Virus Structure and Replication

Virus Basics

• Viruses are not cells
• Viruses lack a cell membrane, nucleus, organelles, and cytoplasm
• Viruses are obligate intracellular parasites, meaning they can only replicate inside a living cell
• Viruses contain either DNA or RNA, enclosed within a protein coat called a capsid
• Some viruses have an envelope, while others are naked (non-enveloped)
• Viruses are extremely small, ranging from 20-30 nm, and are only visible with an electron microscope
• Viruses have specific receptors, and thus can only infect a specific type of cell

Replication of a Lytic Bacteriophage

• Adsorption: The phage attaches to the bacterial cell wall
• Penetration: The phage enters the bacterial cell wall and membrane and injects its DNA
• Bacterial DNA is disrupted
• Synthesis occurs, resulting in the production of new viruses
• Phage DNA directs the cell's metabolism to produce viral components like proteins and copies of phage DNA
• Empty phage heads and pieces of phage DNA are synthesized
• Maturation: Collars, sheaths, and base plates attach to heads, and tail fibers are added last
• Release happens when the bacterial cell lyses, releasing mature phages

Bacteriophages as Drugs

• Bacteriophages can kill bacteria and have been used to treat antibiotic-resistant infections, particularly in Russia
• Bacteriophages are cheap to produce, have few side effects, and are very specific

The Lysogenic Cycle

• In the lysogenic cycle, viruses do not immediately kill their host cells
• Lysogeny occurs when a virus does not lyse the cell, but instead becomes part of the host cell's genome, called a prophage or provirus
• A provirus copies itself instead of killing its host cell

Adsorption and Recognition in Animal Viruses

• Animal viruses recognize proteins on cell membranes, such as the rhinovirus capsid
• Targeting the receptor protein for HIV is effective
• No specific drug exists for rhinovirus

Penetration of an Animal Cell

• Viruses must enter cells to be copied
• Sometimes, they can enter all the way into the nucleus to get themselves copied
• Every virus is different, making the treatment of viral infections difficult
• Endocytosis is a mechanism by which viruses enter animal cells

Maturation of Animal Viruses

• Maturation involves the "growth" or replication of viruses
• Maturation depends on the type of virus
• Maturation happens either in the nucleus or cytoplasm of host cells and uses host cell machinery

Release of New Viruses

• New viruses may be released through budding, also known as shedding
• Host cells may or may not be killed during viral release
• Cell lysis can cause clinical symptoms, such as skin destruction in herpes and polio, and nerve cell destruction in polio

Significance of Understanding Viral Life Cycles

• Understanding viral life cycles allows for the identification of targets for antiviral agents

Studying Viruses

• Animal viruses are studied through culturing in whole animals, chicken embryos (for influenza), and cell cultures in the lab

Viruses and Teratogenesis

• A teratogen is an agent that causes defects during embryonic development
• Damage caused by teratogens is most extensive earlier in development
• Many viruses are teratogens, including cytomegalovirus (CMV), rubella (German measles), chickenpox, erythrovirus (fifth disease), herpes, and Zika

Viruses and Cancer

• Cancer involves the uncontrolled growth of cells
• Cancer is not a single disease
• Approximately 15% of cancers are strongly linked to viruses

Virus-like Agents

• Viroids and prions are virus-like agents, but not viruses or cells

Viroids

• Viroids consist of a single strand of RNA only, with 300-400 nucleotides
• Viroids are smaller than viruses
• Viroids lack a protein coat
• Viroids are infectious agents in plants and can cause plant diseases
• Viroids have not been found in animals

Prions: Infectious Protein Particles

• Prions consist only of protein and can be passed among animals
• Prions lack nucleic acid
• Prions are a mutant form of a normal brain protein
• Prions cause disease by accumulating in cells, eventually killing them and creating holes, primarily in brain cells
• Prion diseases are associated with a sponge-like appearance of the brain

Diseases Associated with Prions

• Mad Cow Disease
  • Bovine spongiform encephalopathy
• Creutzfeldt-Jakob disease
  • Humans
• Scrapie
  • Sheep
• Chronic wasting disease
  • Elk, deer
• Kuru
  • (Cannibals)

Other Microbes

• Protozoa are single-celled eukaryotes
• Fungi are typically single-celled eukaryotes
• Helminths are parasitic and consist of some members of this group

Symbiosis Types include

• Parasitism: One organism lives at the expense of another
• Commensalism: Living in or on without harming
• Mutualism: Both organisms benefit, such as E. coli in the human gut producing vitamins

Terms describing the location of a parasite

• Ecto-: Outside the host
• Endo-: Living inside the host

Modes of Survival

• Obligate: Can only live that way
• Facultative: May act as parasite or not, depends on circumstances
• Vector-transmitter: Something that transmits
  • Biological vector: Parasite develops within
  • Mechanical vector: Only a carrier

Studying Parasitology

• Approximately 15 million people die each year from parasitic infections
• Infections are more prevalent in areas with poverty
• Malaria alone accounts for over one million deaths, mostly in children
• About 3 billion people worldwide harbor parasitic worms, with approximately ⅓ of the world's population- being affected
• 30 million are severely affected and 50% are children
• Worm infestations occur in the US

Protozoa Characteristics

• Protozoa are unicellular eukaryotes that can be free-living, commensals, or parasites
• Some protozoa exhibit motility
• Plasmodium (malaria) is not motile
• Some protozoa are medically important

Fungi Characteristics

• Fungi are eukaryotes
• Fungi are mostly saprophytes that digest dead organic matter, first killing cells upon infection
  • Some fungi are facultative parasites
• Are beneficial as decomposers, providing nutrients
• Some fungi produce antibiotics
• Mycology is the study of fungi

Worms

• Helminthiasis is the most widespread parasitic infection
• Impacts approximately ⅓ of the world's population, including the U.S
• There can be multiple infestations
• Helminths are animal-multicellular eukaryotes that can be free-living or parasitic, and parasitize humans, with examples being flatworms (like tapeworms) and roundworms
• Animals can be infected by worms through microscopic cysts
• Insects can sometimes transmit worms