prokaryotic cells and viruses

prokaryotic cell structure

- prokaryotes are single-celled organisms. - bacteria (like e.coli) are examples of prokaryotes.- prokaryotic cells are much smaller and simpler than eukaryotic cells ; and they don't have any membrane-bound organelles (like a nucleus) in their cytoplasm. 

prokaryotic cell structure

- just like in a eukaryotic cell, the cell-surface membrane is mainly made of lipids and proteins. it controls the movement of substances into and out of the cell. - the cell wall supports the cell and prevents it from changing shape. it's made of a polymer called murein. murein is a glycoprotein (a protein with a carbohydrate attached). - some prokaryotes (e.g. bacteria) also have a capsule made up of secreted slime. it helps to protect the bacteria from attack by cells of the immune system. 

prokaryotic cell structure

- plasmids are small loops of dna that aren't part of the main circular dna molecule. plasmids contain genes for things like antibiotic resistance, and can be passed between prokaryotes. plasmids are not always present in prokaryotic cells. some prokaryotic cells have several. - unlike a eukaryotic cell, a prokaryotic cell doesn't have a nucleus. instead, the dna floats free in the cytoplasm. it's circular dna, present as one coiled up strand. it's not attached to any histone proteins. - the flagellum (plural flagella) is a long, hair-like structure that rotates to make the prokaryotic cell move. not all prokaryotes have a flagellum. some have more than one. - cytoplasm contains ribosomes; but they're smaller than those in a eukaryotic cell.- prokaryotic cells are extremely small; less than 2μm in diameter (that's two millionths of a metre or 0.002 mm). eukaryotic cells can be up to 50 times bigger (although still only around 0.1mm).

prokaryotic cell replication

- prokaryotic cells replicate by a process called binary fission. in binary fission the cell replicates (makes copies of) its genetic material , before physically splitting into two daughter cells: step one ; the circular dna and the plasmids replicate. the main dna loop is only replicated once, but plasmids can be replicated many times.  step two ; the cell gets bigger and the dna loops move to opposite 'poles' of the cell. step three ; the cytoplasm begins to divide and new cell walls begin to form.  step four ; the cytoplasm divides and two daughter cells are produced. each daughter cell has one copy of the circular dna, but can have a variable number of copies of the plasmids.  

viruses

- viruses are acellular - they're not cells. in fact, viruses are just nucleic acids surrounded by protein - they're not even alive. examples of viruses include hiv (which causes aids), influenza (which causes the flu) and rhinoviruses (which cause colds). all viruses invade and reproduce inside the cells of other organisms. these cells are known as host cells. - unlike bacteria, viruses have no cell-surface membrane, no cytoplasm and no ribosomes. they do have a protein coat, called a capsid, with attachment proteins sticking out from it. the attachment proteins let the virus cling onto a suitable host cell. viruses are even smaller than bacteria. 

viral replication

- because they're not alive, viruses don't undergo cell division. instead, they inject their dna or rna into the host cell - this hijacked cell then uses its own 'machinery' (e.g. enzymes, ribosomes) to do the virus's dirty work and replicate the viral particles. - in order to inject their dna and rna, viruses first have to attach to the host cell surface. to do this they use their attachment proteins to bind to complementary receptor proteins on the cell surface membrane of the host cells. different viruses have different attachment proteins and therefore require different receptor proteins on host cells. as a result, some viruses can only infect one type of cell, while others can infect lots of different cells.