Monera, Viruses, Plasmids, Viroids, and Prions: Characteristics and Classification

Posted on Jan 5, 2025 in Biology

Monera: Structure, Morphology, and Reproduction

Monera are the simplest organisms, lacking a nucleus and other membrane-bound organelles. They are divided into two main domains: Bacteria and Archaea.

Eubacteria

Structure and Composition:

  • Capsule: Present in some bacteria, composed of polysaccharides (glycocalyx). It aids in attachment, pathogen penetration, protection against viruses, resistance to phagocytosis, and water accumulation. It also protects against pH changes and host defenses.
  • Cell Wall: A rigid structure that protects the cell and determines its shape.
  • Plasma Membrane: A lipid bilayer without sterols. It has invaginations (mesosomes) for increased metabolic activity.
  • Cytoplasmic Matrix: Contains no membrane-bound organelles, mostly water. It includes the bacterial chromosome in the nucleoid, plasmids, ribosomes, and granules of organic or inorganic substances. Many have gas vesicles.

Morphology:

  • Cocci: Spherical, including diplococci, streptococci (long chains), staphylococci (clusters), tetrads, and cubic arrangements.
  • Bacilli: Rod-shaped (cylindrical), occurring in pairs.
  • Vibrios: Comma-shaped.
  • Spirillum: Elongated and coiled.
  • Spirochetes: Flexible and coiled.
  • Pleomorphic: Irregularly variable shapes.

Bacterial Reproduction:

Asexual reproduction occurs through binary fission. Genetic material is duplicated, and each chromosome attaches to the membrane. The membrane and wall lengthen, separating the chromosomes. Diversity arises from gene transfer mechanisms (parasexual):

  • Conjugation: Transfer via sex pili.
  • Transformation: Uptake of free DNA fragments.
  • Transduction: DNA transfer by viruses.

Archaea

Archaea are a heterogeneous group found in extreme environments. They can be isolated or grouped and reproduce by binary fission, budding, or fragmentation. They lack an external membrane but have a plasma membrane with proteins, diethers, and glycerol tetraethers.


Viruses: Structure, Classification, and Multiplication

Viruses are acellular entities with genetic material (DNA or RNA) that require living cells for reproduction. They lack their own metabolism and use host cell machinery.

Structure and Composition:

A complete virus particle is called a virion. The genetic material is enclosed in a capsid made of proteins (protomers, capsomeres). Viruses have enzymes for replication within host cells.

Classification

Viruses are classified based on:

  • Nucleic Acid:
    • DNA: Single-stranded (parvoviruses) or double-stranded (herpesviruses).
    • RNA: Single-stranded (positive or negative polarity, segmented or non-segmented) or double-stranded (segmented).
  • Morphology: Helical, icosahedral, enveloped, or complex.

Multiplication Cycles

Lytic Cycle:

  1. Adsorption and fixation to specific receptors.
  2. Penetration and injection of nucleic acid.
  3. Replication of nucleic acid.
  4. Transcription and protein synthesis.
  5. Assembly and packaging of structural units.
  6. Lysis and release of mature virions.

Lysogenic Cycle:

Some viruses integrate their DNA into the host chromosome (provirus) and replicate with it. Under certain conditions, the viral DNA can become active and enter the lytic cycle.

Origin and Evolution

  1. Viruses may have originated from prokaryotic cells that simplified and became dependent.
  2. Viruses may have originated from independent nucleic acids that mutated and became infectious.


Acellular Entities: Plasmids, Viroids, and Prions

Plasmids

Small, circular, double-stranded DNA molecules that replicate independently of the chromosome. They can integrate into the chromosome and are transmitted to subsequent generations. They often carry beneficial properties.

Types:

  • Conjugative: Encode sex pili for conjugation.
  • Resistance: Confer resistance to antibiotics.
  • Col: Produce bacteriocins.
  • Virulence: Increase pathogenicity.

Viroids

Small, circular, single-stranded RNA molecules that cause diseases in plants. They interfere with RNA processing and gene regulation.

Prions

Infectious protein particles that cause neurodegenerative diseases.