BIO chapter 17 pt 1

Chapter 17: The Microorganisms: Viruses, Bacteria, and Protists

The Viruses

  • Definition and Classification:

    • Not classified as organisms.

    • Characteristics: Noncellular, obligate intracellular parasites, and uncertain vitality (living or non-living).

    • Viruses have significantly contributed to our understanding of diseases, genetics, and the characteristics of living organisms.

    • Size: Extremely small at around $0.2 ext{ µm}$, which is $ rac{1}{5}$ the size of bacteria.

Structure of Viruses

  • Primary Structural Components:

    1. Outer Capsid: Protein shell that encases the virus.

    2. Inner Core: Composed of either DNA or RNA, which serves as the genetic material.

  • Features of Animal Viruses:

    • Many animal viruses possess:

    • Spike-shaped glycoproteins for attaching to host cells.

    • An outer envelope made of the host’s plasma membrane, interspersed with virus proteins.

    • Various proteins and enzymes within the viral interior.

Debate Over Viral Life

  • Key Considerations:

    • Viruses act as obligate intracellular parasites.

    • They lack the ability to acquire nutrients or utilize energy independently.

    • It can be synthetically produced in laboratories.

    • It possesses a genome that can mutate and directly controls the virus's reproduction.

Viral Reproduction

  • Host Specificity:

    • Viruses show specificity toward their host cells, such as:

    • HIV infects immune cells.

    • The chickenpox virus targets nerve cells.

    • Process within the host involves:

    • Viral genome takeover of the host cell's machinery.

    • Utilization of the host's enzymes, ribosomes, tRNA, and ATP for synthesizing new viral particles.

Bacteriophages

  • Definition: A bacteriophage (or phage) is a virus that specifically reproduces within bacteria.

  • Reproductive Cycles: Two cycles in phage reproduction are:

1) Lytic Cycle Breakdown

Stages of the Lytic Cycle:

  1. Attachment:

  • The capsid molecule attaches to the receptor on the host cell's surface.

  1. Penetration:

  • Viral enzymes degrade the bacterial cell wall, enabling viral DNA injection.

  1. Biosynthesis:

  • Virus disables non-essential host genes.

  • The virus hijacks the host cell's machinery and resources to begin making its own parts.

  1. Maturation:

  • The virus's components assemble into new viral particles that can infect other cells.

  1. Release:

  • Viral lysozymes cause the bacterium to lyse (rupture), releasing new virus particles.

    • Outcome: The lytic cycle results in the death of the host bacteria.

2) Lysogenic Cycle Overview

  • Characteristics:

    • Initial steps mirror the lytic cycle but do not immediately cause lysis. ( first 2 steps as lytic)

    • Integration:

    • Viral DNA integrates into the host DNA, forming a latent viral state called a prophage.

    • Replication:

    • Prophage replicates along with the host's DNA and passes to daughter cells (lysogenic cells).

    • Trigger for Lytic Cycle:

    • These lysogenic cells can be activated to enter the lytic cycle under certain conditions.

Plant Viruses

  • Infection Mechanism:

    • Enter through the plant's damaged tissues.

    • Spread through plasmodesmata (tunnels connecting plant cells).

  • Transmission Mechanisms:

    • Insects, horticultural tools, seeds, and pollen serve as pathways for plant virus transmission.

    • Control Notions: Viral diseases resist chemical control but can be managed through biotechnological methods.

Animal Viruses

  • General Reproduction:

    • The process is similar to bacteriophage replication.

    • Entry into host cells is through endocytosis.

    • The outer membrane is shed post-entry into the host.

    • Example: Herpesviruses.

Retroviruses

  • Definition and Function:

    • Class of RNA viruses characterized by the presence of a DNA stage.

    • Use reverse transcriptase to convert RNA into complementary DNA (cDNA).

    • Example: HIV.

Emerging Viruses

  • Definition: Diseases recently emerging with significant human infection rates.

  • Pathways of Emergence:

    • Relocation to new locations.

    • Ability to infect new host species or change modes of transmission.

    • Examples of emerging viruses include HIV, West Nile Virus, SARS Virus, avian influenza (H5N1), hantavirus, and SARS-CoV-2.

  • Incidence Factors:

    • High mutation rates among viruses.

    • Some viruses transition between host species.

    • Alterations in transmission modes can further elevate disease incidence.

Drug Control of Human Viral Diseases

  • Challenges:

    • Developing antiviral drugs is complex since viruses utilize host cells for replication.

    • Categories of Drugs:

    • Some impede viral genome replication.

    • Others inhibit HIV-specific reverse transcriptase enzymes.

Viroids

  • Definition: Simple structures consisting of naked RNA not encapsulated by a capsid.

  • Relation to Viruses: While similar to viruses, they primarily induce crop diseases, such as Hepatitis D.

Prions

  • Historical Context:

    • Discovered in the Fore tribe, known for cannibalistic traditions.

  • Definition: Infectious proteins that can lead to fatal neurodegenerative disorders.

  • Mechanism: They alter the structure of other proteins, which can lead to diseases like Mad Cow Disease.