Structure of Bacteria & Viruses
Bacteria
- Bacteria are small, single-celled prokaryotes
- Prokaryotic cells are much smaller than eukaryotic cells
- They also differ from eukaryotic cells in having
- A cytoplasm that lacks membrane-bound organelles
- Ribosomes that are smaller (70 S) than those found in eukaryotic cells (80 S)
- No nucleus, instead having a single circular bacterial chromosome that is free in the cytoplasm and is not associated with proteins
- A cell wall that contains the glycoprotein murein
- Murein is sometimes known as peptidoglycan
- A cell membrane that contains folds known as mesosomes
- In addition, many prokaryotic cells have a few other structures
- Loops of DNA known as plasmids
- Capsules
- This is sometimes called the slime capsule
- It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism
- Flagella (singular flagellum)
- Long, tail-like structures that rotate, enabling the prokaryote to move
- Some prokaryotes have more than one
- Pili (singular pilus)
- Thread-like structures on the surface of some bacteria that enable the bacteria to attach to other cells or surfaces
- Involved in gene transfer during sexual reproduction
- Thread-like structures on the surface of some bacteria that enable the bacteria to attach to other cells or surfaces
Prokaryotic cells are often described as being 'simpler' than eukaryotic cells, and they are believed to have emerged as the first living organisms on Earth
Viruses
- Viruses are non-cellular infectious particles
- They are relatively simple in structure, and much smaller than prokaryotic cells
- Structurally they have
- A nucleic acid core
- Their genomes are either DNA or RNA, and can be single or double-stranded
- A protein coat called a ‘capsid’
- A nucleic acid core
- They do not possess a plasma membrane, cytoplasm, or ribosomes
- Some viruses have an outer layer called an envelope formed from the membrane-phospholipids of the cell they were made in
- Some contain proteins inside the capsid which perform a variety of functions
- E.g. HIV contains the enzyme reverse transcriptase which converts its RNA into DNA once it's infected a cell
- Viruses also contain attachment proteins that sticks out from the capsid or envelope
- This enables the virus to attach itself to a host cell
- Viruses can only reproduce by infecting living cells and using their protein-building machinery to produce new viral particles
HIV is a virus; it is contains RNA and enzymes surrounded by a protein capsid, as well as having an outer lipid envelope and attachment proteins