The viruses you will meet in this chapter and the concepts they illustrate
In this book, you will learn how miniscule viruses enter, take over, and kill our cells. A molecular understanding of these processes provides insights into how both viruses and their host cells function at a molecular level. It is an exciting time to study viruses because of the breadth and depth of techniques available to investigate them, and the increasing number of known viruses and viral genome sequences (including those for viruses that have never been cultivated in the laboratory). In addition, there are many practical applications of virology. For example, basic research on the molecular biology of the human immunodeficiency virus (HIV) ultimately led to the first treatments that moved HIV infection away from being a death sentence to a chronic, controllable illness (Figure 1.1
). Many applications such as these attract people to the field of virology.
In this chapter, we examine the origins of virology in order to explain how molecular and cellular virology fit into the broader discipline of virology. Molecular biology is fundamentally concerned with how macromolecules, especially proteins and nucleic acids, function to control the structure and behavior of cells. By extension, molecular and cellular virology studies focus particularly on the interactions among viral proteins, viral nucleic acids, cellular proteins, cellular nucleic acids, and cellular organelles. In Technique Box 1.1
, we will see that some of the consequences of viral infection can be
observed with a light microscope. After, we consider the characteristics shared by all viruses (Section 1.5
). We will then discuss viral diversity (Section 1.6
), especially with respect to their genomes and the mechanisms by which they synthesize mRNA (Section 1.7
). We will also explain, in Section 1.7
, how diverse viruses have been named and classified. We will encounter the general method of propagating viruses in a laboratory setting in Section 1.8
. The chapter continues with a consideration of the abundance of viral sequences in the human genome (Section 1.9
); indeed, DNA of viral origin is found in almost every known cellular genome, where it contributes to the evolution of organisms, including humans. The chapter concludes with a consideration of how sequences of viral nucleic acids and proteins can be used to generate hypotheses about the evolution, structure, and function of viruses and their component parts.
Our goal in this chapter is to prepare you for the rest of the book. Chapter 2
explains how we will divide the virus replication cycle into several parts. Chapters 3
will address each of these parts of the cycle in turn, with Chapters 5
focused on the different Baltimore classes of viruses and how they express and replicate their genomes. In Chapter 12
, we will learn how viruses generally interact with host processes such as translation and apoptosis, and in Chapter 13
, we will see how viruses can cause integrated and persistent infections that can last for the entire life span of their hosts. In Chapters 14
we will examine how hosts fight back against viral infections. Chapter 16
is about clinical applications of virology, such as vaccines, gene therapy, and antiviral drugs. Chapter 17
concludes with a discussion of the diversity and evolution of viruses.
Molecular and cellular virology
investigates the molecular and cellular aspects of virus infection. For example, a typical research project in molecular and cellular virology would be to determine the function of each protein encoded by a viral genome. Another typical project would be to determine the subcellular localization of virus assembly in infected cells and the cellular factors that are required for that subcellular localization. Molecular and cellular virology studies have provided a molecular explanation for why HIV infects cells of the immune system but not other types of cells. Other studies conducted through the lenses of molecular and cellular biology have helped us begin to understand why the closely related severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses cause similar yet distinct diseases (Figure 1.2
Molecular and cellular virology also has practical applications such as the design of antiviral medicines
). Antiviral medications typically interfere with viral enzymes not normally found in human cells, whereas vaccines cause a protective immune response to develop without making the treated person sick. Furthermore, viruses can be genetically engineered to serve as agents of gene therapy
, which includes the introduction of a functional gene to reverse the effects of nonfunctional or dysfunctional inherited genes. Although we will not address in detail how translational scientists
take findings from basic research and turn them into treatments or vaccines, the book will point out many instances where basic research has been used to better human health.
The meaning of the Latin word virus
is poison. The name indicates that viruses were first discovered because of their role in producing disease in agriculturally important plants or animals, such as tobacco or livestock. In the late nineteenth century, European microbiologists invented a filter with pores smaller than the diameter of a bacterium that could remove all bacteria from a liquid suspension (Figure 1.4
). Using this device to study tobacco mosaic disease revealed that the infectious agent could not be removed by filtration, and so the infectious substance must be smaller than a bacterium. Soon thereafter, other microbiologists showed that viruses were particles, not liquids, and that a virus also causes the agriculturally important foot-and-mouth disease, which can infect a...