4.6.4 Structure of bacteria and viruses

A typical prokaryot

Ribosomes. Same function as eukaryotic cells (protein synthesis), but are smaller (70s rather than 80s).


Nuclear Zone. The region of the cytoplasm that contains DNA. There is no nuclear membrane.

DNA. Always circular, and not in chromosome form.


Plasmid. Very small circles of DNA, containing non-esential genes. Can be exchanged between different bacterial cells.


Cell membrane. made of phospholipids and proteins, like eukaryotic membranes.


Mesosome. Tightly-folded region of the cell membrane containing all the proteins required for respiration and photosynthesis.

Cell Wall. DIFFERENT from plant cell wall. Made of murein (a protein). There are two kinds of cell wall, which can be distinguished by a Gram stain:


A: Gram positive bacteria have a thick cell wall and stain purple


B: Gram negative bacteria have a thin cell wall with an outer lipid layer and stain pink.

Capsule (or Slime Layer). Thick polysaccharide layer outside of the cell wall. Used for;


  1. Sticking cells together


  1. As a food reserve


  1. As protection against desiccation (drying out) and chemicals, and as protection against phagocytosis (being broken down by a white blood cell).


Flagellum. A rotating tail used for propulsion.

A typical virus

Viruses have a wide range of different structures. Some viruses are about 100nm in diameter, whilst others can range from 20 – 3000nm.


All viruses have a protein coat (the capsid), which contains genetic material. The genetic material is either DNA or RNA, and can be single or double-stranded.

Figure 6.24. A – The basic structure of a virus. B – Some examples of different viruses.


The virus genetic material (the viral genome) contains only a few genes, from about 20 in the polio virus to more than 200 in the herpes virus (human genome contains ~80,000 genes). The viral genome codes for the proteins required to manufacture the virus.


The protein capsid is made from identical subunits (called capsomeres). The capsomeres can be arranged into an icosahedral shape (e.g. polio & herpes), or a cylindrical shape (e.g. TMV & rabies) or a loose containment structure (e.g. measles & influenza).


In addition, some viruses also have an outer membrane envelope, which allows the virus to penetrate the host cell membrane by endocytosis. Influenza, HIV and measles virus all have membrane envelopes.

Viral Damage – What do viruses actually do to us?

Like bacteria, viruses have protein ligands on their capsid that attach to ligand receptors on eukaryotic cells. After a virus ligand attaches to a host cell ligand receptor it becomes anchored to the host cell. The virus attempts to get its viral genome into the host cell,  usually through endocytosis using its lipid membrane. Viruses without lipid membranes may have specialised proteins designed to help inject the viral genome into the cell cytoplasm.


  • Virus RNA enters host cell


  • Virus may also inject RNA Polymerase into host cell as


  • Viral RNA and RNA Polymerase enter host cell nucleus via nuclear pores


  • Viral RNA is copied in nucleus


  • Viral RNA is transcribed using viral RNA Polymerase


  • Viral mRNA is translated in the cytoplasm


  • New Virus proteins formed


  • Viral proteins associate with copied RNA forming new complete viruses


  • New viruses leave host cell to infect other cells


Viruses that have a DNA code instead of an RNA code often insert their viral DNA into the host cell’s DNA. Other RNA viruses inject the enzyme Reverse Transcriptase, which makes a cDNA copy of the viral RNA. The cDNA copy is then inserted into the host cell’s DNA. Other viruses (e.g. HIV) also inject the enzyme integrase, which helps insert the viral cDNA into the host’s DNA


Be sure you can recall what the 3 viral enzymes do;


  • DNA Polymerase:
  • RNA Transcriptase:
  • Integrase:

Some viruses target specific tissues (e.g. Poliomyelitis virus targets motor neurones, HIV targets helper T cells, Influenza targets epithelial cells & rabies virus targets specific brain cells). If lots of new virus is being made, these host cell may lyse (burst) and die.