Nucleic Acid Structure

Overview

This module looks at evidence that nucleic acids are the genetic material, and examines the structure of nucleic acids (DNA and RNA), which underlies their function.

Objectives

1. Understand the experiments that led DNA to be identified as the genetic material.
2. Know that DNA is a polymer of nucleotides, and that each nucleotide consists of a sugar, a phosphate, and a nitrogenous base. Know that there are four types of bases routinely used in DNA, and that they fall into two basic types: purines and pyrimidines. It is not necessary to know the exact molecular structure of each base.
3. Understand that RNA is similar in structure to DNA, but know the differences between the two types of molecules.
4. Understand how nucleic acids denature.

Genetic Material

When Mendel proposed the existence of his 'factors' (see the discussion of Mendel's postulates), he had no idea exactly what form those factors might have. Even in the first half of this century, there was considerable debate about the nature of the genetic material. Some thought it must be made protein, whereas others believed it was made of nucleic acid. In spite of this uncertainty, there was consensus about what properties the genetic material must have.

Properties of the Genetic Material

• The genetic material must be able to store hereditary information.
• The genetic material must be able to be replicated (reproduced).
• The information stored in the genetic material must be able to be expressed.
• The genetic material must be able to undergo change (mutation/recombination).

These properties can be grouped into three essential functions of the genetic material: the genotypic function (storage and replication of hereditary information to be passed on to future generations), the phenotypic function (expression of the stored information as a physical or behavioral trait), and the evolutionary function (change in the information over time so that organisms can adapt to environmental changes).

The first real evidence of the nature of the genetic material came from studies of genetic changes in bacteria.

Bacterial Transformation

The discovery of the principle of bacterial transformation provided strong evidence to support DNA as the genetic material. This discovery came from research on Diplococcus pneumoniae, a bacterium that causes pneumonia. There are two strains of this bug: the type 'S' strain, which forms smooth colonies on agar plates and is virulent (causes disease) because the bacteria are covered with a polysaccharide capsule; and the type 'R' strain, which forms rough colonies on agar plates and is avirulent (does not cause disease) because the bacteria do not have a capsule. The virulence properties are summarized in the series of diagrams below.

When mice are injected with type S cells (from smooth colonies), the mice die of pneumonia.

When mice are injected with type R cells (from rough colonies), the mice survive.

When type S cells are heat killed and then injected, the mice survive, indicating that the cells must be alive to be virulent.

When living type R cells were mixed with heat-killed type S cells and injected, the mice surprisingly died. Even though either cell type alone is harmless, when combined, they become lethal. What's more, live type S cells could be isolated from the dead mice! It was concluded that the presence of the dead type S cells was somehow transforming the live type R cells into type S cells. In other words, the dead type S cells were causing a genetic change in the type R cells. But what was the transforming substance?