Structure of bacteria and viruses

Microbiology, Immunity and Forensics · Bacteria, viruses and infection · note 1 of 4

Spec 6.5(i)

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Structure of bacteria and virusesSpec 6.5(i)

In short

Bacteria are single prokaryotic cells with a cell wall, cytoplasm, 70S ribosomes and circular DNA. Viruses are acellular: nucleic acid enclosed in a protein capsid, sometimes surrounded by an envelope from the host membrane. Viruses have no ribosomes or metabolism and replicate only inside host cells.

Bacteria are living prokaryotic cells, about 0.5 to 5 µm long. Viruses are far smaller (most are about 20 to 300 nm across; Ebola filaments are about 80 nm wide but often over 1 µm long) and are not cells: they have no cytoplasm, no ribosomes and no metabolism of their own, and they can reproduce only inside a host cell.

Bacteria compared with viruses
Feature	Bacterium	Virus
Cell structure	Prokaryotic cell: cell-surface membrane, cell wall of peptidoglycan, cytoplasm	Acellular: no membrane or cytoplasm of its own
Genetic material	Circular DNA (nucleoid), often plasmids too	DNA or RNA (single or double stranded), never both, in small amounts
Protein coat	None (cell wall is a different structure)	Capsid made of protein subunits (capsomeres) around the nucleic acid
Envelope	None	Present in some viruses: lipid bilayer from the host membrane with viral glycoproteins
Ribosomes	70S ribosomes	None; uses host ribosomes
Reproduction	Binary fission; can reproduce outside a host	Only inside a host cell, using its enzymes, ribosomes and ATP
Susceptible to antibiotics	Yes	No

Four named viruses

Structure of the viruses named in the specification
Virus	Nucleic acid	Capsid	Envelope	Notes
Ebola virus	Single-stranded RNA	Helical capsid forming a long filament	Yes, with glycoproteins	Infects human cells; filamentous particle
Tobacco mosaic virus (TMV)	Single-stranded RNA	Helical (rod-shaped) capsid of identical protein subunits	No	Infects plant cells, causing mosaic discolouration of leaves
HIV	Two copies of single-stranded RNA, plus reverse transcriptase	Cone-shaped capsid	Yes, with glycoproteins (gp120, gp41) that bind to host cells	Infects helper T cells
Lambda phage (λ phage)	Double-stranded DNA	Icosahedral head with a tail and tail fibres	No	A bacteriophage: infects bacteria (Escherichia coli)

Five structures drawn side by side: Ebola virus (helical capsid, envelope with glycoprotein spikes, single-stranded RNA), tobacco mosaic virus (helical capsid, RNA, no envelope), HIV (envelope with gp120 and gp41, cone-shaped capsid, two RNAs, reverse transcriptase), lambda phage (icosahedral head with double-stranded DNA, long non-contractile tail, single tail fibre) and a bacterium. — The viruses named in the specification compared with a bacterium (not drawn to scale).

Common mistake:

Viruses are not cells and are not made of 'a nucleus and cytoplasm'. Say 'nucleic acid in a capsid, with or without an envelope'. Never say viruses are killed by antibiotics.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between bacteria and viruses?

Bacteria are living prokaryotic cells with a cell wall, cytoplasm, ribosomes and circular DNA, and reproduce by binary fission. Viruses are acellular: nucleic acid in a protein capsid, sometimes with an envelope. They have no metabolism, reproduce only inside host cells, and antibiotics do not work against them.

What do lytic and latent mean for a virus?

In the lytic cycle a virus replicates immediately, makes many new particles and bursts the host cell. In latency the viral genome stays in the host cell without making new virus, often integrated into the host DNA as a prophage or provirus, until something triggers the lytic cycle.

How does HIV cause AIDS?

HIV binds to the CD4 receptor on helper T cells, enters, uses reverse transcriptase to make DNA from its RNA, integrates it and then makes new viruses that burst out. As helper T cells are destroyed, the immune system fails, and opportunistic infections such as TB cause death.

All 5 questions on Bacteria, viruses and infection