Overview: A Borrowed Life

¥            Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli

¥            Viruses lead Òa kind of borrowed lifeÓ between life-forms and chemicals

¥            The origins of molecular biology lie in early studies of viruses that infect bacteria


Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat

¥            Viruses were detected indirectly long before they were actually seen

The Discovery of Viruses: Scientific Inquiry

¥            Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration

¥            In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease

¥            In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

Structure of Viruses

¥            Viruses are not cells

¥            A virus is a very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

Viral Genomes

¥            Viral genomes may consist of either

         Double- or single-stranded DNA, or

         Double- or single-stranded RNA

¥            Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

Capsids and Envelopes

¥            A capsid is the protein shell that encloses the viral genome

¥            Capsids are built from protein subunits called capsomeres

¥            A capsid can have various structures

¥            Some viruses have membranous envelopes that help them infect hosts

¥            These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals

¥            Viral envelopes, which are derived from the host cellÕs membrane, contain a combination of viral and host cell molecules

¥            Bacteriophages, also called phages, are viruses that infect bacteria

¥            They have the most complex capsids found among viruses

¥            Phages have an elongated capsid head that encloses their DNA

¥            A protein tail piece attaches the phage to the host and injects the phage DNA inside


Concept 19.2: Viruses replicate only in host cells

¥            Viruses are obligate intracellular parasites, which means they can replicate only within a host cell

¥            Each virus has a host range, a limited number of host cells that it can infect

General Features of Viral Replicative Cycles

¥            Once a viral genome has entered a cell, the cell begins to manufacture viral proteins

¥            The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules

¥            Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

Replicative Cycles of Phages

¥            Phages are the best understood of all viruses

¥            Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

The Lytic Cycle

¥            The lytic cycle is a phage replicative cycle that culminates in the death of the host cell

¥            The lytic cycle produces new phages and lyses (breaks open) the hostÕs cell wall, releasing the progeny viruses

¥            A phage that reproduces only by the lytic cycle is called a virulent phage

¥            Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

The Lysogenic Cycle

¥            The lysogenic cycle replicates the phage genome without destroying the host

¥            The viral DNA molecule is incorporated into the host cellÕs chromosome

¥            This integrated viral DNA is known as a prophage

¥            Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

¥            An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

¥            Phages that use both the lytic and lysogenic cycles are called temperate phages

Replicative Cycles of Animal Viruses

¥            There are two key variables used to classify viruses that infect animals

         DNA or RNA?

         Single-stranded or double-stranded?

Viral Envelopes

¥            Many viruses that infect animals have a membranous envelope

¥            Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell

¥            Some viral envelopes are formed from the host cellÕs plasma membrane as the viral capsids exit

¥            Other viral membranes form from the hostÕs nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

RNA as Viral Genetic Material

¥            The broadest variety of RNA genomes is found in viruses that infect animals

¥            Retroviruses use reverse transcriptase to copy their RNA genome into DNA

¥            HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

¥            The viral DNA that is integrated into the host genome is called a provirus

¥            Unlike a prophage, a provirus remains a permanent resident of the host cell

¥            The hostÕs RNA polymerase 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

Evolution of Viruses

¥            Viruses do not fit our definition of living organisms

¥            Since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid

¥            Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments

¥            Plasmids, transposons, and viruses are all mobile genetic elements

¥            Mimivirus, a double-stranded DNA virus, the largest virus yet discovered, is the size of a small  bacterium

¥            There is controversy about whether this virus evolved before or after cells


Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants

¥            Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide

¥            Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

Viral Diseases in Animals

¥            Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes

¥            Some viruses cause infected cells to produce toxins that lead to disease symptoms

¥            Others have molecular components such as envelope proteins that are toxic

¥            Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen

¥            Vaccines can prevent certain viral illnesses

¥            Viral infections cannot be treated by antibiotics

¥            Antiviral drugs can help to treat, though not cure, viral infections

Emerging Viruses

¥            Emerging viruses are those that suddenly become apparent

¥            Recently, a general outbreak (epidemic) of a flu-like illness appeared in Mexico and the United States, caused by an influenza virus named H1N1

¥            Flu epidemics 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 global

¥            New viral diseases can emerge when viruses spread from animals to humans

¥            Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

¥            These strains can cause pandemics, global epidemics

¥            The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the Òswine fluÓ

Viral Diseases in Plants

¥            More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots

¥            Most plant viruses have an RNA genome

¥            Plant viruses spread disease in two major modes

         Horizontal transmission, entering through damaged cell walls

         Vertical transmission, inheriting the virus from a parent

Viroids and Prions: The Simplest Infectious Agents

¥            Viroids are small circular RNA molecules that infect plants and disrupt their growth

¥            Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

¥            Prions propagate by converting normal proteins into the prion version

¥            Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions