Viruses & "Prokaryotes"

Virus make-up

• An obligate intracellular parasite– they have to live inside of a cell and use that cell to produce, to function

• Virion: the complete package outside of a cell– a virus not inside of a cell

• Have a nucleic acid core (single stranded DNA, double stranded DNA, single stranded RNA, double stranded RNA)-- present in all viruses

• Capsid: a protective protein coat that surrounds the nucleic acid core– comes in various shapes and sizes-- present in all viruses 

• Envelope: present in SOME but not all viruses– contains proteins, lipids (fatty structures), carbohydrates, and trace metals…usually made up of pieces of the host plasma membrane  (ex: HIV)

  • Viruses are obligate intracellular parasites, so they use the host cell to reproduce themselves, but when they need to leave,  some will take part of the plasma membrane of the host cell with them, so then you have this virus surrounded by this envelope 

    • Think of a wolf in sheep’s clothing– the predator is hiding inside of the outer layer of its prey 

• Tail: present in SOME but not all viruses (ex: bacteria phages) 

• Viruses range in size 

  • Smaller viruses (ex: polio– 40 nanometers)

  • Bigger viruses (ex: pox viruses– 300 by 200 nanometers, Mimi virus– 400 nanometers)

Virus Origin Hypotheses

• Cellular origin hypothesis: suggests that viruses originate from pieces of broken cells that came together– might explain why some viruses only attack very specific hosts and will not attack others, they may be attacking cells similar to the ones they originated from

  • Some viruses genetically (in proteins) can be more like the host that a different kind of virus 

• Co-evolution hypothesis: suggests that viruses evolved early in life’s history, around the time the first life appeared – could explain the similarities in why between viruses that infect archaea and viruses that affect the bacteria (two earliest branches of life) 

• Regressive hypothesis: suggest that the viruses are some sort of regression from a cell– idea that their origin is that of small parasitic cells that were residing in larger cells 

  • Before there were viruses, there were cells. And there were some cells that were parasites in bigger cells

  • And bc there was so much that those parasitic cells were getting from the host cells, it might've been a waste of energy to build up all the things they need to function as a living thing– causing them to lose the genes and traits that would’ve allow them to be individual cells  

    • Mimi virus– has a genome larger than some procaryotes, and some of those genes can’t perform code for protein synthesis. So they can’t make proteins, but they have genes that can. 

    • This could explain how a virus could evolve before their specific hosts, bc they may have evolved inside a another cell and became an infectious agent that could infect other cells

Bacteriophages

• Bacteriophages: eaters of bacteria, bacteria killers

  • Some can be used in medicine and food safety by killing harmful bacteria

  • Does not infect humans, just bacteria

How? Lytic Cycle

Lytic Cycle

refers to bursting of a cell

• Attachment

  • Phages attach to the surface of the bacterial cell, the cell membrane 

• Penetration

  • The bacterial phages will penetrate the cell membrane and inject its nucleic acids into the host cell

• Replication & Synthesis

  • Using the materials in the host cell, the phages replicate their nucleic acid and synthesize their parts– making their capsids and tails

• Assembly

• Release

  • Causes cell to lye breaking apart and releasing all of the virions into the environment, where each of those can infect a new bacterial cell

Lysogenic Cycle

Temperate Viruses; can be stimulus based or spontaneous

• Attachment

  • Phages attach to cell surface of bacterium

• Penetration

  • Phage DNA enters bacterial cell

• Integration

  • The DNA of the bacterial phage integrates itself into the chromosome of the bacterial DNA– hides itself within the bacteria and the DNA just sits there and the host, the bacteria, may continue to replicate through mitosis–  replicate both the viral and bacterial DNA (does this for possibly generations)

• Replication

  • The viral DNA pops out of the bacterial DNA and starts to replicate some more, assembles, then bursts out of the cell…killing the cell (basically ending in a lytic cycle)

Lysogenic Conversion

• If the viral DNA integrates itself back into the bacterial DNA, the bacterial DNA now has DNA it didn’t have before ... .so will the bacteria be 100% itself or will it be altered by the integration of the foreign DNA?   

  • It would be different

  • Some bacteria that are infected with a temperature virus will exhibit lysogenic conversion

  • And the host cells starts to exhibit new properties it didn’t before

    • Ex: Diphtheria– a disease caused by a bacterium that wouldn’t normally get you sick, but when it goes through lysogenic conversion it makes a harmful toxin

Ex: Botulism– botulinum bacterium doesn’t naturally make that toxin, but when infected with a virus, it goes through lysogenic conversion and makes the botulism toxins, a type of food poisoning

Animal Viruses

• Including retroviruses…will enter the host cell by merging (the envelope has proteins on it the cell can accept) 

  • Ex: the coronavirus has glycoproteins around it that bind to receptors in cells and then the virus can enter into the host cell

    • If it’s going into a non bacterium, non arcadian, that cell will have a nucleus (a eukaryotic cell) and the nucleic acid will work its way into the nucleus (where the DNA of the host is) and will make viral DNA…then the various structures within the cell will make the necessary components of the virus to assemble to virus and send it out of the host cell

  • HIV,a retrovirus, will get into the host’s DNA for a while, then create the viral RNA (HIV has two single strands of RNA) and makes DNA, putting it into the host chromosome and that codes for viral RNA– making more virus 

Plant Viruses

• Plant cell walls contain cellulose, which it thick and strong

• Most have RNA, not DNA

• Due to the difficulty it takes to get through the cell wall, usually infection occurs to damaged cells, and that’s how the virus will penetrate into the plant

  • From there, the disease becomes an inherited disease– as cells replicate and make seeds, the virus can continue to spread through the host. Usually no cure– usually when this happens, the plants/ crops need to be burned. 

  • Genetically resistant plants are attempting to be developed

“Prokaryotes”-- Bacteria & Archaea

• Represent the branches of the earliest forms of life

• Their make-up

  • Small

    • circular bacteria: 0.5-1 micrometer diameter, elongated bacteria: 1-5 micrometers in length

    • Mimi virus is 400 nanometers– aka 0.4 micrometers…just under the size of the small bacteria of 0.5 micrometers, so the Mimi virus is pretty close to the size of a small bacterium

  • Shapes 

    • Cocci bacteria (balls) 

      • Can be in pairs (diplococcus)

      • chains (streptococcus)--strep throat 

      • clumps (staphylococcus)-- infections

    • Bacilli bacteria (rods)

    • Spirochete bacteria / spirillum (spiral)

  • Lack Organelles

    • Bacteria & archaea do not have 

      • Nuclei– they have a nuclear area where the DNA will cluster, forming their circular DNA molecule 

      • Membrane bound organelles– things like chloroplast, mitochondria, endoplasmic reticulum, or GOGI apparatus…none of which are in bacteria or archaea  

  • Structures / Cell Wall

    • Capsule: jelly like structure surrounding bacteria (only present in very few bacteria) 

    • Peptidoglycan: present in bacteria ONLY

      • Gram positive... if there’s a lot of peptidoglycan, the gram stain will stain quite well 

      • Gram negative…very thin layer of peptidoglycan and don’t stain well, and so they’re gram negative 

    • Cell membrane is flexible

    • Cell wall is sturdy (provides rigidity, strength)

      • If a bacterium ends up in a highly hypotonic situation, the fresh water will go into the cell and it wasn’t for a sturdy cell wall, the cell could burst due to the water pressure 

  • Flagella: acts like a propeller, it spins 

    • Made of three parts

      • Filament (long stringy-like part)  

      • Hook (creates the bend)

      • Basal body (the swivel– structure that penetrates through the capsule, cell wall, and cell membrane) 

    • Penetrates all the way through the capsule, cell wall, and cell membrane 

  • DNA

    • In a single circular molecule