10) Bacteria Summary 9744 2018

The Cell & Biomolecules of Life | Genetics & Inheritance (9744) Bacteria 2018 Prepared by: Mr Low Chor Meng and Mrs Selvamani Nair 1 Structure of Bacterial Cell Binary Fission – asexual reproduction produces genetically identical/clonal bacteria 1. DNA replication begins at the origin of replication (ori) where DNA is unzipped by breaking hydrogen bonds between bases of the 2 strands to form a replication bubble 2. DNA replicates by semi-conservative replication where each original strand serves as template for synthesis of daughter strands by complementary base pairing 3. 2 newly formed ori move to opposite poles of the cell and attach to the plasma membrane 4. Cell elongates to prepare for division. 5. DNA is circular with no free ends, and the 2 daughter DNA molecules will be interlocked with the completion of replication. 6. Enzyme topoisomerase cut, separate and reseal the two DNA molecules 7. Invagination of the plasma membrane and the deposition of new cell wall (division septum) eventually divide the parent cell into two daughter cells  each inherits a complete genome (genetically identical) Compare Binary Fission & Mitosis  Produces genetically identical offspring: selective advantage in a stable, favourable environment as it allows successful genotypes to rapidly colonise a habitat Point of Comparison Binary Fission Mitosis End Product 2 genetically identical cells 2 genetically identical cells Amount of DNA Same in each daughter cell as compared to parent cell Daughter cells have the same amount of DNA as parent cells DNA Replication DNA replication occurs during binary fission DNA replication occurs during S phase of interphase before mitosis Behaviour of Chromosomes Attachment of chromosomes to plasma membrane No attachment of chromosomes to plasma membrane Formation of entangled rings made up of 2 DNA molecules to be separated DNA molecules are not entangled No chromosome condensation into sister chromatids Chromosome condensation into sister chromatids No specific positioning of chromosomes in the cell that characterizes the different stages Specific positioning of chromosomes in the cell characterize the different stages Spindle Fibres No spindle fibres involved Spindle fibres involved Chromosome: > circular DNA which contains essential genes for survival Peptidoglycan cell wall: > protects the cell from osmotic lysis Fimbria: > for attachment to surfaces 70S ribosome: > site of protein synthesis Flagellum: > for motility Capsule (distinct layer) / Slime layer (diffused): > protects against phagocytosis Storage granule Plasmid: > extrachromosomal circular DNA > replicate autonomously > genes may confer advantages e.g. antibiotic resistance Cytoplasm Plasma Membrane: > Phospholipid bilayer with the electron transport chains and ATP synthase are embedded to produce ATP

The Cell & Biomolecules of Life | Genetics & Inheritance (9744) Bacteria 2018 Prepared by: Mr Low Chor Meng and Mrs Selvamani Nair 2 Genetic variation in bacteria arise from lateral gene transfer: Transformation, Conjugation & Transduction  resulting in change of the bacterial cell’s genotype and phenotype Transformation Definition: Transformation is the uptake of naked, foreign DNA from the surrounding environment, resulting in a change of the bacterial cell’s genotype and phenotype Process: 1. Fragments of foreign naked DNA from dead lysed bacterial cells 2. Naturally competent bacteria with cell-surface proteins bind and transport DNA into the cell. 3. Artificially bacteria can be made competent through immersion in a medium with CaCl2 followed by a heat shock treatment 4. Foreign DNA incorporated into chromosome through crossing over at 2 homologous regions found on the bacterial chromosome 5. Result: recombinant cell 6. If different alleles for a gene were exchanged, the new allele will be expressed  permanent change in genotype & phenotype 7. Recombinant genome will be passed on to all subsequent offspring through binary fission Conjugation Definition: Direct transfer of genetic material from one bacterial cell to another through a mating bridge between the two cells via the transfer of F plasmid from an F+ donor to F– recipient cell Process: 1. Sex pilus (coded for by F factor) of F+ bacterial cell makes contact with a F- cell and retracts to bring the 2 cells closer 2. The hollow pilus then acts as a cytoplasmic mating bridge between the 2 cells 3. One of the 2 strands of the plasmid DNA is nicked and transferred from the F+ cell to the F- cell through the bridge 4. The single stranded F plasmid DNA circularizes in F - cell and is used as a template to synthesize a complementary strand for a double-stranded plasmid DNA. The F- recipient cell is now a F+ cell 5. Replication of the plasmid occurs via rolling circle DNA replication occurs a) One strand of ds F plasmid is nicked by a nuclease  free 3’OH end is then used as a primer for strand elongation by DNA polymerase using the unnicked/intact strand as a template  elongation process is facilitated by the displacement of the 5’ end of the nicked strand and is transferred across the mating bridge to the recipient bacterium  Upon completion of a unit length of the plasmid DNA (after 1 round), another nick occurs to release the original strand b) In the recipient cell, the single strand of F p lasmid DNA re-circularises and serves as a template for the synthesis of a complementary daughter stand to form a double stranded circular DNA. Transduction Definition: Transduction is the process by which bacterial DNA from one host cell is introduced into another bacterial host cell by a bacteriophage due to aberrations in the phage reproductive cycle Generalised Transduction 1. A phage infects a bacterium, injecting its viral genome(DNA) into the host cell 2. The bacterial DNA is degraded into small fragments , one of which may be randomly packaged into a capsid head during the spontaneous assembly of new viruses 3. Upon cell lysis, the defective phage will infect another bacterium and inject bacterial DNA from the previous host cell into the new bacterium 4. Foreign bacterial DNA can replace the homologous region in the recipient cell’s chromosome through homologous recombination, allowing the expression of a different allele from the previous host Specialised Transduction 1. Temperate phage infects a bacterium, injecting its viral genome into the host cell 2. The viral DNA is integrated into bacterial chromosome forming a prophage 3. which may be improperly excised to include adjacent segment of bacterial DNA and not the entire phage DNA during an induction event 4. Hence phage-bacterium hybrid DNA may be packaged into a capsid head during the spontaneous assembly of new viruses 5. Upon cell lysis, the defective phage will infect another bacterium and inject bacterial DNA from the previous host cell into the new bacterium 6. New alleles from the previous bacterial cell can be incorporated into the genome of the new host by homologous recombination or integration of phage - bacterium hybrid DNA as defective phage enters the lysogenic cycle Compare the similarities and differences between the mechanisms of transformation, generalized, specialised transduction and conjugation Poin