Introduction to Viruses

Viruses

• Viruses are nonliving infectious particles.
• The word "virus" comes from the Latin word meaning "poison."
• All viruses are parasites that require a host.
• A parasite lives in or on a host organism, harming it.
• The host is the organism the parasite lives on.
• Martinus Beijerinck (1898, Dutch scientist) is considered the founder of virology, the study of viruses.
• He used filtration experiments to prove an agent smaller than bacteria caused tobacco mosaic disease.
• He named these small particles "viruses."
• Wendell Stanley (1904-1971, American biochemist) isolated crystals of the tobacco mosaic virus in 1935.
• This proved viruses are not alive since living things do not crystallize.
• He was awarded a Nobel Prize in chemistry.
• Viruses are extremely small and can only be seen with an electron microscope.
• Viruses are active only when inside a living cell.
• Outside a living cell, viruses cease all activities but retain their ability to infect a cell.
• Viruses can be crystallized, and stored indefinitely, while retaining infection capability.
• Viruses enter cells and use the host cell's machinery to produce more viruses.
• Viruses have a capsid (protein coat) around their DNA or RNA.
• The capsid has a specific shape that must match receptors on the host cell surface.
• The host cell is tricked into letting the virus in by the matching shapes.
• Viruses reproduce only within a living host cell. They force the host cell to make viral parts instead of its own parts.
• Viruses are highly specific to the types of cells they infect (e.g., plant viruses only infect plant cells).
• Viruses are not affected by antibiotics. Anything that kills a virus will also kill the host.

Viral Characteristics

• Viruses can reproduce and mutate; they have DNA or RNA.
• Viruses are noncellular and lack metabolism (no food or energy requirements).

Viral Reproduction

• A viral infection begins when the viral DNA or RNA enters a host cell.
• The virus hijacks the host cell.
• The viral genome re-programs the host cell.
• The host cell produces viral parts.
• The host cell assembles the parts into viruses.
• The reproductive cycle often destroys the host cell, releasing hundreds or thousands of new viruses.

Viral Reproduction Cycles

• Lytic cycle: The virus enters a cell, replicates, and causes the cell to burst, releasing new viruses. Phage T4 is an example of a virus that follows a lytic cycle.
• Lysogenic cycle: A lysogenic virus integrates its DNA into the host's DNA (becoming a prophage), replicates with the host's DNA, and may remain dormant or inactive until triggered by environmental conditions to enter the lytic cycle.

Retroviruses

• Retroviruses use RNA as their genetic material instead of DNA.
• They have reverse transcriptase (an enzyme).
• Reverse transcriptase converts RNA into DNA, which then integrates into the host cell's DNA.
• Retroviruses may remain dormant, but eventually become active and cause the host cell to produce new viruses. They often lead to host cell death.

Bacteria

• Bacteria are prokaryotes.
• Prokaryotic cells do not have a true nucleus or membrane-bound organelles.
• Bacteria are abundant, outnumbering eukaryotes.
• Some bacteria are harmful pathogens while most are beneficial or harmless.

Classification of Prokaryotes

• Bacteria are divided into two domains: Archaea and Bacteria.

Archaea

• Archaea are similar in size and structure to bacteria, lacking nuclei and having cell walls, but chemically different from bacteria.
• They lack peptidoglycans; have different membrane lipids, and their DNA sequences are more like eukaryotes than bacteria.
• They live in extreme environments (swamps, salt lakes, hot springs) and are considered ancient.

Archaeal Groups

• Methanogens: Use hydrogen and carbon dioxide to produce methane gas (anaerobic environments).
• Halophiles: "Salt-loving" archaea that thrive in high-salt environments.
• Thermoacidophiles: Thrive in hot, acidic environments (e.g., Yellowstone hot springs).

Eubacteria

• Eubacteria, the "true bacteria," have a cell wall containing peptidoglycan, and are diverse and abundant in environments on Earth.

Bacterial Characteristics

• Bacteria have three basic shapes: cocci (spherical), bacilli (rod-shaped), and spirilla (spiral).
• Some bacteria are motile (move).
• Movement can involve flagella or gliding over secreted slime.
• Bacteria can be identified by shape, cell wall materials, movement, and energy acquisition.

Bacterial Structure

• Bacteria have a capsule (polysaccharide) for protection and attachment.
• Bacteria have flagella (movement), pili (attachment and reproduction), and ribosomes (protein synthesis).
• Their DNA is in a single, circular strand. Plasmid DNA is separate from the main chromosome.

Bacterial Metabolism

• Most bacteria are heterotrophs (cannot make their own food).
• Some bacteria are saprophytes (feed on dead organic matter) or parasites (live off another organism).
• Some bacteria are autotrophs (make their own food).
• Some are photoautotrophs (use sunlight).
• Others are chemoautotrophs (use inorganic reactions for energy).
• Some need oxygen (obligate aerobes); others cannot tolerate oxygen (obligate anaerobes). Facultative anaerobes can survive with or without oxygen.

Bacterial Reproduction

• Bacteria reproduce asexually by binary fission, creating two identical daughter cells.

Bacterial Conjugation

• Conjugation involves a hollow bridge between two cells, allowing gene transfer.

Bacterial Endospores

• Under unfavorable conditions, bacteria form protective endospores, which can survive extreme conditions and form new cells later.

Importance of Bacteria

• Bacteria are vital for decomposition, recycling essential elements, and nitrogen fixation (converting nitrogen gas into usable forms for plants).

Human Uses For Bacteria

• Bacteria are used in food production (yogurt, cheese).
• Some bacteria are used in oil spill cleanup.

Viral and Bacterial Diseases

• Bacterial diseases can be caused by damage to cells and tissues by feeding on the cells or toxins produced by bacteria.
• Viral diseases cannot be cured by antibiotics. Instead, vaccines can prevent viral diseases by exposing the body to a harmless form of the virus so the immune system learns how to combat it.

Symbiotic Relationships

• Symbiosis is a close, permanent relationship between organisms of different species. There are three types:
  • Mutualism (both benefit)
  • Commensalism (one benefits, the other is unaffected)
  • Parasitism (one benefits, the other is harmed)

This quiz covers the fundamental aspects of viruses, including their characteristics as nonliving infectious agents and their dependence on host organisms. It highlights historical contributions to virology from scientists like Martinus Beijerinck and Wendell Stanley. Test your knowledge on the nature of viruses and their role in disease.