Understanding Viruses: Structure and Types

Viruses

• Non-cellular particle of genetic material and protein that invades living cells
• Come in different shapes
• Very small, approximately 10 to 400 nanometers (nm)
• 1 nm = 1,000,000 nm
• Infectious agents reproduce inside living organism cells
• Hijack a host cell's tools to make copies

Virus Structure

• Genetic material (DNA or RNA); its blueprint carrying instructions
• Some viruses have DNA (herpes), some have RNA (flu, COVID-19)
• Capsid protects genetic material & attach to host cells
• Lipid envelope is an outer fat layer; derived from host cell, helps virus enter new cells easily. Some viruses like the common cold virus lack this
• Antigens are proteins that attach to host cells; trigger immune response

Types of Viruses

• Influenza (flu) and common cold viruses
• Protein spikes mutate regularly, so humans must build new immunity

Other Virus Types

• Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) causes COVID-19
• Bacteriophage is a virus that invades bacteria with cone of nucleic acid and capsid
• Some treat bacterial infections that are resistant to antibiotics

Are Viruses Alive?

• Biologists debate whether viruses are alive
• Viruses aren't in any biological kingdom
• Genetic material (DNA or RNA) surrounded by protein
• Characteristics of living things include movement and reproduction

Key points when determining living status

• Made of cells - No
• Move - No
• Reproduce - No
• Respond to environment - Yes
• Grow and develop - No
• Respire - No
• Excrete - No
• Obtain and use energy - No
• Genetic material - Yes
• Evolve - Yes
• No cytoplasm or organelles
• Cannot do biochemistry on their own

Additional Characteristics

• Reproduce only with a host cell
• Contain DNA or RNA, never both
• Contain genes and can mutate
• Can take over a host cell's machinery
• Cause contagious diseases
• Much smaller than cells, visible only with strong microscopes
• Can respond by infecting hosts

Virus Specificity

• Viruses are said to be specific to type of cell or organism
• Viruses reproduce in certain organisms and in certain tissues with exceptions
• Usually specific to only humans or certain animals
• Some viruses can evolve/change

How viruses are carried

• Can be carried by an unaffected host to an organism, example being the monkey carrying killer strain of Ebola

Virus Replication

• Occurs in two ways: lytic or lysogenic

Lytic Infections

• Known as virulent & are quick
• Host cell is invaded, lysed (burst), and destroyed
• The viral DNA is translated (read)
• Process involves attaching to host and injecting DNA
• Replication occurs as the cell can't differentiate between DNA strands
• Viral mRNA uses the host cell's tools, called machinery, to make copies of it's proteins
• Assembly occurs as the host becomes filled with viral DNA molecules and it serves as the starting points around which new virus particles are assembled
• Release is when the infected cell bursts

Lytic Cycle Summary

• Only takes 20 minutes
• Life-cycle of a lytic virus consists of acts of infection, growth, and replication
• In the cycle, the virus takes over a cell, makes copies & destroys the cell, this cycle happens quickly
• Cell destruction can cause a disease

Lysogenic Infections

• Known to be latent (dormant)
• The viral DNA is inserted into a host cell and can remain for generations before becoming active
• Lysogenic cycles are found more in prokaryotes or bacteria

Lysogenic Cycle Summary

• Viral DNA becomes a prophage
• Prophage may remain as part of the cell for generations that can benefit the cell
• Eventually, the prophage DNA becomes active, removes itself from the host cell
• Envoirmental cues are stress, sunlight, temperature change

CHAT-GPT Version Compared

• Lytic Cycle is the rapid production of new viruses/host cell lysis
• Lysogeneic cycle is more latent & Virus integrates into DNA silently without harming the cell

Stages in Lysongenic Cycle

• Virus binds to specific reporters on cells, its genome enters
• Integrates into chromosome to become a prophage or provirus
• Viral DNA replicates along with the host, dormant that doesn't cause immediate disease
• Specific conditions (stress/UV) trigger cycle
• Leading to viral replication, assembly, & cell lysis.

Retroviruses

• Use RNA as genetic information.
• Produce DNA from RNA (retro- in latin means backwards)
• The DNA is inserted into the host cell's DNA
• Enzyme reverse transcriptase converts RNA to DNA once inside a host
• Common retrovirus is HIV

SARS-CoV-2 Spike Proteins

• Attach to ACE2 receptors
• ACE2 is present in many cell types and tissues
• ACE2 can be located in the lungs, heart, blood, and other organs
• Present in cells, which present protectine barriers

How Viruses Cause Diseases

• Disease causing viruses pathogens change cells and make you sick
• The immune system causes infections
• Oncogenic (tumour-making)
• Science can not stop an infection
• The best defense is the vaccination
• Antiviral and anti-retroviral drugs developed stop the virus and stops the pogression of the disease with rates of death decline

Interferons

• Proteins released make it difficult for viruses to infect other cells
• They interfere by sending a signal to nearby cells to boost defenses
• Of the 1000 to 1500 types of known viruses, approximately 250 cause diseases in humans

Virus-Like RNA Particles

• Viroids contain RNA that cause diseases but have no cure
• Prions contain proteins that cause some diseases in animals such as mad cow

Explore the structure of viruses, from their genetic material (DNA or RNA) to the capsid and lipid envelope. Learn about common types like influenza, the common cold, and SARS-CoV-2, and how they infect host cells.