Prokaryotic Cells and Viruses

What is the structure & function of the capsule?

It’s a slimy layer made of polysaccharides

Prevents desiccation (drying out) and protects the bacteria against the host’s immune system

What is the structure of the DNA (prokaryotic cells)?

DNA is in a single circular loop (free in the cytoplasm, not associated with proteins)

What is the structure of the plasmid?

Small circular loops of DNA that are separate from the main circular DNA

Exam Tip: (They carry non-essential genes that can be passed between bacteria, such as antibiotic resistance genes)

What is the function of the flagellum?

It’s used for locomotion (movement)

What are the similarities between Eukaryotic and Prokaryotic Cells?

• Both contain a plasma membrane
  
  

• Both contain RNA and DNA
  
  

• Both contain ribosomes to make proteins from amino acids
  
  

• Both contain a cytoplasm

What are the differences between Eukaryotic and Prokaryotic Cells?

• Prokaryotic cells are much smaller than Eukaryotes
  eukaryotic cells.
  
  

• Prokaryotes lack a nucleus (DNA is free-floating), whereas eukaryotes have a nucleus.
  
  

• Prokaryotes lack membrane-bound organelles, whereas eukaryotes have them (e.g., mitochondria, Golgi).
  
  

• Prokaryotic cell walls are made of murein, whereas plant cell walls are made of cellulose (and fungi of chitin).
  
  

• Prokaryotes have smaller 70S ribosomes, whereas eukaryotes have larger 80S ribosomes.

Why do prokaryotes divide by binary fission?

They do not have a nucleus or linear chromosomes wrapped around histones

They also lack spindle fibres to pull the sister chromatids apart

Describe the process of Binary fission

1. Circular DNA and plasmids replicate
   
   

2. The cell elongates, and 2 copies of DNA move to opposite poles of the cell
   
   

3. Cytoplasm divides to form two daughter cells
   
   

4. Each daughter cell receives a single copy of the circular DNA and a different number of plasmids.

What are viruses?

Acellular (not made of cells), non-living particles

They are segments of nucleic acid (either RNA or DNA), enclosed within a protein coat called a capsid

What are the properties of viruses?

• They cannot replicate independently; they must inject their genetic material into a host cell to replicate.
  
  

• They contain attachment proteins on their surface that bind to specific, complementary receptors on host cells
  
  

• Some viruses are surrounded by a lipid envelope (stolen from host cell’s membrane)

Describe the process of virus replication

1. A virus uses attachment proteins to bind to specific, complementary receptors on the surface of a host cell
   
   

2. The virus injects its nucleic acid into the host cell
   
   

3. Virus uses the host cell’s machinery to replicate viral nucleic acid and synthesise viral proteins
   
   

4. New viral particles are assembled and are then released from the host cell (often bursting it)