SBI3U - Unit 1 - Viruses

Virus

Virus

A structure that contains strands of DNA or RNA surrounded by a protective protein coat; it cannot live independently outside of host cells

Why viruses are not living

They create no waste, produce no energy, have no membranes, organelles, or cytoplasm, are dormant outside of living host cells, and are not made of cells

What viruses are made of

A protein capsid, genetic material, and possibly spike proteins or tail fibres

Capsid

The outer protein coat of a virus that surround the genetic material, and can be spherical, crystaline, cylindrical, etc.

How viruses can be classified

Size and shape, or the types of viruses they cause

Replication

The fundamental process of all cells, in which the genetic material is copied before the cell reproduces

How viruses can replicate

As a DNA virus or RNA retrovirus

Central Dogma of Biology

DNA in the nucleus, through transcription, creates mRNA which leaves the nucleus, finds ribosomes, and makes proteins

Process of DNA viruses

Attachment

Viral DNA is inserted to the host cell and deactivates host mRNA

Viral mRNA uses host cell machinery to create viral proteins

New virus proteins are assembled into viruses

Cell lyses, which releases cell proteins

Examples of DNA viruses

Papillomavirus (warts) and Variola virus (smallpox)

Process of RNA retroviruses

Retroviral RNA is inserted into the host

Reverse transcriptase is activated, and viral RNA becomes DNA

The new DNA splices into host chromosome and becomes dormant

The cell replicates with the viral DNA

The virus is triggered and DNA is spliced out and becomes active

Viral mRNA uses host cell ribosomes to create viral proteins

New virus proteins are assembled into viruses

Cell lyses, which releases cell proteins

RNA retrovirus examples

HIV (AIDS), Poliovirus (Polio), and SARS-CoV-2 (Covid-19)

Types of virus cycles

Lytic and lysogenic

Lytic Cycle

The replication process in viruses in which the virus’s genetic material uses the copying machinery of the host cell to make new viruses

Lytic cycle process

Attachment, insertion, replication, virus assembly, lysis

Lysogenic Cycle

The replication process in viruses, in which the viral DNA enters the host cell’s chromosome; it may remain dormant and later activate and instruct the host cell to produce more viruses

Lysogenic cycle process

Attachment, insertion, provirus formation, lysogeny, provirus separation, replication, assembly, lysis

Provirus

A virus which can invade a cell, but does not kill it

Temperate Virus

A virus that replicates using the lysogenic and lytic cycle, where the virus lays dormant, and cell lyses is not immediate

Virulent Virus

A virus that replicates using the lytic cycle, where cell lyses is immediate

Methods of transmitting viruses

Through air, physical contact, and insect bites

Power of a Virus

How easily a virus can be transmitted

Epidemic

A large-scale transmission confined to a geographical area

Pandemic

A large-scale transmission that is global

Vaccines

Mixtures containing weakened versions of a dangerous virus

How vaccines work

The antigen triggers the immune system (B Plasma Cells) to produce specific antibodies. Antibodies will recognize foreign substances and mark them for destruction by other immune cells. When real viruses enter the body, immune system will remember the specific antibody to produce. Vaccines can prevent and eradicate viruses

How SARS-CoV-2 infects cells

Uses spike proteins to gain entry into epithelial cells via a receptor called ACE2. The virus “tricks” the cell into swallowing it through endocytosis. ACE2 becomes occupied and does not perform its regular function

Prion

An infectious particle that causes damage to nerve cells in the brain, and that appears to consist mostly or entirely of a single protein

Diseases caused by prions

Brain diseases such as Creutzfeldt-Jakob Disease, scrapie, and bovine spongiform encephalopathy (mad cow disease)

Uses for viruses

Genetic engineers can inject genes into a virus, which then replicates inside of a host cell