Cell: The Unit of Life · Lesson 2 of 16
Prokaryotic Cell Organisation and Genetic Material
“Examine bacterial shapes, nucleoid, plasmids and the cellular distinction between prokaryotes and eukaryotes.”
• Recognise the common bacterial cell shapes. • Describe the nucleoid and distinguish chromosomal DNA from plasmids. • Explain which cell structures are membrane enclosed in prokaryotes. • Use structural evidence rather than size alone to identify prokaryotic cells.
Prokaryotic cells are organised, not empty
A bacterium can be only a few micrometres across, yet it carries out transport, growth, reproduction and often remarkable metabolic activities. Prokaryotes include bacteria, including cyanobacteria, and archaea; the textbook introduces bacteria, cyanobacteria and mycoplasmas as examples. Their major distinction from eukaryotes is the absence of a membrane-bound nucleus and typical membrane-enclosed organelles such as mitochondria and the Golgi apparatus. Their cytoplasm is nevertheless structured and active, with DNA, ribosomes, enzymes and stored substances.
Common bacterial forms include bacilli, which are rod-like; cocci, which are spherical; vibrios, which are curved like commas; and spirilla, which have a helical appearance. The apparent shape can help describe a bacterium but does not uniquely identify its species or metabolic abilities. For instance, two rod-shaped bacteria may have different nutritional modes and arrangements of surface structures.
| Shape | Appearance | Useful observation |
|---|---|---|
| Coccus | Approximately spherical | Cells may form pairs, chains or groups |
| Bacillus | Rod-shaped | Length exceeds width |
| Vibrio | Curved rod or comma | One prominent curve |
| Spirillum | Rigid spiral form | Multiple helical turns |
The nucleoid is DNA without a nuclear envelope
Many bacteria have a principal chromosome consisting of a double-stranded DNA molecule, frequently circular, concentrated in a nucleoid region. The nucleoid does not have its own surrounding bilayer membrane. The word 'naked' in older descriptions means that this DNA is not enclosed within a nucleus; it does not mean the DNA is completely devoid of associated proteins. Bacterial DNA is compacted and functionally organised despite the absence of a nuclear envelope.
An irregularly defined region containing the main chromosomal DNA of a prokaryotic cell, without an enclosing nuclear membrane.
Plasmids carry additional hereditary information
Many bacteria also carry plasmids, smaller DNA molecules distinct from the main chromosome. A plasmid often replicates independently and can encode proteins that provide useful traits in a particular environment. Some plasmids carry resistance genes that alter bacterial sensitivity to antibiotics; others help exploit additional resources or exchange genetic material. The plasmid is not a second nucleus and does not replace the main chromosome.
It is inaccurate to assume every prokaryote has one circular chromosome and a plasmid. Many familiar textbook bacteria follow that plan, but bacterial genomic arrangements show exceptions and some cells have no plasmids. Using the typical model helps build a foundation; distinguishing the model from universal claims prevents incorrect generalisations. The core identification criterion remains the lack of a membrane-bound nuclear compartment.
Cytoplasm, ribosomes and surface organisation
The cytoplasm is the semi-fluid region where many metabolic reactions occur. Prokaryotic ribosomes synthesise proteins, often in association with messenger RNA and sometimes near the plasma membrane. The cell envelope is the barrier separating the living interior from external conditions; in most bacteria it includes a cell membrane and a wall. Mycoplasmas are a notable wall-less exception. Some bacteria also have flagella, pili or fimbriae extending outwards from the cell surface.
An electron micrograph may reveal internal membrane profiles in bacteria, but these should not automatically be labelled mitochondria or endoplasmic reticulum. Bacterial respiration and photosynthesis, where present, use specialised proteins and membranes organised differently from eukaryotic organelles. The existence of a respiratory pathway does not require a mitochondrial organelle.
How to compare different cell types
Compare cells by asking whether DNA is surrounded by a double nuclear envelope, whether large membrane-bound compartments occur, what the ribosome type is, and which surface coverings are present. Prokaryotic and eukaryotic cells both contain DNA, plasma membranes, cytoplasm and ribosomes. Their comparison is therefore a study of structural organisation, not a claim that one cell lacks the basic machinery of life.
A visible DNA-containing region should not be called a nucleus unless a nuclear envelope surrounds it. Likewise, a circular DNA drawing does not necessarily represent a plasmid; distinguish the main chromosome from separate accessory DNA.
Quick Quiz
Quiz
Which feature makes a bacterial nucleoid different from a nucleus?
Which shape describes a vibrio?
What is a plasmid?
Which of these occurs in both prokaryotic and eukaryotic cells?
Why does antibiotic resistance sometimes spread through bacteria?
Practice Problems
- Describe the principal structural differences between a bacterium and a eukaryotic cell.
- Distinguish a chromosome from a plasmid in a typical bacterium.
- Why should a bacterium's shape not be treated as a complete identification?
- Give two features shared by prokaryotes and eukaryotes and two distinguishing features.
Model Answers
1. Bacteria lack a membrane-enclosed nucleus and typical endomembrane organelles; their DNA is in a nucleoid, with 70S cytosolic ribosomes. Eukaryotes have a nucleus and membrane-bound compartments.
2. The main chromosome carries much essential hereditary information and is concentrated in the nucleoid. A plasmid is an additional DNA element that may carry accessory traits such as resistance.
3. Coccus, bacillus, vibrio and spirillum describe external geometry but not genome, metabolism, ecology or molecular relationships.
4. Shared: cytoplasm, plasma membrane, DNA and ribosomes. Distinctions: eukaryotes typically have a membrane-enclosed nucleus and multiple membrane-bound organelles; prokaryotes do not.
Key Takeaways
A prokaryote has an organised cytoplasm but no membrane-bound nucleus. Nucleoid DNA is not enclosed by a nuclear membrane; plasmids are additional hereditary elements in many bacteria. Common forms include cocci, bacilli, vibrios and spirilla. Prokaryotes and eukaryotes share DNA, ribosomes, cytoplasm and a plasma membrane.