CHAPTER 19: VIRUSES
 OVERVIEW
 Viruses called _________________________can infect and set in motion a genetic takeover of
bacteria, such as Escherichia coli
 Viruses lead “a kind of __________________________ life” between life-forms and chemicals
 The origins of molecular biology lie in early studies of __________________ that infect bacteria
I. Concept 19.1: Structure of a Virus
 Most viral research has been done with the __________________________ virus (TMV).
A. Discovery of Viruses
1. _______________ (1883)
 Demonstrated that TMD (tobacco mosaic disease) was _______________________
 Proposed that the infectious agent was a small __________________
2. D. ___________________ (1890’s)
 Proposed that TMV was caused by a __________________ too small to be trapped by a
filter or it produced a filterable toxin
3. M. _______________________(1897)
 Proposed that TMD was caused by a reproducing particle much smaller and
________________ than a bacterium
 Noted that the pathogen:
a. reproduced only within a ________________cell
b. could not be cultured on media
c. could not be killed by ________________
4. _________________________ (1935)
 _________________ the TMV
 Viruses were first observed in 1950’s with ___________________ microscope.
B. Structure of Viruses
 A virus is not a cell but a very small infectious particle consisting of ___________________ surrounded
by a _______________________ and, in some cases, a membranous ____________________
1.Viral Genomes
 Viral genomes may consist of either
• Double- or single-stranded ____________, orDouble- or single-stranded ________________
• Depending on its type of ___________________, a virus is called a DNA virus or an RNA virus
2. Capsids and Envelopes
 A __________________ is the protein shell that encloses the viral genome
 Capsids are built from protein subunits called ________________________
 A capsid can have various structures
 Some viruses have membranous envelopes that help them infect ______________
 These viral envelopes surround the capsids of _____________ viruses and many other viruses found in
animals
 Viral envelopes, which are derived from the host cell’s membrane, contain a combination of
__________ (protein and glycoproteins) and ___________ (phospholipids and membrane proteins )
cell molecules
 ___________________, also called phages, are viruses that infect bacteria
 They have the most ___________________ capsids found among viruses
 Phages have an elongated capsid head that encloses their __________________
 A protein _________________ piece attaches the phage to the host and ______________the phage
DNA inside
II. Concept 19.2 Viral Reproduction
 Viruses are _________________ intracellular parasites, which means they can reproduce only within a
________________ cell
 Each virus has a ________________________, a limited number of host cells that it can infect
A. General Features of Viral Reproductive Cycles
1. Once a viral genome has entered a cell, the cell begins to manufacture ________________________
2. The virus makes use of host _________________, ribosomes, tRNAs, _______________, _________,
and other molecules
3. Viral nucleic acid molecules and capsomeres spontaneously _____________________ into new viruses
B. Viral Reproductive Cycles
 ________________ are the best understood of all viruses
 Phages have two reproductive mechanisms: the ___________ cycle and the _________________cycle
1. The Lytic Cycle
 The lytic cycle is a phage reproductive cycle that culminates in the ________________ of the host cell
 The lytic cycle produces new ________________ and digests the host’s _______________, releasing
the progeny viruses
 A phage that reproduces only by the lytic cycle is called a _______________phage
 Bacteria have _________________ against phages, including restriction enzymes that recognize and
____________ up certain phage DNA
 Stages of Lytic Cycle:
a. _____________________
b. _______________of phage DNA and _____________________ of host DNA
c. __________________ of viral genomes and proteins
d. ___________________
e. _________________ (ruptures) and __________________ phage particles
2. The Lysogenic Cycle
 The lysogenic cycle replicates the phage genome _____________________________ the host
 The viral DNA molecule is incorporated into the host cell’s ____________________
 This integrated viral DNA is known as a ______________________
 Every time the host divides, it copies the phage DNA and passes the copies to ________________cells
 An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to
the ________________ mode
 Phages that use both the lytic and lysogenic cycles are called ______________________phages
C. Reproductive Cycles of Animal Viruses
 Two key variables in classifying viruses that infect animals:
 _________________?
 Presence or absence of membranous ________________ derived from host cell membrane
1. Viral Envelopes
 Many viruses that infect animals have a membranous ___________________
 Viral ______________________ on the envelope bind to specific ______________________ molecules
on the surface of a host cell
 Some viral envelopes are formed from the host cell’s _______________________ as the viral capsids
exit
 Other viral membranes form from the host’s _____________________ envelope and are then replaced
by an envelope made from Golgi apparatus membrane
2. RNA as Viral Genetic Material
 The broadest variety of RNA genomes is found in viruses that infect _____________________
 Retroviruses use ________________________ to copy their RNA genome into DNA
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_____________ (human immunodeficiency virus) is the retrovirus that causes _____________
(acquired immunodeficiency syndrome)
The viral DNA that is integrated into the host genome is called a _________________
Unlike a prophage, a provirus remains a _________________resident of the host cell
The host’s ____________________________ transcribes the proviral DNA into RNA molecules
The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus
particles released from the cell
D. Evolution of Viruses
 Viruses do not fit our definition of _____________________ organisms
 Since viruses can reproduce only _________________ cells, they probably evolved as bits of cellular
nucleic acid
 Candidates for the _________________ of viral genomes are plasmids, circular DNA in bacteria and
yeasts, and transposons, small mobile DNA segments
 Plasmids, transposons, and viruses are all ____________________ genetic elements
III. Concept 19.3: Viruses, Viroids, and Prions
 ________________ caused by viral infections affect humans, agricultural crops, and livestock
worldwide
 Smaller, less complex entities called _____________ and ____________ also cause disease in plants
and animals, respectively
A. Viral Diseases in Animals
 Viruses may damage or kill cells by causing the release of ___________________________ from
lysosomes
 Some viruses cause infected cells to produce ___________________ that lead to disease symptoms
 Others have envelope __________________ that are toxic
 _______________________ are harmless derivatives of pathogenic microbes that stimulate the
immune system to mount defenses against the actual pathogen
 Vaccines can __________________ certain viral illnesses
 Viral infections cannot be treated by _____________________
 Antiviral drugs can help to _____________, though not cure, viral infections
B. Emerging Viruses
 ___________________ of “new” viral diseases in humans are usually caused by existing viruses that
expand their host territory
 _________________________ are caused by new strains of influenza virus to which people have little
immunity
 Viral diseases in a small isolated population can emerge and become __________________
 New viral diseases can emerge when viruses spread from ______________ to __________________
 Viral strains that jump _______________________ can exchange genetic information with other
viruses to which humans have no immunity
 These strains can cause ______________________, global epidemics
C. Viral Disease in Plants
 Most plant viruses have an _________________ genome
 Plant viruses spread disease in two major modes:
 Horizontal transmission-entering through ___________________________
 Vertical transmission-inheriting the virus from a ______________________