformation. He believed that the answer to this question would lead him to identifying the genetic information. This question would eventually be answered by some of his graduate students; Avery, McCarty and MacLeod in 1944. This group of researchers decided to introduce purified cell extracts of the heat-killed S cells into the R cells. The one fragment which was able to cause the transformation was the one which contained the DNA. This led the researchers to the conclusion that DNA carried the genetic information.

Figure 1

- Shortly after the transformation experiment was complete, Alfred Hershey and Martha Chase conducted an experiment with the same goal. The focus of their experiment was the bacteriophage, which is a type of virus that infects bacteria. It consists entirely of DNA and protein. The way that a virus works is by injecting its genetic information into its host cell in order to take over its host cell machinery in order to make copies of itself. Hershey and Chase used radioisotopes to label the DNA component as well as the protein component of the virus. When they traced the radioisotopes, they observed that it was the DNA fragment that ended up inside the host cell, thus, coming to the same conclusion that DNA carries the genetic information.

See Figure 1: Griffiths transformation experiment was integral to the discovery of DNA as the genetic material. You can see in the image here that the R strain is not normally pathogenic but when combined with the heat-killed S strain, they can be transformed into a pathogenic form.

DNA Structure

Shortly after the experimentation which elucidated DNA as the molecule which carries our genetic information, came research into the discovery of the structure of DNA. Key to the discovery of the structure of DNA was an x-ray photograph of a DNA molecule known as photo 51, taken by Rosalind Franklin. Another individual who is primarily responsible for our current understanding of DNA was Erwin Chargaff. After examining the content of the DNA of several organisms, he came up with what is now referred to as Chargaff’s rules. These rules are as follows: the composition of DNA varies from one species to another; purines and pyrimidines will always be present in equal amounts, with the amount of adenine equaling the amount of thymine and the amount of cytosine equaling the amount of guanine. Using Franklin’s photograph, along with the valuable information about the content of DNA contributed by Erwin Chargaff, researchers were able to determine the features of DNA structure.
See Figure 2: The famous photo 51, taken by Rosalind Franklin was instrumental in elucidating the structure of DNA.

Figure 2

Figure 3

In 1953, a double helix model of DNA structure was introduced by James Watson and Francis Crick. There are several key characteristics of DNA.
See Figure 3: This model illustrates some of the features of DNA structure. Note the gray, yellow and white components which represent the sugar-phosphate backbone, while the blue and red components represent the paired nitrogenous bases.
- DNA (deoxyribose nucleic acid) is a double helix composed of two strings of nucleotides twisted into a ladder structure.

- A nucleotide is made of a phosphate, a sugar, and a nitrogenous base.
- There are four types of DNA bases: adenine (A), guanine (G), cytosine (C), thymine (T). A and G are considered to be purine nucleotides, while C and T are pyrimidines.
- The sugar is known as deoxyribose and it contains 5 carbon atoms, which are labeled 1’- 5’.
- The two strands of the DNA molecule are said to be anti-parallel, where one strand is oriented in one direction (5’ to 3’; pronounced 5 prime to 3 prime) and the parallel strand runs in the opposite direction (3’ to 5’).
- The rungs of the DNA “ladder” are made of bases held together with hydrogen bonds. These bases exhibit complementary base pairing where A always bonds to T, C always bonds to G. A is bound to T with two hydrogen bonds, while C is bound to G with three hydrogen bonds. This complementary base pairing is important because if you know the sequence of one strand of DNA, then you know the sequence of the complementary strand.
- The sides of the ladder are made of alternating phosphates and sugars of the nucleotides. The nucleotides are held together via a strong, covalent bond known as a phosphodie

See Figure 4: Highlighted here is a close-up image of a DNA molecule. Note the bonds between the sugar and phosphate of different nucleotides as well as the hydrogen bonds between the complementary nitrogenous bases. The 5’ and 3’ nature of each strand is also observable in this image.

See Figure 4: