Viruses: Structure, Bacteriophages, and Life Cycles

Viruses

• Whether viruses are considered living or non-living is a matter of debate.

• Viruses possess genetic material (DNA or RNA) and can evolve.

• Viruses cannot reproduce on their own; they require a host cell.

• Viruses lack the cellular machinery found in living cells such as ribosomes

• A virus has a basic structure including genetic material (DNA or RNA) surrounded by a protein coat called a capsid.

• A bacteriophage is a virus that infects bacteria.

• Bacteriophages have a complex structure with a head containing genetic material, a tail sheath, and tail fibers for attachment to the host cell.

• A virus that attacks bacteria is called a bacteriophage.

• The lytic cycle involves the virus infecting a host cell, replicating, and lysing (bursting) the cell to release new viruses.

• The lysogenic cycle involves the virus integrating its DNA into the host cell's DNA.

• In the lysogenic cycle the viral DNA replicates along with the host cell DNA without causing immediate harm.

• The lytic cycle results in the death of the host cell, while the lysogenic cycle allows the virus to remain dormant for a period.

• Lytic infection steps:

  • Attachment: Virus attaches to host cell.
  • Entry: Virus injects its genetic material into the cell.
  • Replication: Viral DNA replicates and synthesizes viral proteins.
  • Assembly: New viral particles are assembled.
  • Release: New viruses are released by lysis of the host cell.

• During lysogenic infection, viral DNA integrates into the host cell's DNA.

• While integrated in the lysogenic cycle, the viral DNA is called a prophage.

• When the DNA of the prophage becomes active, it excises from the host cell's DNA.

• When the DNA of the prophage becomes active, it enters the lytic cycle.

• The host cell will then begin to produce new viral particles, leading to cell lysis.

• A retrovirus is a virus that uses RNA as its genetic material.

• Retroviruses use reverse transcriptase to convert their RNA into DNA.

• HIV (Human Immunodeficiency Virus) is an example of a retrovirus.

• Viruses are obligate parasites because they cannot replicate without a host cell.

• Viruses need the host cell's machinery (ribosomes, enzymes) to produce new viral particles.

• Latent viruses can become active due to various factors, such as stress, weakened immune system, or changes in the environment.

• Viruses cause disease by infecting and damaging host cells.

• Viruses can disrupt normal cell function, cause cell lysis, or trigger an immune response that damages tissues.

• Viruses harm cells by using the cell's resources to replicate, leading to cell death or dysfunction.

Bacteria

• Bacterial cells are prokaryotic, lacking a nucleus and membrane-bound organelles.

• Eukaryotic cells have a nucleus and membrane-bound organelles.

• Bacterial cells have a cell wall made of peptidoglycan.

• Eukaryotic cells have a cell wall made of cellulose (in plants) or chitin (in fungi).

• Bacterial cells are smaller and simpler in structure than eukaryotic cells.

• Gram stain helps identify bacteria based on their cell wall structure.

• Gram-positive bacteria have a thick peptidoglycan layer and stain purple.

• Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane and stain pink or red.

• Bacteria can have various shapes, including cocci (spherical), bacilli (rod-shaped), and spirilla (spiral).

• Bacteria can be arranged in pairs (diplo-), chains (strepto-), or clusters (staphylo-).

• Bacteria can move using flagella (whip-like structures), pili (hair-like structures), or by gliding.

• Not all bacteria are motile; some are non-motile.

• An obligate aerobe requires oxygen to grow.

• An obligate anaerobe cannot survive in the presence of oxygen.

• A facultative anaerobe can grow with or without oxygen.

• Bacteria grow through binary fission, a form of asexual reproduction.

• Bacteria divide into two identical daughter cells.

• Conjugation is the transfer of genetic material between bacterial cells through direct contact or a bridge-like connection.

• Conjugation can be thought of as a "pseudo" sexual process because it involves the exchange of genetic material.

• Conjugation does not result in the formation of a new organism.

• Bacteria can cause disease by producing toxins or by directly invading and damaging tissues.

• Bacteria obtain energy and nutrients through various methods:

  • Some are autotrophs, producing their own food through photosynthesis or chemosynthesis.
  • Some are heterotrophs, obtaining nutrients from organic matter.
  • Some are saprophytes, feeding on dead organic matter.
  • Some are parasites, obtaining nutrients from a living host.

• Bacteria can survive unfavorable conditions by forming endospores.

• Endospores are dormant, highly resistant structures that can withstand extreme temperatures, radiation, and chemicals.

• Positive impacts of bacteria:

  • Nutrient cycling
  • Production of antibiotics and certain foods (yogurt, cheese)

• Negative impacts of bacteria:

  • Causing diseases
  • Spoilage of food

Disease and the Immune System

• A pathogen is a disease-causing organism, such as a bacterium, virus, fungus, or parasite.

• The first line of defense against pathogens includes physical and chemical barriers.

• These barriers prevent pathogens from entering the body.

• Skin acts as a physical barrier, preventing pathogen entry.

• Tears, saliva, and mucus contain enzymes (e.g., lysozyme) that break down bacterial cell walls.

• Cilia in the respiratory tract trap and remove pathogens.

• Stomach acid kills many ingested pathogens.

• Mucous membranes trap pathogens and prevent them from entering the body.

• A pathogen might enter the body through breaks in the skin, the respiratory tract, the digestive tract, or the urogenital tract.

• The white blood cells (leukocytes) involved in the innate immune response include:

  • Neutrophils
  • Macrophages
  • Natural killer cells

• The inflammatory response is a localized reaction to infection or injury.

• Histamines are released, causing blood vessels to dilate and become more permeable.

• Increased blood flow brings more immune cells to the site of infection.

• Pus contains dead cells, bacteria, and debris.

• A fever is beneficial because it inhibits the growth of some pathogens and increases the rate of immune reactions.

• An antigen is a substance that triggers an immune response.

• An antibody is a protein produced by the immune system that recognizes and binds to a specific antigen.

• The humoral response events:

  • B cells recognize and bind to a specific antigen.
  • B cells are activated and differentiate into plasma cells.
  • Plasma cells produce antibodies that bind to the antigen.
  • Antibodies neutralize the antigen, mark it for destruction, or activate complement.

• The two types of lymphocytes involved in the humoral response are B cells and T helper cells.

• B cells produce antibodies, and T helper cells help activate B cells and other immune cells.

• B cells recognize antigens directly, while T helper cells recognize antigens presented by other cells.

• The immune system creates a "memory" of an infection by producing memory cells.

• Memory cells are long-lived lymphocytes that can quickly recognize and respond to an antigen if it is encountered again in the future

• The primary immune response occurs when the body is first exposed to an antigen.

• The secondary immune response is faster and stronger than the primary response.

• The secondary response is due to memory cells that recognize the antigen and quickly produce antibodies

• The cell-mediated response involves T cells that directly kill infected cells.

• Killer T cells (cytotoxic T lymphocytes) recognize and bind to infected cells.

• Killer T cells release substances that cause the infected cells to undergo apoptosis (programmed cell death).

• Passive immunity is temporary immunity acquired by receiving antibodies from another source.

• Examples of passive immunity include:

  • Antibodies passed from mother to fetus through the placenta.
  • Antibodies received through breast milk.
  • Injection of antibodies (e.g., immunoglobulin).

• Active immunity is long-lasting immunity acquired by being exposed to an antigen and producing antibodies.

• Active immunity can be acquired through:

  • Natural infection
  • Vaccination

• A vaccine is a substance that contains antigens that stimulate an immune response, leading to active immunity.

• Types of vaccines:

  • Live attenuated vaccines contain weakened pathogens that can still stimulate an immune response but are less likely to cause disease.
  • Inactivated vaccines contain killed pathogens that cannot replicate but can still stimulate an immune response.
  • Subunit vaccines contain only specific parts of a pathogen (e.g., proteins) to stimulate an immune response.
  • DNA/RNA vaccines contain genetic material that codes for specific antigens, stimulating an immune response.

• All vaccine types lead to active immunity by stimulating the body to produce antibodies and memory cells.

• Herd immunity is the protection of a population from an infectious disease when a large percentage of individuals are immune.

• Herd immunity protects vulnerable populations, such as infants, the elderly, and immunocompromised individuals who cannot be vaccinated.

Explore the structure of viruses, including the genetic material and capsid. Learn about bacteriophages and their role in infecting bacteria. Understand the lytic and lysogenic cycles of viral replication and their impact on host cells.