Difficulty: Introductory | Prerequisites: None, though familiarity with basic cell biology vocabulary is helpful.
Prokaryotic cells are the simplest and most ancient form of cellular life on Earth. Understanding their structure is the natural starting point before diving into the complexity of eukaryotic cells, which is where most introductory biology courses spend the bulk of their time.
Prokaryotes include all bacteria and archaea. They lack the internal membrane-bound compartments that define eukaryotes, yet they are extraordinarily successful, colonising virtually every habitat on the planet. If you have covered basic chemistry and macromolecules (DNA, proteins, lipids), you have everything you need for this topic.
Prokaryotic cells (bacteria and archaea) are small, structurally simple cells with no nucleus and no membrane-bound organelles. Their DNA sits in an open region called the nucleoid, they reproduce by binary fission, and they gain genetic diversity through horizontal gene transfer rather than sexual reproduction.
Prokaryote
An organism whose cells lack a membrane-bound nucleus and other membrane-bound organelles. Think of it as the "before the nucleus" branch of life (from Greek pro, before, and karyon, nut/kernel).
Nucleoid
The region within a prokaryotic cell where the DNA is concentrated. It is not enclosed by a membrane. In simple terms, this is where the chromosome lives, just without a wall around it.
Plasmid
A small, circular piece of DNA separate from the main chromosome. Plasmids often carry genes for antibiotic resistance or other survival advantages. Think of it as a bonus flash drive of genetic information.
Peptidoglycan
A polymer of sugars and amino acids that forms the rigid mesh in bacterial cell walls. In simple terms, it is the scaffolding that keeps a bacterial cell from bursting.
Binary fission
The asexual reproduction method used by prokaryotes, in which one cell divides into two genetically identical daughter cells. Think of it as the cell copying its DNA and then splitting in half.
Horizontal gene transfer (HGT)
The movement of genetic material between organisms by means other than parent-to-offspring inheritance. Includes transformation, conjugation, and transduction. In simple terms, bacteria can swap DNA with neighbours, which is one reason antibiotic resistance spreads so quickly.
70S ribosome
The smaller type of ribosome found in prokaryotic cells (compared with 80S ribosomes in eukaryotes). The "S" stands for Svedberg units, a measure of sedimentation rate, not size. Both types do the same job: translating mRNA into protein.
Transformation
Uptake of free DNA fragments from the environment by a bacterial cell.
Conjugation
Direct transfer of DNA between two bacterial cells through a physical connection called a pilus.
Transduction
Transfer of DNA from one bacterium to another via a bacteriophage (a virus that infects bacteria).
Prokaryotic DNA is not enclosed in a membrane-bound nucleus.
Instead it occupies the nucleoid, an irregularly shaped region in the cytoplasm.
The nucleoid is visible under electron microscopy as a lighter, less dense zone, but there is no physical barrier between it and the rest of the cell.
Most prokaryotes carry one circular DNA molecule as their main chromosome.
Many also harbour plasmids, which are small, independently replicating circles of DNA.
Plasmids frequently carry genes for antibiotic resistance, toxin production, or metabolic tricks that help in specific environments.
Prokaryotes lack mitochondria, endoplasmic reticulum, Golgi apparatus, and other compartments found in eukaryotic cells.
All metabolic processes happen in the cytoplasm or at the plasma membrane.
Some prokaryotes do have internal membrane folds (e.g. thylakoids in cyanobacteria), but these are not equivalent to true eukaryotic organelles.
Every prokaryote has a plasma membrane (phospholipid bilayer) controlling what enters and exits.
Most also have a rigid cell wall outside the membrane.
In bacteria, the wall typically contains peptidoglycan.
In archaea, the wall uses different molecules (pseudopeptidoglycan, S-layers, or polysaccharides), which is one reason archaea are classified separately.
The Gram stain distinguishes bacteria by wall thickness: Gram-positive cells have a thick peptidoglycan layer; Gram-negative cells have a thinner layer plus an outer membrane.
Prokaryotic ribosomes are 70S (composed of 30S and 50S subunits).
Eukaryotic ribosomes are 80S (40S + 60S subunits).
This size difference is clinically important: several antibiotics (e.g. tetracycline, chloramphenicol) target the 70S ribosome specifically, killing bacteria without harming human cells.
Typical range: 1 to 5 micrometres in diameter.
Most eukaryotic cells are 10 to 100 micrometres, roughly an order of magnitude larger.
Smaller size gives prokaryotes a higher surface-area-to-volume ratio, which supports faster nutrient exchange and growth.
The cell copies its single circular chromosome.
The two copies move to opposite ends of the cell.
The cell elongates and a septum (dividing wall) forms at the midpoint, splitting the cell into two identical daughters.
Under ideal conditions, some bacteria (e.g. E. coli) can divide every 20 minutes.
Prokaryotes do not reproduce sexually, but they still generate genetic variation through three mechanisms:
Transformation: a cell picks up free DNA fragments from the surrounding environment (e.g. from dead cells).
Conjugation: one cell transfers DNA directly to another through a pilus, a temporary protein bridge.
Transduction: a bacteriophage (bacterial virus) accidentally packages host DNA and delivers it to a new cell.
All three increase genetic diversity within populations and are a major reason antibiotic-resistance genes spread between species.
The structural simplicity of prokaryotes is directly exploited in medicine and biotechnology. Antibiotics such as penicillin target peptidoglycan synthesis, which is why they kill bacteria without harming human cells (we have no peptidoglycan). The difference between 70S and 80S ribosomes is another drug target: antibiotics like erythromycin bind the prokaryotic ribosome and block bacterial protein synthesis while leaving ours alone.
Plasmids are the workhorse of genetic engineering. Scientists insert genes of interest into plasmids and introduce them into bacteria, which then produce the desired protein at scale. Insulin, for example, is manufactured this way.
Students often think "no nucleus" means prokaryotes have no DNA. They do have DNA; it is simply not enclosed in a membrane-bound compartment.
Students often confuse the nucleoid with a nucleus. The nucleoid is an open region, not an organelle with its own membrane.
Students often assume prokaryotes cannot evolve quickly because they reproduce asexually. Horizontal gene transfer introduces significant genetic variation and can spread traits like antibiotic resistance across species in a single generation.
Students often think all prokaryotes are bacteria. Archaea are a separate domain with distinct cell-wall chemistry and membrane lipids.
⚠️ Be ready to compare prokaryotic and eukaryotic cells in a table format. Common columns: nucleus, organelles, ribosome size, DNA shape, cell wall composition, reproduction method.
⚠️ Know all three mechanisms of horizontal gene transfer by name and be able to distinguish them (transformation = free DNA; conjugation = pilus; transduction = phage).
⚠️ The 70S vs 80S ribosome distinction is a frequent exam question because of its clinical relevance to antibiotics.
⚠️ Gram-positive vs Gram-negative is a classic short-answer or labelling question. Know which has the thicker peptidoglycan layer (Gram-positive) and which has an outer membrane (Gram-negative).
True or false: Prokaryotic cells contain mitochondria.
False. Prokaryotes lack all membrane-bound organelles.
Fill in the blank: The region where prokaryotic DNA is concentrated is called the ________.
Nucleoid.
True or false: Archaea and bacteria have identical cell-wall compositions.
False. Bacterial walls typically contain peptidoglycan; archaeal walls use different molecules.
Fill in the blank: Prokaryotic ribosomes are ____S, while eukaryotic ribosomes are ____S.
70S; 80S.
True or false: Binary fission produces genetically diverse offspring.
False. Binary fission produces two genetically identical daughter cells.
Q: Name three structural features that distinguish prokaryotic cells from eukaryotic cells.
A: Prokaryotes lack a membrane-bound nucleus (DNA is in the nucleoid), lack membrane-bound organelles (no mitochondria, ER, or Golgi), and have smaller 70S ribosomes instead of 80S.
Q: A bacterium is found to be resistant to an antibiotic it has never been exposed to. Explain how this could occur.
A: The resistance gene could have been acquired through horizontal gene transfer: transformation (uptake of free DNA), conjugation (direct transfer via a pilus from a resistant bacterium), or transduction (delivery by a bacteriophage).
Q: Why does penicillin kill bacteria but not human cells?
A: Penicillin inhibits peptidoglycan synthesis. Human cells do not have peptidoglycan (or cell walls at all), so penicillin has no target in human cells.
Q: Describe the steps of binary fission.
A: The cell replicates its circular chromosome. The two copies move to opposite poles. The cell elongates and a septum forms at the midpoint, dividing the cell into two identical daughter cells.
Q: What is the functional difference between the nucleoid and a true nucleus?
A: Both house the cell's DNA, but the nucleus is enclosed by a double membrane (nuclear envelope) that controls molecular traffic in and out. The nucleoid has no surrounding membrane; DNA is in direct contact with the cytoplasm.
This material connects directly to eukaryotic cell structure, which is typically covered next in introductory biology. The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells, so understanding prokaryotic features helps explain where those eukaryotic organelles came from.
Horizontal gene transfer ties into genetics and evolution units. The mechanisms (transformation, conjugation, transduction) reappear when studying microbial genetics, antibiotic resistance in medical microbiology, and recombinant DNA technology in biotechnology courses.
Prokaryote, prokaryotic cell, bacteria, archaea, nucleoid, plasmid, peptidoglycan, binary fission, horizontal gene transfer, HGT, transformation, conjugation, transduction, 70S ribosome, 80S ribosome, Gram-positive, Gram-negative, cell wall, plasma membrane, pilus, bacteriophage, endosymbiotic theory, asexual reproduction, circular chromosome, Svedberg unit, antibiotic resistance, introductory biology, cell structure comparison, prokaryote vs eukaryote