Viruses, Cellular Structures, and Processes

Posted on Nov 7, 2024 in Biology

Viruses

Viruses originate in aquatic environments and can associate with particular ways of life. They are free, evolving entities that survive by becoming parasites. Essentially, they are organisms that have lost their cellular machinery through adaptation to parasitism or are genetic fragments detached from cellular organisms. Their specificity lies in their nucleic acid, which allows them to parasitize cells.

Each virion consists of a nucleic acid (DNA or RNA, never both simultaneously), a capsid that protects and transports it, and sometimes a lipoprotein envelope derived from the host cell’s membrane. The nucleic acid and capsid together form the nucleocapsid.

Virion Structures

  • Helical virions: The capsomers (protein subunits) are arranged in a helix, forming a cylindrical capsid with the nucleic acid inside. They can be naked or enveloped.
  • Icosahedral virions: The capsid is icosahedron-shaped and contains the nucleic acid. These can also be naked or enveloped.
  • Complex virions: Primarily bacteriophages, these have distinct parts:
    • Head: Icosahedral capsid containing the nucleic acid.
    • Tail: Hollow tube through which the nucleic acid passes, often surrounded by a sheath.
    • Basal plate: Contains caudal fibers that bind to the bacterial cell wall and spines for attachment.

Lytic Cycle

The most frequent viral reproduction cycle, leading to host cell destruction:

  1. Absorption: Caudal fibers and basal spines attach to the bacterial cell wall.
  2. Penetration: The phage sheath contracts, injecting viral DNA into the cytoplasm.
  3. Multiplication: Viral DNA takes over cellular metabolism, producing viral proteins and enzymes that destroy bacterial DNA. Viral nucleic acid replicates.
  4. Assembly: New virions are formed from the synthesized components.
  5. Liberation: The cell membrane ruptures, releasing new virions.

Lysogenic Pathway

Viral DNA integrates into the bacterial chromosome, replicating with it. External stimuli can trigger the prophage to separate and initiate a lytic cycle.

Viroids

Parasites exclusive to plants, formed by circular, single-stranded DNA fragments. They interfere with cellular genes, causing necrosis, chlorosis, and stunted growth. They are transmitted by insects or contaminated materials.

Prions

Infectious protein particles without nucleic acid. Their altered secondary structure causes spongiform encephalopathies (e.g., mad cow disease, Creutzfeldt-Jakob disease), leading to lack of coordination, itching, and dementia.

Provirus

Mobile DNA fragments that carry genes between cells.

Plasmids

Circular DNA fragments that replicate independently of the bacterial chromosome.

Cell Wall

A rigid, thick cover of polysaccharides surrounding plant cells, providing support and allowing the cytoplasm to be hypertonic. It consists of a matrix of hemicellulose and pectin, with primary and secondary layers. Plasmodesmata connect adjacent cells.

Ribosomes

Non-membranous granules responsible for protein synthesis (translation). Composed of rRNA and proteins.

Endoplasmic Reticulum

Two types: rough ER (with ribosomes, synthesizes and transports proteins) and smooth ER (synthesizes, stores, transports, and metabolizes lipids and toxins).

Golgi Apparatus

A set of dictyosomes that process and package proteins and lipids, form lysosomes, repair cell membranes, and contribute to cell secretions.

Cell Membrane Transport

  • Active Transport: Requires ATP to move molecules against their concentration gradient.
  • Passive Transport: Occurs spontaneously without energy expenditure, following the concentration gradient. Can be simple (through channel proteins) or facilitated (by permeases).

Metabolism

  • Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.
  • Catabolism: Breakdown of complex molecules, releasing energy.

Cell Cycle

The sequence of growth and division:

  • Interphase:
    • G1: Cell growth and increase in biomolecules.
    • S: DNA replication.
    • G2: Preparation for division.
  • Division:
    • Mitosis: Nuclear division.
    • Cytokinesis: Cytoplasmic division.

Mitosis

Cell division resulting in two identical daughter cells:

  1. Prophase: Chromosomes condense, nucleoli disappear, mitotic spindle forms.
  2. Metaphase: Chromosomes align at the equatorial plane.
  3. Anaphase: Chromatids separate and move to opposite poles.
  4. Telophase: Chromosomes decondense, nuclear envelope reforms, cytokinesis begins.