International A Level Biology / Microbiology

Bacteria and viruses compared revision notes

Edexcel International A Level Biology (YBI11), chapter 17: Microbiology, specification point 6.5. Free lesson.

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Bacteria are living prokaryotic cells, while viruses are not cells and can only be copied inside a host cell. Comparing their structures, and the structures of four named viruses, helps you identify their key features accurately.

How bacteria and viruses differ

A bacterium is a prokaryotic cell with cytoplasm. A virus is not a cell: it has no cytoplasm or ribosomes and depends on a host cell to make more viruses. Bacteria are also much larger than most viruses. A bacterium is about 1 to 5 µm long. Most viruses are about 20 to 300 nm across. Bacterial DNA is a circular chromosome, often with extra small DNA loops called plasmids. A virus contains DNA or RNA, never both. Bacteria have a peptidoglycan cell wall. Some also have a capsule. Viruses have a protein coat called a capsid, and some have an envelope. Bacteria have 70S ribosomes and…

The parts of a bacterial cell

The bacterial cell has the structures it needs to live and divide, but no nucleus. Its circular DNA is free in the cytoplasm, and its ribosomes make the cell's proteins. Capsule: a slimy outer layer found in some bacteria. Cell wall: made of peptidoglycan; it helps stop the cell bursting. Cell surface membrane: just inside the wall, enclosing the cytoplasm. Circular DNA: one long loop of DNA free in the cytoplasm. Plasmid: a smaller, separate loop of DNA. 70S ribosomes: the sites where the bacterium makes its own proteins.

HIV and λ phage have different structures

HIV is an RNA virus with an envelope. Its reverse transcriptase enzyme makes DNA using viral RNA as a template. This makes HIV a retrovirus. λ phage infects the bacterium E. coli and has DNA inside a head, with a tail for attachment and injection. HIV has two single strands of RNA, which contain its genes. Its capsid is a protein coat made of capsomeres and is roughly cone-shaped. HIV's envelope is a lipid membrane taken from the host cell surface membrane as the virus leaves. Glycoproteins on the envelope bind to receptors on a host cell. From the inside out, HIV has RNA and reverse…

TMV and Ebola have helical capsids

Both tobacco mosaic virus (TMV) and Ebola virus have single-stranded RNA inside a helical capsid, where protein subunits follow a spiral. Their outer structures differ: TMV is a stiff rod with no envelope, while Ebola is an enveloped, flexible thread. TMV infects tobacco plant leaves. About 2,000 identical capsomeres stack around its single RNA strand to form a stiff, hollow rod. It has no envelope and no enzymes. Ebola has a single RNA strand running along the length of the virus. Its protein subunits spiral around the RNA, and an envelope wraps around the capsid. Ebola's envelope has…

Compare the four named viruses

For each virus, learn its nucleic acid, capsid and whether it has an envelope. This makes it easier to give precise structural comparisons. Ebola: single-stranded RNA; a helical capsid in a long, filamentous thread; envelope present. TMV: single-stranded RNA; a helical capsid forming a rigid rod; no envelope. HIV: two single strands of RNA and reverse transcriptase; cone-shaped capsid; envelope with glycoproteins. λ phage: double-stranded DNA; head-and-tail capsid; no envelope. Ebola and HIV have envelopes. TMV and λ phage do not. λ phage is the only one of the four with DNA;…

Do not confuse RNA viruses with retroviruses

Having RNA does not mean a virus uses reverse transcriptase. Only HIV is a retrovirus in this group. Ebola and TMV are RNA viruses, but their RNA is copied as RNA and they do not make DNA.