Structure of prokaryotic cells and viruses (3.2.1.2)
An overview of the structure of prokaryotic cells and viruses (3.2.1.2) from AQA A level Biology
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Prokaryotes are unicellular organisms that include bacteria and archaea. They differ from eukaryotes in several ways:
- Prokaryotic cells are smaller.
- They lack membrane–bound organelles, including a nucleus.
- Their ribosomes are smaller.
- They have a peptidoglycan cell wall made of murein.
- Their DNA consists of a circular molecule, free in the cytoplasm and they may also contain additional plasmids.
- Some can be surrounded by a capsule.
- Some can possess more than one flagella.
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Prokaryotic DNA consists of a single circular DNA molecule free in the cytoplasm, not associated with histone proteins and does not contain introns, making it shorter.
Prokaryotes may also contain additional small circular DNA molecules called plasmids.
Like all DNA, it is composed of nucleotides joined by phosphodiester bonds and is similar to the circular DNA found in mitochondria and chloroplasts.
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Viruses are acellular and non-living infectious agents.
- Their only way of replicating is inside living host cells.
- They consist of a core of nucleic acid, which can be either DNA or RNA, and may be single or double stranded. This is surrounded by a capsid (protein shell).
- They possess attachment proteins on their surface that can bind to host cells, to initiate infection. For some viruses, these proteins are embedded in a lipid envelope.
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