Viruses, Bacteria, and Cell Signaling: Study Notes for General Biology

Viruses & Bacteria

Infectious Agents and Pathogens

Infectious agents are organisms or particles that cause disease in hosts. Diseases can result from genetic, environmental, or pathogenic factors.

• Disease: A condition affecting an entire body or some of its parts.

• Pathogen: Infectious agent that causes disease.

• Prions: Infectious proteins that cause proteins to misfold; considered non-living.

• Viruses: Simple infectious agents that have no activity outside of a host; non-living.

• Bacteria: Single-celled prokaryotes.

• Protists: Single-celled eukaryotes.

• Fungi: Eukaryotes.

• Parasitic worms: Eukaryotes.

• Very few archaea are pathogenic to humans.

Agent	Cell Type	Living?
Prion	None	No
Virus	None	No
Bacteria	Prokaryote	Yes
Protist	Eukaryote	Yes
Fungus	Eukaryote	Yes
Parasitic worm	Eukaryote	Yes

Example: Leprosy is caused by a pathogen, but only certain people are susceptible.

Viruses

Viruses are extremely simple, tiny infectious agents composed of genetic material (DNA or RNA) surrounded by a protein coat. They lack organelles and are neither prokaryotic nor eukaryotic.

• Associated with diseases such as cold sores (herpes), chicken pox.

• Force host cells to assemble new virus particles (virions).

• Can be as simple as containing only 3 genes.

Special Types of Viruses

• Bacteriophages: Infect specific bacteria.

• Viruses as medicine: Used to defeat bacterial infections.

• Retroviruses: Use reverse transcriptase to convert viral RNA into DNA, which is then inserted into the host's DNA.

• Estimated 5-8% of human DNA consists of viral DNA remnants.

• Reverse transcriptase is important in biotechnology.

Example: HIV (antiretroviral drugs target reverse transcriptase).

Pathogenic Viruses

• Most viruses do not infect humans; notable exceptions include HIV, ZIKA, rabies, hepatitis, influenza, warts, chicken pox, Ebola.

• Viruses are host- and tissue-specific due to protein/carbohydrate interactions on cell and virus surfaces.

Example: Bacteriophage will not infect human cells; ACE2 is a receptor for coronavirus.

Bacteria

Bacteria are single-celled prokaryotes (distinct from archaea) with a rigid cell wall, plasma membrane, ribosomes, and DNA. Some have cilia or flagella for movement.

• Obtain energy by consuming organic molecules, breaking down inorganic molecules, or photosynthesis.

• Can reproduce rapidly via asexual reproduction; under ideal conditions, one bacterium can produce over 8 billion progeny in 11 hours.

Bacterial Diversity

• Diversity in energy and carbon sources.

• Most pathogenic bacteria consume organic molecules.

Genetic Diversity

• Despite asexual reproduction, bacteria introduce genetic diversity via plasmids (small, circular DNA).

• Plasmids can be exchanged, conferring antibiotic resistance.

• Genes on plasmids are often used in unusual circumstances.

• Viruses can leave genetic material in bacteria.

• Bacteria can take up DNA from the environment under stress.

Helpful Bacteria

• 99.36% of bacteria are not pathogenic.

• Roles include nitrogen fixation and bioremediation.

• Many bacteria live on/in humans (microbiota).

• Extract otherwise inaccessible calories/nutrients.

• Outcompete harmful bacteria.

Pathogenic Bacteria

• Examples: tetanus, cholera, tuberculosis, UTIs, pneumonia, salmonella, anthrax, leprosy, strep throat.

• Pathogenic bacteria produce destructive enzymes/toxins to spread through tissues.

Example: Bacterial gangrene: enzymes dissolve connections between muscle cells, then digest cells.

Antibiotics

Antibiotics are drugs that kill or inhibit the growth of bacteria, generally without affecting human cells. They exploit differences between bacterial and eukaryotic cells.

• Work against cell wall (e.g., penicillin, vancomycin), bacterial DNA, and ribosomes (e.g., tetracycline).

• Some are broad-spectrum; others target specific bacteria.

• Target is modified, immunity bypass, efflux, inactivating enzymes, protein synthesis.

Antibiotic Resistance

• Antibiotic resistance is a growing problem; some bacteria are resistant to all common antibiotics.

• Antibiotic-resistant staph infections (MRSA) are common in hospitals.

• Causes include incomplete courses of antibiotics and overuse in agriculture.

Important: Antibiotics are only effective against bacteria, not viruses.

Cell Signaling

Cell-Cell Communication

Cells communicate using various signals to coordinate activities. Most responses involve regulating gene expression or activating/inactivating proteins.

• Regulating gene expression (protein synthesis).

• Activating/inactivating already existing proteins (often by phosphorylation).

• Signals can be adjacent (gap junctions) or distant (hormones).

Examples of Signals

• Ions

• Small molecules

• Steroid hormones

• Nonsteroid hormones

Hormones

Hormones are chemical messengers that facilitate cell communication over long distances. In animals, the nervous and endocrine systems are the main communication systems.

• Hormones reach nearly every cell via blood.

• Each hormone/signaling molecule only works on certain target cells.

• Different target cells may exhibit different responses to the same hormone.

Example: Adrenaline/epinephrine affects the liver and heart differently.

Steroid Hormones

Steroid hormones are lipid-soluble and can diffuse through cell membranes to bind intracellular receptors, activating genes and stimulating protein production.

• Enter cells by diffusion.

• Bind to intracellular receptors.

• Activate genes and stimulate protein production.

• Change cellular activity.

• Similar in structure to cholesterol.

• Transported in blood via carrier proteins.

Examples: Testosterone, cortisol.

Nonsteroid Hormones

Nonsteroid hormones are proteins or similar molecules that cannot cross cell membranes. They bind to receptors on the cell surface, triggering a cascade of intracellular events.

• Bind to hormone receptors at the surface of target cells.

• Initiate a chain of events inside the cell, often involving enzyme activation.

• Cannot cross membranes.

Examples: Insulin, epinephrine.

Summary Table: Steroid vs Nonsteroid Hormones

Hormone Type	Structure	Receptor Location	Mechanism	Examples
Steroid	Lipid-soluble	Inside cell	Activate genes, stimulate protein production	Testosterone, cortisol
Nonsteroid	Protein/peptide	Cell surface	Activate enzymes via signaling cascade	Insulin, epinephrine

Key Equations and Concepts

• Viral Replication: Viruses use host cell machinery to replicate their genetic material and assemble new virions.

• Bacterial Growth Rate: Under ideal conditions, bacterial population doubles every 20 minutes.

• Gene Activation by Steroid Hormones: $\text{Steroid hormone} + \text{Receptor} \rightarrow \text{Hormone-Receptor Complex} \rightarrow \text{Gene Activation}$

• Antibiotic Mechanism: $\text{Antibiotic} + \text{Bacterial Target} \rightarrow \text{Inhibition of Growth or Death}$

Additional info: Some context and examples were expanded for clarity and completeness, including the summary tables and equations.