Prokaryotic and Eukaryotic cells

A comparison of prokaryotic and eukaryotic cells It was realised that two basic cell types existed. One was a small, simple cell with very little internal compartmentalisation. These cells did not have a true, membrane-bound nucleus and there are no membrane-bound organelles in their dense cytoplasm. These cells were called PROKARYOTIC CELLS (before nucleus) and consisted of the bacteria and blue green algae. The second type of cell was more complex with a nucleus, organelles and probably evolved from prokaryotic cells. These were termed EUKARYOTIC CELLS. These include all plant, animal, fungi and protoctist cells.

The bacterial / prokaryotic cell Naked DNA

The bacterial / prokaryotic cell Electron microscope picture of a bacterial cell

Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles Ribosomes. The smaller (70 S) type. Nucleoid The region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane. DNA. Always circular, and not associated with any proteins to form chromatin. Plasmid. Small circles of DNA, used to exchange DNA between bacterial cells, and very useful for genetic engineering. Cell membrane. made of phospholipids and proteins, like eukaryotic membranes. Mesosome. A tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis. Can also be associated with the nucleoid. Cell Wall. Made of murein (not cellulose), which is a glycoprotein (i.e. a protein/carbohydrate complex, also called peptidoglycan). Capsule (or Slime Layer). A thick polysaccharide layer outside of the cell wall, Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. Flagellum. A rigid rotating helical-shaped tail used for propulsion.

Process of conjugation Some bacteria can have an extra small loop of DNA called a plasmid. They can pass this to each other called conjugation. This is how antibiotic resistance is transferred.

A eukaryotic plant cell

Eukaryotic animal cell

Prokaryotes v Eukaryotes Bacteria and blue green algae Smaller 0.1-10µm No nucleus No membrane bound organelles DNA lies free in the cytoplasm Cell Wall (peptidoglycan/murein) Dense cytoplasm Divide by binary fission Slime capsule Bacteriochlorophyll Mesosomes (site of respiration) Free-floating ribosomes Smaller ribosomes Have plasmids Have pilli Eukaryotes Plants, animals, fungi and protoctists Larger 10-100µm True nucleus Membrane bound organelles DNA in chromosomes Cellulose CW in Plants Less dense cytoplasm Divide by mitosis / meiosis No slime capsule True chlorophylls a and b Mitochondria (site of respiration) Free-floating and attached ribosomes Larger ribosomes Do not have plasmids No pilli

Viruses Viruses are extremely small and can only be seen using an electron microscope They can be called non cells since they have no organelles and no chromosomes Outside a host cell they are called a virion They consist of a protein coat (capsid) and a core of nucleic acid (either RNA or DNA)

Flu virus Bacteriophage

Structure of a virus T2 phage (bacteriophage) infecting an E.coli bacteria.

Viruses are host specific Viruses are host specific. They have receptors that recognise specific markers on host cells. The virus injects its nucleic acid into the cell and uses the cell’s nucleic acid to make viral proteins (both DNA and RNA are needed to make protein). The virus takes over the cell’s metabolism. The viral particles break out of the cell (destroying it in the process) and infect other cells to make more viral particles. Sometimes the viral DNA can join to the host’s chromosome and lay dormant for many years before beginning to make viral proteins.

Virus replication