Dead or alive: Viruses

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.

Flashcards

Concept or Term	Description or Answer
Viruses	Microscopic molecules containing proteins and nucleic acids
Antibiotics	Drugs that target the cell wall of bacteria and are not effective against viruses
Prion	Infectious particle made up of protein only and fatal to the body
Gene therapy	Technique used to correct defective genes responsible for disease development
Antibodies	Proteins developed by the body to fight infections
Capsid	Protein coat of a virus
Helical capsid	Capsid that forms a twisted helix around a core of nucleic acids
Icosahedral capsid	Capsid with a polygonal shape and a closed shell made up of identical subunits
Nucleic acid core	Genetic material of a virus, either DNA or RNA
Baltimore classification	Classification system of viruses categorizing them into classes based on their genetic material
Viral lytic life cycle	Stages of viral replication: Attachment, Penetration, Biosynthesis, Maturation, Lysis
Viral lysogenic life cycle	Stages of viral replication: Attachment, Penetration, Lysogeny and Integration, Cell reproduction, Induction, Lysis
Bacteriophage	Virus that infects bacteria
Lytic cycle	Replication cycle of bacteriophages that involves host cell destruction
Lysogenic cycle	Replication cycle of bacteriophages where the viral DNA is incorporated into the bacterium’s genetic material
Retrovirus	Virus that can reverse transcribe RNA into DNA and integrate into the host cell genome
Latent virus	Virus with a dormant period within cells
Provirus	Dormant virus within eukaryotic cells
Acute disease	Illness caused by viruses that replicate rapidly and cause a short, rapid illness
Fusion	Viral envelope melting into the eukaryotic cell
Receptor-mediated endocytosis	Entry of virus into the host cell through binding to host cell receptors
Viral infection stages	Free virus, Binding and fusion, Penetration, Reverse transcription, Recombination, Transcription, Assembly, Budding, Release, Maturation
Latency	Ability of a virus to remain integrated into the host cell genome without causing immediate effects
Vertical transmission	Transmission of retroviruses from parent cells to daughter cells
Horizontal transmission	Transmission of retroviruses from one cell to another directly
Virus	Microscopic particle that gets inside a cell and often destroys the cell
Reasons viruses are difficult to fight	Small size and rapid, frequent changes
Characteristics of living things that viruses have	Contain protein and genetic material
Characteristics of living things that viruses do not have	Eat, grow, break down food, use oxygen
Where viruses reproduce	Only in a living (host) cell
Characteristics used to group viruses	Shape, type of disease they cause, life cycle, and genetic material
Four basic shapes of viruses	Crystals, Spheres, Cylinders, Spacecraft
DNA Viruses	Viruses with stable genetic material that can use the same vaccine for years
RNA Viruses	Viruses with less stable genetic material that requires new vaccines
Lytic cycle	Replication cycle of viruses involving attachment, entry, replication, assembly, and release
Lysogenic cycle	Replication cycle of viruses involving attachment, entry, integration, cell reproduction, induction, and release
Ways to prevent viral infections	Vaccinations, handwashing, avoiding contact with wild animals
Edward Jenner	Developed the vaccine for smallpox
Jonas Salk	Developed the polio vaccine
Major functions of the immune system	Antibodies (recognition), White blood cells (destruction), Memory cells (remembering and attacking)