state that all viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids

Cells as the Basic Units of Living Organisms

Learning Objective

State that all viruses are non‑cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids.

Key Points

• Viruses are not cells; they lack the machinery required for independent metabolism and reproduction.
• Every virus contains a nucleic acid genome:

  • DNA viruses – double‑stranded or single‑stranded DNA.
  • RNA viruses – double‑stranded, single‑stranded positive‑sense (+) RNA, or single‑stranded negative‑sense (–) RNA.

• The genome is enclosed within a protein shell called a capsid. The capsid is composed of repeating protein subunits called capsomeres.
• Some viruses possess an additional outer layer called an envelope, which is derived from the host cell’s phospholipid membrane and contains viral glycoproteins.

Structural Summary of a Typical \cdot irus

Why \cdot iruses Are Considered Non‑Cellular

1. They lack a cytoplasm and organelles.
2. They cannot carry out metabolic reactions on their own.
3. Replication occurs only inside a host cell, using the host’s biosynthetic machinery.

Comparison: Cells vs. Viruses