Genetic Material
Prokaryotic cells, like bacteria, are structurally simpler than eukaryotes. Their genetic material floats freely in the cytoplasm rather than being enclosed in a nucleus, and they lack membrane-bound organelles. Despite this simplicity, features like plasmids, a rigid cell wall, and surface appendages make prokaryotes remarkably adaptable across a wide range of environments.
Nucleoid Structure and Function
The nucleoid is the region of the cytoplasm where a prokaryote keeps its chromosome. Unlike eukaryotic chromosomes, this DNA is not enclosed by a nuclear membrane.
- The bacterial chromosome is a single, circular molecule of DNA
- It lacks the histone proteins that eukaryotic cells use to package their DNA; instead, other proteins help compact the DNA so it fits inside the cell
- The nucleoid controls cellular processes through gene expression and regulation, determining which proteins the cell produces and when
Because there's no nuclear envelope separating the DNA from the ribosomes, transcription and translation can happen simultaneously in prokaryotes. That's a key difference from eukaryotic cells, where mRNA must exit the nucleus before translation begins.
Plasmids: Extrachromosomal DNA
Plasmids are small, circular pieces of DNA that exist separately from the main bacterial chromosome. They aren't required for normal cell function, but they often carry genes that give the bacterium a survival advantage.
- Plasmid genes can code for traits like antibiotic resistance, toxin production, or the ability to break down unusual nutrients
- Plasmids replicate independently of the bacterial chromosome, so they can increase in number within a single cell
- They can be transferred between bacteria through horizontal gene transfer processes like conjugation (more on this below)
- In biotechnology, scientists use plasmids as vectors to introduce foreign genes into bacteria, which is how organisms are engineered to produce proteins like human insulin
Ribosomes: Protein Synthesis Machinery
Ribosomes are the structures responsible for translating mRNA into proteins. They're composed of ribosomal RNA (rRNA) and proteins, and they're found free-floating in the cytoplasm or attached to the cell membrane.
- Prokaryotic ribosomes are 70S, which is smaller than the 80S ribosomes found in eukaryotic cells. The "S" stands for Svedberg units, a measure of sedimentation rate during centrifugation, not a simple addition of subunit sizes.
- Each 70S ribosome consists of a small 30S subunit and a large 50S subunit
- During translation, the ribosome reads mRNA codons and assembles a polypeptide chain using tRNA molecules that carry specific amino acids
This size difference between prokaryotic and eukaryotic ribosomes is medically significant. Many antibiotics (like tetracycline and erythromycin) specifically target 70S ribosomes, which means they can kill bacteria without harming human cells.
Cell Envelope Structures
Cell Wall Composition and Function
The cell wall is a rigid layer surrounding the plasma membrane that gives a prokaryotic cell its shape and structural support.
- It's composed of peptidoglycan, a mesh-like polymer made of sugar chains cross-linked by short amino acid bridges
- The cell wall prevents the cell from bursting (lysing) in hypotonic environments, where water would otherwise rush in by osmosis
- It also acts as a physical barrier against some harmful substances
The Gram stain, one of the most important techniques in microbiology, divides bacteria into two major groups based on cell wall structure:
Gram-positive bacteria have a thick peptidoglycan layer (many layers). They retain the crystal violet stain and appear purple.
Gram-negative bacteria have a thin peptidoglycan layer (few layers) plus an additional outer membrane containing lipopolysaccharides. They lose the crystal violet stain during washing and pick up the counterstain, appearing pink.
This distinction matters clinically because Gram-negative bacteria are often harder to treat with antibiotics, partly because that outer membrane acts as an extra barrier.
Capsule: Protective Coating
Some bacteria have a capsule, a thick, slimy layer of polysaccharides (or sometimes proteins) that surrounds the cell wall.
- Protects the bacterium from phagocytosis by immune cells, making encapsulated bacteria harder for the body to fight off
- Helps bacteria adhere to surfaces and to other cells, which contributes to biofilm formation (structured communities of bacteria embedded in a sticky extracellular matrix)
- In pathogenic species like Streptococcus pneumoniae, the capsule is a major virulence factor, meaning it directly contributes to the organism's ability to cause disease
Not all bacteria have capsules. Those that do tend to be more dangerous as pathogens precisely because the capsule helps them evade the immune system.
Cell Surface Appendages
Pili: Attachment and DNA Transfer
Pili (singular: pilus) are short, hair-like protein structures extending from the cell surface. They serve two main roles: attachment and DNA transfer.
- Common pili help bacteria attach to host cells, tissues, or environmental surfaces. This attachment is often the first step in establishing an infection.
- Sex pili (also called F pili) are specialized, longer structures used during conjugation, a form of horizontal gene transfer:
- A donor cell extends a sex pilus toward a recipient cell
- The pilus makes contact and draws the two cells together
- A cytoplasmic bridge forms, and DNA (often a plasmid) transfers from donor to recipient
- Type IV pili enable a jerky surface movement called twitching motility, where the pilus extends, attaches, and then retracts to pull the cell forward
- Some pili also serve as receptors for bacteriophages (viruses that infect bacteria)
Conjugation is one reason antibiotic resistance can spread so quickly through bacterial populations. A single resistant bacterium can share its resistance plasmid with neighboring cells.
Flagella: Bacterial Locomotion
Flagella (singular: flagellum) are long, whip-like protein structures that allow bacteria to swim through liquid environments. They rotate like a propeller rather than beating back and forth like eukaryotic flagella.
A bacterial flagellum has three structural components:
- Filament: the long, helical portion that extends outward from the cell surface
- Hook: a flexible joint connecting the filament to the basal body
- Basal body: a motor complex embedded in the cell membrane and cell wall that anchors the flagellum and drives its rotation
Bacteria use flagella to move toward favorable conditions (like nutrients) and away from harmful ones (like toxins). This directed movement in response to chemical signals is called chemotaxis.
Flagellar arrangement varies by species:
- Monotrichous: a single flagellum at one end
- Lophotrichous: a cluster of flagella at one end
- Amphitrichous: flagella at both ends of the cell
- Peritrichous: flagella distributed all around the cell surface