Ch 3: Dead or alive: Viruses


  • Are microscopic molecules containing proteins and nucleic acids
  • Attack cells and replicate within them
  • They are not classified as cells but as particles
  • They cannot reproduce on their own nor live on their own

For the virus to recognize host cells, it uses the capsid which has a specific structure to unlock a receptor site on the host cell membrane. There are DNA viruses and RNA viruses. RNA viruses are known to mutate faster with minimal immunity while DNA viruses mutate after a long period of time, allowing the body to develop substantial immunity.

Antibiotics are not used to treat viruses because these types of drugs target the cell wall of bacteria, and viruses lack a cell wall.

A prion is an infectious particle that is made up of protein only and which is fatal to the body because the body lacks an immune response against a protein. Gene therapy is a common technique used in relation to viruses, to correct defective genes that are responsible for disease development. The body produces antibodies (proteins developed to fight infections antigens) against foreign substances identified by the immune system as non-self.

Structure of viruses

They contain: nucleic acid core and a protein capsid

Protein capsid

Classified into various categories

  • Helical RNA viruses
  • Polyhedral DNA virus
  • Enveloped RNA virus
  • Complex DNA virus

Helical viruses have a capsid that enables twisted helix around a core of nucleic acids. Polyhedral viruses consist of a geometric shape. Complex viruses have patches of proteins that are separate forming unique structures

The function of virion proteins is to protect the genome by forming a stable protein shell and packaging the genome, delivery of the genome by binding to host receptors and fusion with the host cell membrane eventually transferring the genome, and also is responsible for various host interactions with cellular components to ensure the infectious cycle is maintained and evasion from the host immune cells.

Capsid can either be helical or icosahedral. Helical capsids have identical monomers of a single polypeptide that self-assemble to for helical structure. The icosahedral is polygonal and is the most common type of capsid found in viruses, with a closed shell made up of a number of identical subunits. Examples of viruses with helical capsids are ebola (filoviruses) and TMV. Examples of viruses with icosahedral capsids are proviruses and adenoviruses.

Nucleic acid core

The viral genome is either DNA or RNA. The Baltimore classification system of viruses categorizes viruses from classes I-VII.

  • Class I & VII – dsDNA (+/-) virus
  • Class II ssDNA (+) virus
  • Class III dsRNA (+/-) virus
  • Class IV ssRNA (+) virus
  • Class V ssRNA (-) virus
  • Class VI ssRNA (+) retrovirus

How viruses enter into the cell

  • Viral protein particles bind to specific  receptors on host cells
  • The virus forces itself into the host cell via the plasma membrane
  • The virus then replicates within the cell using enzymes of the host cell
  •  New viral particles are assembled inside the cell
  • New viruses produced infect more cells

Viruses of bacteria

  • Viruses attack bacteria just as they infect eukaryotic cells.
  • These viruses are called bacteriophages
Bacteriophage replicates through two cycles:
  • Lytic cycle-uses host cells to generate many copes and finally destroy the host cell
  • Lysogenic cycle– The phage enters a latent phase and its nucleic material is incorporated into bacterium genetic material. During replication of bacterial DNA, the phage DNA is multiplied leading to the generation of bacterial cells infected with the virus.

Lytic cycle

  • Attachment-viral surface proteins attach to host cell receptors
  • Penetration– viral genetic material enters the target host cell
  • Biosynthesis-viral genome replicates using host cell transcription and translation mechanisms.
  • Maturation-some viral particles assemble to form new phages
  • Release- new virus generated exits the host cell, the host cell  is destroyed as the bacteriophage leaves

The five stages of the viral lytic life cycle are attachment, Penetration, Biosynthesis, Maturation, Lysis. A virus attaches on the host cell through a receptor, enzymes break down the cell and insert the genes, the viral genes are copied and used to synthesize proteins, the copies of the gene are packaged into capsids to form a mature virion which lysis the membrane of the cell and into the environment.

Lysogenic cycle

  • Attachment- viral surface proteins attach to host cell receptors
  • Penetration- viral genetic material enters the target host cell
  • Recombination-phage DNA is pasted into bacterial DNA
  • Replication-bacterial cell replicates hence generating copies of the phage alongside

Stages of a viral lysogenic life cycle are attachment, Penetration, Lysogeny, and Integration, Cell reproduction, Induction, Lysis. The virus inserts a prophage into the host cell which then remains within the host cell’s DNA without immediate effects. The lysogenized host cell continues to multiply normally, carrying with its genome, the viral genes.

A change in factors such as UV light exposure, changes in temperature triggers the prophage to become active and is used to synthesize new virions which are then released to infect more cells. Retroviruses are special types of viruses because they carry enzymes that can reverse transcribe RNA into DNA and integrate into the host cell genome, an example is HIV.

Viruses of eukaryotic cells

  • Latent instead of lysogenic – some eukaryotic virus like herpes have a dormant period within cells hence called latent viruses
  • Provirus instead of prophage – a provirus is a dormant virus within eukaryotic cells
  • Acute disease instead of lytic cycle – acute illness is caused by viruses that strike faster, has a quick replication, and causes a short rapid illness.

How virus attacks eukaryotic cells

  • Fusion- viral envelope melts into the eukaryotic cell
  • Receptor-mediated endocytosis- caused by binding of various viral proteins to host cell receptors

Stages of viral infection

  • Free virus
  • Binding and fusion- viral proteins bind to various receptors
  • Penetration- viral genetic material enters the host cell
  • Reverse transcription- reverse transcriptase converts viral RNA to DNA
  • Recombination- viral DNA is pasted into the host cell DNA
  • Transcription- cell DNA undergoes normal transcription
  • Assembly-  new viruses assembled into virions
  • Budding- immature virus released
  • Release-the immature virus breaks from the host cell
  • Maturation-viral protease finishes cutting of viral protein to form a mature virus

Latency is the ability of a virus to remain integrated into the host cell genome and cause no signs or effects. The virus can be transmitted through the carrier shows no symptoms of infection. When retroviruses are transmitted from parent cells to daughter cells, this is described as vertical transmission. If the transmission is from one cell to another directly, this is the horizontal transmission.

Final Revision

The protein coat of a virus is known as a

A phage that becomes part of a bacterial chromosome is a

The bursting of a host cell is also called

What type of life cycle is characteristic of temperate viruses?
Lysogenic cycle

Viruses are classified according to which genetic material,_ or _, makes up its core

Early work on viruses was done on the
Tobacco mosaic virus

New, complete virus particles are known as

Viruses are much __ than bacteria in size

If a virus does not cause disease immediately after it enters the organism, it is said to be

Which of the following is a helical virus?
Tobacco mosaic virus

What does the solid line in box 3 of the diagram represent?
Viral DNA

The term viroid refers to a
Disease-causing particle made of RNA that has no capsid

What is the structure of the virus that causes the common cold?

Louis Pasteur’s treatment for rabies worked by
Injecting weakened virus into infected people to stimulate disease-fighting mechanisms

Transduction is the process in which
A phage picks up a portion of host cell DNA and transfers it into another host cell

Which of the following is represented by steps 1, 2, 3, and 4 in this diagram?
The lysogenic cycle

At which stage in this process does phage DNA first become a prophage?

Sometimes a virus can carry some of its host’s genes with it into the next host cell it infects. This process is known as

Which of the following is often used to determine an organisms antibody response to viruses

Retroviruses are the only viruses that
Make DNA from RNA

The T-even phases can infect and destroy
Bacteria cells

Which structures in T4 pages contain proteins that have a chemical affinity with bacterial cell walls?
Tail fibers

When cold viruses invade the human body, most are quickly destroyed by the body’s immune system. A few of the viruses will not be destroyed because they
are resistant to immediate attack by the immune system

The genomes of some RNA viruses take over the machinery of the host cell by
inactivating the host cell ribosomes

Scientists think that viruses evolved after cells because
viruses require cells to survive

AIDS is caused by a

Viruses that introduce viral genes into the bacteria without causing cell lysis or the production of virions are
temperate phages

Viruses are classified based on all of the following except
type of cell membrane

Viruses can reproduce themselves
only inside a living cell

The ability of a virus to cause disease is called

What is the basic structure of a virus?
DNA or RNA surrounded by a protein coat

Bacteriophages infect
bacteria only

viruses can cause infection by
entering body cells and replicating

In composition, a rod shaped virus is about % protein and % nucleic acid
95, 5

The protein produced by animal cells to fight off viral infections is known as

A person could get AIDS from an HIV carrier in all of the following ways except
using the same toilet

During a lytic infection, the host cell is

T/F Another name for obligate intracellular parasite is host cell

T/F Viral DNA undergoes mitosis during the replication phase of the lytic cycle

T/F Most icosahedral viruses are less than 200nm in diameter

T/F Reverse transcriptase is an enzyme that makes DNA from RNA

T/F Viruses are the smallest disease-causing agents known to man

T/F A temperate virus become virulent when it enters the lytic cycle

T/F The rabies virus specifically attacks nerve tissue

T/F The sheath of a T4 phage attaches to a receptor site in the cell wall of the bacterial host

T/F A prion is a disease-causing particle that contains no nucleic acid

T/F The term “prophage” refers to the earliest fossil evidence of viruses, thought to be ancestors of bacteriophages

T/F Viruses contain either DNA or RNA, but never both

T/F Electrophoresis is the study of biological fluids

T/F Viruses can reproduce outside a host cell only if they undergo transduction

T/F Obligate intracellular parasites are bacteria commonly found in the human digestive tract

T/F Some lysogenic phages can cause genetic recombination among their bacterial hosts

T/F Upon entering a host cell, viral DNA goes directly to the ribosomes to stat manufacturing viral proteins

T/F A prion is a glycoprotein particle

T/F The rabies virus has a polyhedral structure

T/F During entry the T4 phage injects its DNA into the host cell through its tail sheath

T/F During the replication phase of the lytic cycle of the T4 phage, the host cell DNA is inactivated

T/F Temperate viruses released during cell lysis may take with them a portion of the DNA of the host cell

T/F Scientists infer that viruses evolved after cells

T/F Retroviruses are a type of RNA virus

T/F Louis Pasteur’s treatment for rabies worked by killing the rabies virus

T/F Tissue culture is a technique that virologists use to grow a large number of cells in a controlled medium

T/F A temperate phage can undergo both the lytic cycle and the lysogenic cycle

T/F The host cell recovers after the visions are released

T/F A person could get AIDS from donating blood

T/F A person could get AIDS from kissing an infected person

T/F A person could get HIV by having sex with a carrier, even if it is their first time

T/F Only homosexuals, prostitutes, and drug addicts get AIDS

T/F Men can get AIDS by having sex with an infected woman

T/F People with HIV or AIDS should be isolated from public

T/F The clear areas in a petri dish where bacteria have been destroyed are called plaques

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