Viruses

Study Guide on Viruses

Distinguishing Viruses from Cells

• Size: Viruses are much smaller than cells, typically ranging from 20 nm to 300 nm, while cells are generally 1 µm or larger.

• Structural Elements Present:

  • Viruses: Have a protein coat (capsid), nucleic acid (RNA or DNA), and sometimes an envelope made of lipids.

  • Cells: Composed of membranes, organelles (like nucleus, mitochondria), cytoplasm, and a cell wall (in plants and bacteria).

• Structural Elements Absent:

  • Viruses: Lack cellular structures (no organelles or metabolic machinery).

  • Cells: Do not have a protein capsid or viral genome.

Structures of Typical Virus Types

• Core Components:

  • Nucleotide Genomes: Present in all viruses as DNA or RNA (single-stranded or double-stranded).

  • Protein Capsids: Present in all viruses, serving as protection for the genetic material.

  • Phospholipid Envelopes: Present in some viruses (e.g., enveloped viruses) but not in all (e.g., non-enveloped viruses).

• Variations: Different types of viruses may have differences in genome type, size, and presence or absence of an envelope.

Dependence on Host Cells for Reproduction

• Viruses lack the cellular machinery necessary for reproduction (e.g., ribosomes, enzymes). Therefore, they must infect a host cell and hijack its metabolic processes to replicate and produce new virus particles.

Lytic vs. Lysogenic Cycles in Phages

• Lytic Cycle:

  • Phage attaches to host, injects genetic material, uses host machinery to replicate, assembles new phages, and then lyses the host cell to release new phages.

• Lysogenic Cycle:

  • Phage integrates its DNA into the host's genome (provirus) and replicates along with it without causing immediate lysis. The phage can enter the lytic cycle later under certain conditions.

Animal Viruses with Envelopes vs. Phages

• Life Cycle Differences:

  • Enveloped animal viruses enter host cells via fusion with the cell membrane or receptor-mediated endocytosis, while phages inject their genome directly into the host. Enveloped viruses may also acquire their envelope from the host cell membrane during budding.

Retroviruses Compared to Other Virus Types

• Retroviruses: Contain RNA genomes and replicate through a DNA intermediate via reverse transcription. They incorporate into the host DNA, which is a key distinction from other viruses that typically do not reverse transcribe their genomes.

Variation in Viral Genomes

• Major Ways of Differentiation:

  • Type of Nucleic Acid: RNA vs. DNA, single-stranded vs. double-stranded.

  • Genome Structure: Circular vs. linear genomes.

  • Size of Genome: Varies significantly among types of viruses.

  • Presence of Additional Features: e.g., segmented genomes (as in some RNA viruses).