YIJC [H2] CI2.11 Genetics of Bacteria (N)(Student)

2024 JC1 BIOLOGY LECTURE NOTES 1 CORE IDEA 2: GENETICS AND INHERITANCE TOPIC 2.11: GENETICS OF BACTERIA Learning Outcomes: Core Idea 1 [Recap – Lecture notes CI 1.1 Organelles and Cellular Structures] (d) describe the structure of a typical bacterial cell (small and unicellular, peptidoglycan cell wall, circular DNA, 70S ribosomes and lack of membrane-bound organelles) Core Idea 2: (d) [modified] describe the structure and organisation of prokaryotic genomes (including DNA/RNA, single-/double-stranded, number of nucleotides, packing of DNA, linearity/circularity and presence/absence of introns) (g) 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 (i) explain how gene expression in prokaryotes can be regulated, through the concept of simple operons (including lac and trp operons), including the role of regulatory genes, and distinguish between inducible and repressible systems (knowledge of attenuation of the trp operon is not required) Use the knowledge gained in this section in new situations or to solve related problems. Textbooks and References Campbell, Urry, Cain, Wasserman, Minorsky, Reece (2018) Biology – A Global Approach (11 th Edition) (Global Edition) Chapter 27: Prokaryotes pg. 625 – 633, Chapter 18: Control of Gene Expression pg. 413 - 417 (Pearson Publication) ISBN-10 1-292-17043-3 Note: This textbook is available in our library. You may wish to borrow them to supplement your reading when necessary. H2

2 Contents 1. Bacterial Cell Structure 2. Bacteria Genome 2.1 Bacterial Chromosome 2.2 Bacterial Plasmid 3. Bacteria Reproduction via Binary Fission 3.1 Process of Binary Fission 4. Genetic Variation in Bacteria 4.1 Mutation 4.2 Genetic Transfer between Bacteria 5. Regulation of Gene Expression in prokaryotes 5.1 The Operon Model 5.2 Repressible operon (e.g. trp operon) 5.3 Inducible operon (e.g. lac operon) 5.4 Comparing Inducible and Repressible System

3 1. Bacteria Cell Structure Learning Outcome 1(d) Describe the typical structure of a bacterial cell (small and unicellular, peptidoglycan cell wall, circular DNA, 70S ribosomes and lack of membrane-bound organelles) • Bacteria (singular: bacterium) are small, unicellular prokaryotes. • They lack a true nucleus and membrane -bound organelles (e.g. rER , Golgi apparatus, mitochondrion, chloroplast). • They possess one circular double-stranded DNA in the nucleoid region and 70S ribosomes in the cytoplasm. Structure Function 1. Nucleoid Non-membrane bound. A region of cytoplasm where the bacterial chromosome is found. Appears lighter than the surrounding cytoplasm in electron micrographs. Contains one double -stranded, circular DNA molecule. Presence of a large amount of RNA and RNA polymerase involved in transcription and translation. 2. Plasmids Extra-chromosomal genome. Small circular DNA molecule, not part of the bacterial chromosome. Replicates independently from the bacterial chromosome. Carries a few genes which code for proteins that are not required for survival. These proteins, when expressed, might result in characteristics that confer advantages in stressful environments (e.g., antibiotic resistance). May or may not be present in all bacteria 3. Ribosomes 70S (unlike 80S in eukaryotes). Gives cytoplasm a granular appearance. Required for polypeptide synthesis. 4. Cell Surface Membrane Phospholipid bilayer containing proteins. Selectively permeable barrier that regulates the transport of material in and out of the bacterial cell. Proteins involved in the transport of ions, nutrients, and waste across the membrane. Energy transduction as it contains enzymes involved in ATP synthesis 5. Cell Wall Consists of peptidoglycan (sugars cross-linked by peptide bonds between short polypeptide chains) Maintains cell shape Protects bacteria from osmotic lysis

4 6. Flagella (singular: flagellum) Hollow cylindrical thread made of protein Bacteria cells may possess none, one or multiple flagella. For movement 7. Pili (singular: pilus) Hollow hair-like structure made of protein Sex Pili – Long conjugation pili that facilitates transfer of genetic material between two bacteria Fig. 1.1 A typical prokaryotic cell Fig. 1.2 Peptidoglycan Cell Wall in Bacteria; sugars cross-linked by peptide bonds between short polypeptides

5 CHECKPOINT 1 1 Which of the options below correctly list the distinctive features of prokaryotes? A cellulose cell wall, 70S ribosomes, linear DNA B cellulose cell wall, 80S ribosomes, circular DNA C peptidoglycan cell wall, 70S ribosomes, circular DNA D peptidoglycan cell wall, 80S ribosomes, linear DNA 2 The genome of prokaryotes typically comprises a large circular chromosome and smaller plasmids. TRUE FALSE 3 Compare the structure between prokaryotic and eukaryotic genome Feature Prokaryotic Eukaryotic Condition Nucleus DNA Plasmids Chromosomes Genome size

6 Learning Outcome 2(d) [modified] describe the structure and organisation of prokaryotic genomes (including DNA/RNA, single-/double-stranded, number of nucleotides, packing of DNA, linearity/circularity and presence/absence of introns) 2. Structure and Organisation of Prokaryotic Genome 2.1 Bacterial Chromosome • Bacteria have simpler, smaller genomes than eukaryotes . For example, Escherichia coli bacteria has a genome size of 5 million bases compared to yeast which has 12 million bases. • Bacterial cells are monoploid, with only one set of chromosomes • Bacterial cell has a single molecule of circular, double stranded DNA. • Circular DNA is present in nucleoid region, not bound by a membrane. • Circular DNA contains a single origin of replication (oriC). • Circular DNA c ontains very few introns between genes and totally absent within genes (unlike eukaryotes). • Circular DNA is a ssociated with DNA binding proteins (nucleoid-associated proteins) to result in the DNA bending and coiling to form loop domains, which then supercoil to form condensed DNA. Fig. 2.1.1 Compacting of bacterial chromosome

7 2.2 Bacterial Plasmid • Plasmids are small, circular, doubled -stranded, extrachromosomal DNA (outside chromosome) that can self-replicate • Plasmid DNA contains only a few genes: o Genes involved in its own replication and control o Genes that can provide survival advantages for the bacteria under certain environmental conditions o Examples: resistance to antibiotics and heavy metals, resistance to radiation, ability to utilize certain metabolite, production of toxins. • Since plasmid contains genes which provide survival advantages, and not genes which are essential for survival, plasmids may or may not be present in bacteria cells. Fig. 2.2.1 Bacterial chromosome and plasmids CHECKPOINT 2 1 The bacteria chromosome is present in the ________________ region of the cell. 2 DNA compacting occurs due to DNA binding ________________which bend and coil the DNA to form _________ domains which then _________________ to form condensed DNA. 3 The bacterial chromosome contains __________ origin of replication. 4 Plasmids are essential for bacterial growth and survival under all conditions. TRUE FALSE 5 Plasmids replicate only when the bacterial chromosome replicates. TRUE FALSE

8 Learning Outcome 2(g) part (i) Outline the mechanism of asexual reproduction by binary fission in a typical prokaryote 3. Bacteria Reproduction via Binary Fission Binary fission (binary − “two parts”, fission − “split’) is the asexual means by which bacterial cells