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Questions and Answers
What is the main characteristic that distinguishes viruses from cells?
What is the main characteristic that distinguishes viruses from cells?
What type of nucleic acid is found in RNA viruses?
What type of nucleic acid is found in RNA viruses?
What is the name of the disease caused by the tobacco mosaic virus?
What is the name of the disease caused by the tobacco mosaic virus?
Who crystallized the infectious particle of tobacco mosaic virus in 1935?
Who crystallized the infectious particle of tobacco mosaic virus in 1935?
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What is the term used to describe the kind of life that viruses lead?
What is the term used to describe the kind of life that viruses lead?
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What is the name of the bacteria that can be infected by bacteriophages?
What is the name of the bacteria that can be infected by bacteriophages?
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What is the primary function of reverse transcriptase in retroviruses?
What is the primary function of reverse transcriptase in retroviruses?
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What is the term for the process by which a virus forms a membranous envelope from the host cell's plasma membrane?
What is the term for the process by which a virus forms a membranous envelope from the host cell's plasma membrane?
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Which of the following is NOT a characteristic of proviruses?
Which of the following is NOT a characteristic of proviruses?
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What is the term for the small, mobile genetic elements that are thought to be the source of viral genomes?
What is the term for the small, mobile genetic elements that are thought to be the source of viral genomes?
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Which of the following is an example of a disease caused by a prion?
Which of the following is an example of a disease caused by a prion?
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What is the term for the process by which a virus takes control of the host cell's machinery to replicate itself?
What is the term for the process by which a virus takes control of the host cell's machinery to replicate itself?
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What is the term for the RNA-DNA hybrid molecule formed during reverse transcription?
What is the term for the RNA-DNA hybrid molecule formed during reverse transcription?
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Which of the following is a characteristic of viroids?
Which of the following is a characteristic of viroids?
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What is the term for the process by which a virus releases its genetic material from the host cell?
What is the term for the process by which a virus releases its genetic material from the host cell?
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What is the term for the envelope that surrounds some viruses?
What is the term for the envelope that surrounds some viruses?
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What is the primary function of a viral envelope?
What is the primary function of a viral envelope?
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What is unique about the capsids of bacteriophages?
What is unique about the capsids of bacteriophages?
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What is a characteristic of viral replication?
What is a characteristic of viral replication?
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What is the primary difference between the lytic and lysogenic cycles of phage replication?
What is the primary difference between the lytic and lysogenic cycles of phage replication?
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What is a key feature of viral evolution?
What is a key feature of viral evolution?
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Which of the following is NOT a characteristic of bacteriophages?
Which of the following is NOT a characteristic of bacteriophages?
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What is the term for viruses that can only replicate using the lytic cycle?
What is the term for viruses that can only replicate using the lytic cycle?
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What is the primary function of capsomeres?
What is the primary function of capsomeres?
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Study Notes
Replicative Cycles of Animal Viruses
- Viruses that infect animals can be classified based on their genetic material (DNA or RNA) and strandedness (single-stranded or double-stranded)
- Many viruses that infect animals have a membranous envelope, which is formed from the host cell's plasma membrane or the Golgi apparatus membrane
- The viral envelope has glycoproteins that bind to specific receptor molecules on the host cell surface
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 a retrovirus that causes AIDS (acquired immunodeficiency syndrome)
HIV Reproductive Cycle
- The viral DNA that is integrated into the host genome is called a provirus
- A provirus remains a permanent resident of the host cell
- The host's RNA polymerase transcribes the proviral DNA into RNA molecules, which function as mRNA for synthesizing viral proteins and as genomes for new virus particles
- HIV enters a host cell, and its RNA genome is reverse-transcribed into DNA, which is then integrated into the host genome
- The host cell's RNA polymerase transcribes the proviral DNA into RNA molecules, which are used to synthesize new viral proteins and genomes
- New HIV particles are assembled and released from the host cell
Evolution of Viruses
- Viruses do not fit the definition of living organisms
- Viruses probably evolved as bits of cellular nucleic acid
- Candidates for the source of viral genomes are plasmids, transposons, and other mobile genetic elements
Viroids and Prions
- 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
Genetics of Viruses
- Viruses called bacteriophages can infect and take over the genetic machinery of bacteria
- Viruses lead "a kind of borrowed life" between life-forms and chemicals
- The origins of molecular biology lie in early studies of viruses that infect bacteria
The Discovery of Viruses
- Tobacco mosaic disease stunts the 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 may consist of either double- or single-stranded DNA or RNA
Capsid and Envelope
- A capsid is the protein shell that encloses the viral genome
- Capsids are built from protein subunits called capsomeres
- Some viruses have membranous envelopes that help them infect hosts
Bacteriophages
- 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
Viral Replication
- Viruses replicate only in host cells
- Each virus has a host range, a limited number of host cells that it can infect
- Viruses use host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules to replicate
Replicative Cycles of Phages
- Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
- The lytic cycle is a phage replicative cycle that culminates in the death of the host cell
- The lysogenic cycle is a phage replicative cycle that does not kill the host cell immediately
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Description
This quiz covers the classification of viruses that infect animals, including DNA/RNA and single-stranded/double-stranded characteristics, as well as viral envelopes and their interactions with host cells.
