6.2 CaUSE OF DEATH
Bacteria
Bacteria are prokaryotic cells and so have no nucleus, membrane-bound organelles nor do they produce spindle fires during cell division. Their diameter varies between 0.5 and 5 μm. Bacteria reproduce asexually by binary fission - divide into two identical cells after DNA replication. Their structure is normally as follows:
- Flagellum: used for cell movement.
- Pili: protein tubes that allow bacteria to attach to surfaces and are involved in cell-to-cell attachment.
- Capsule or slimy layer: a mucus layer that protects the cell and prevents dehydration; it also allows bacteria to form colonies.
- Cell wall: made of peptidoglycan, a polysaccharide cross-linked with peptide chains. Also protects the cell.
- Ribosomes: site of protein synthesis. They are free in the cytoplasm.
- Plasmid: small circles of DNA.
- Circular DNA: contains the genetic information of the cell.
- Food granule: starch storing lysosome.
- Mesosome: infolding of the cell membrane that is the site of respiration.
Viruses
Viruses are small organic particles which are not prokaryotic nor eukaryotic. They consist of a strand of nucleic acid (RNA or DNA) enclosed within a protein coat (capsid). Viral DNA can be single or double stranded.
Some viruses can have an outer envelope taken from the host cell's surface membrane so it will therefore contain lipids and proteins. Viral envelopes also contain glycoproteins from the virus itself but these are antigens - molecules recognized by the host's immune system as not being part of itself. The envelope allows the virus to penetrate the membrane. Viruses can be different sizes, between 20nm and 1 μm. Viruses are also able to infect plant cells. The HIV virus is an example of an enveloped virus:
Different virus structures include:
Virus Infection
Viruses must enter the cells of the host in order to use their metabolic systems to make more viruses. On their own, viruses lack the internal structures required for growth and reproduction. Once the virus enters the cell, its functioning will become disrupted and the virus is able to reproduce. New virus particles bud from the cell surface or burst out of the cell, splitting it open and so killing the cell. This is known as lysis. The cell's contents are released to surrounding tissue and this can damage neighboring cells. These cause the disease symptoms produced by the virus infection.