BACTERIA

1 EJC H2 Biology T1W2 Bacteria Bacteria 1. Learning Outcomes Candidates should be able to: 1d Describe the structure of a typical bacterial cell (small and unicellular, peptidoglycan cell wall, circular DNA, 70S ribosomes and lack of membrane-bound organelles). (Covered in JC1 Cell Biology) 2d Describe the structure and organisation of prokaryotic genome (including DNA, double-stranded, number of nucleotides, packing of DNA, circularity and absence of introns). (Covered in JC1 Cell Biology) 2g Outline the mechanism of asexual reproduction by binary fission in a typical prokaryote and describe how transformation, transduction and conjugation (including the role of F plasmids but not Hfr) give rise to variation in prokaryotic genomes. 2i Explain how gene expression in prokaryotes can be regulated, through the concept of simple operons (including lac and trp operons), in cluding the role of regulatory genes; and distinguish between inducible and repressible systems. (Attenuation of trp operon is not required). Use the knowledge gained in this section in new situations or to solve related problems. 2. References Campbell and Reece, BIOLOGY, 9th Ed. Brooker, Widmaier, Graham and Stiling, BIOLOGY Green, Stout and Taylor, BIOLOGICAL SCIENCE Vol 1 and 2, 3rd Ed. Soloman, Berg and Martin BIOLOGY 7th Ed. Tortora, Funke and Case, MICROBIOLOGY, 6th Ed. Alberts, Bray, Lewis, Raff, Roberts and Watson, MOLECULAR BIOLOGY OF THE CELL, 3rd Ed. Raven, Johnson, Losos, Mason and Singer, McGraw-Hill, Biology, 8th Edition

2 EJC H2 Biology T1W2 Bacteria 3. Contents 1. Learning Outcomes ................................................................................................... 1 2. References ................................................................................................................. 1 3. Contents .................................................................................................................... 2 4. Bacterial Cells ............................................................................................................ 3 A. Morphology......................................................................................................... 3 B. General Bacterial Structure ………………………………………………………………………..….4 5. Binary Fission ........................................................................................................... 12 6. Ways to generate genetic variation in bacterial genomes by DNA introduction .... 13 A. Transformation …………………………………………………………………………………………….14 B. Transduction ………………………………………………………………………………………………..15 C. Conjugation ………………………………………………………………………………………………….18 7. Gene Regulation in Bacteria .................................................................................... 22 8. The lac Operon in E. coli ......................................................................................... 23 9. The trp operon ……………………………………………………………………………………………………32 10. Glossary .................................................................................................................. 36 11. Links …………………………………………………………………………………………………………………..37

3 EJC H2 Biology T1W2 Bacteria 4. Bacterial Cells (Prokaryotic Cells) Prokaryotes generally lack membrane -bound organelles and the endomembrane systems; but they still survive and reproduce. In the endosymbiont theory, organelles like mitochondria and chloroplasts represent formerly free-living prokaryotes that were taken inside another cell, and this could explain the link between the eukaryotes and prokaryotes. A. Morphology Most bacterial cells are very small (about 0.2µm in diameter and 2-8µm in length) and are unicellular. Bacteria may be classified based on their morphology (“morphe” = “form”, “shape”, “outward appearance”), amount of peptidoglycan in the cell wall or phylogeny (phylogeny = the organization of species to show their evolutionary relationships). Based on morphology, bacteria may be classified into: (a) coccus (spherical), (b) bacillus (rod-like), (c) spiral and (d) filamentous (elongated). Fig. 1 - Different morphology of bacterial cells. Cocci (spherical) Bacilli (rod-like) Budding and appendaged bacteria Others

4 EJC H2 Biology T1W2 Bacteria B. General Bacterial Structure (Ultrastructure) Internal Structure Bacteria are prokaryotes and have a very simple internal structure with no membrane-bound organelles. Structures present include: - The main component of the genome in most bacteria is one double- stranded, circular DNA molecule that is associated with proteins (they are not called histones!). - The DNA forms loop domains with the pr oteins, followed by further supercoiling, forming highly condensed DNA (NB: stretched out, the DNA of E. coli would measure about 1mm in length, 500 times longer than the cell). - The entire structure is referred to as the bacterial chromosome . Bacterium being prokaryote has no intron in its chromosome (non-coding sequences within the gene). - The bacterial chromosome makes up a dense region within the cell called the nucleoid, which is not bound by a membrane. - In addition to the chromosome, some bacteria may also have plasmids, which are much smaller rings of autonomously replicating circular DNA. fimbria bacterial chromosome (in nucleoid region) 70S ribosome storage granule plasmid cell membrane cell wall capsule cytoplasm flagellum Fig. 2 - Diagram of a bacterial cell Notes to Self:

5 EJC H2 Biology T1W2 Bacteria  Nucleoid: - Region in the bacterial cell where chromosomal DNA is generally confined to. - It is not bound by a membrane but is visibly distinct from the rest of the cell interior.  Ribosomes: - 70S (vs. 80S in eukaryotes). They are needed for protein synthesis. - The ribosomes give the cytoplasm of bacteria a granular appearance in electron micrographs.  Storage granules: - Nutrients and chemical reserves may be stored in the cytoplasm in the form of granules e.g. granules of glycogen, lipids and ions like phosphorous and magnesium.  Plasmid(s) (may be present) - A small, circular autonomously replicating DNA molecule. (This is not the bacterial chromosome.) - The plasmid contains genes which may confer advantages on bacteria living in stressful environments e.g. antibiotic resistance genes. - Multiple copies are usually present in a cell. - Plasmids are used extensively in genetic engineering as vectors for carrying and expressing foreign DNA in bacterial cells. - Different bacteria can have different plasmids. - Bacterial genome includes both the bacterial chromosome and the plasmid (if present). Fig. 3 a) The nucleoid region of a bacterial cell is stained less densely than the surrounding cytoplasm b) Condensation of the bacterial DNA is made possible through its association with proteins http://www.agen.ufl.edu/~chyn/age2062/lect/lect_06/5 Nucleoid a) b) Formation of loop domains Loop domains Supercoiling Proteins anchoring loops Notes to Self:

6 EJC H2 Biology T1W2 Bacteria Surface Structure  Cell Membrane - A phospholipid bilayer similar the cell membrane of other cells. - In addition to the roles of a cell membrane which you have learned (See Notes on Cell Membranes), the membrane of a bacteria is also where the electron transport chains, as well as the enzyme ATP synthase are embedded to produce ATP during photosynthesis a nd/or respiration. (How is this different from an eukaryotic cell?)  Cell Wall - Consists of a polymer called peptidoglycan – long chains of sugars cross-linked by short peptide chains. (compare: what is the cell wall in plants made up