Microbial Genetics Lecture Notes PDF

Microbial Genetics Instructions for building microbes Introduction to Genetics and Genes Genetics: The study of heredity that explores: – The transmission of biological properties (traits) from parents to offspring – The expression and variation of those traits –...

Microbial Genetics Instructions for building microbes Introduction to Genetics and Genes Genetics: The study of heredity that explores: – The transmission of biological properties (traits) from parents to offspring – The expression and variation of those traits – The structure and function of genetic material – How this material changes Introduction to Genetics and Genes  Structure and Function of the Genome The Levels of Structure and Function of the Genome Genome: – The sum total of genetic material of an organism – Most exists in the form of chromosome(s) Prokaryotes (usually) have one large circular chromosome Eukaryotes have multiple linear chromosomes – Some may appear in non- chromosomal sites Plasmids: tiny extra pieces of DNA Eukaryotic organelles: mitochondria and chloroplasts – Genomics – study of structure, function, evolution and mapping of genomes The Levels of Structure and Function of the Genome Genes: basic informational packets – Classical genetics: a functional unit of heredity – Molecular and biochemical genetics: site on the chromosome that provides information for a certain cell function – Preferred definition: a segment of DNA that contains the necessary code to make a protein or an RNA  The Levels of Structure and Function of the Genome Genotype: – the sum of all types of genes constituting an organism’s distinctive genetic makeup Phenotype: – the expression of the genotype that creates certain structures or functions The DNA Code: A Simple Yet Profound Message Basic unit of DNA is a nucleotide: – Phosphate – Deoxyribose sugar – Nitrogenous base (A, C, G, T) Antiparallel arrangement: – One side of the helix runs in the opposite direction of the other: 5’ to 3’ in one direction and 3’ to 5’ in the other direction – Significant factor in DNA synthesis and protein production The Central Dogma is an oversimplification The “central dogma” of genetics: – Transcription: DNA is used to synthesize RNA – Translation: RNA used to produce proteins The “central dogma” is oversimplified. – A wide variety of RNAs are used to regulate gene function – The DNA that codes for these crucial RNA molecules was once called “junk” DNA. The Master Genetic Code The Master Genetic Code This genetic code is (virtually) universal – what are the implications of this for research, GMOs, cloning, etc.? What are the implications for viral diseases Genetic Regulation of Protein Synthesis Operons: – Found only in bacteria and archaea – Coordinated set of genes regulated as a single unit – Can be inducible or repressible A Repressible Operon Repressible operons are usually in the on mode Will only be turned off when synthesis is no longer required Excess product serves as a corepressor to block the action of the operon Generally anabolic genes An Inducible Operon The lac operon is inducible – Regulator: composed of the gene that codes for the repressor, a protein capable of repressing the operon – Control locus: Promoter: recognized by RNA polymerase Operator: acts as an on/off switch for transcription – Structural locus: made up of three genes, each coding for a different enzyme needed to catabolize lactose The Lac Operon The repressor protein is allosteric: – Binding sites for the operator sequence on the DNA and lactose – In the absence of lactose, the repressor binds to the operator, blocking transcription of structural genes – The regulator gene lies upstream of the operator and is transcribed constitutively Alterations in growth – external perturbations Normal growth can be altered by changes to the growth environment – Addition of antibiotics – New media – Different media – Change in carbon source utilized DNA Recombination Events Recombination: – An event in which one bacterium donates DNA to another bacterium – The end result is a strain different from both the donor and the recipient strain – Plasmids: extrachromosomal DNA that is transferable between cells – Recombinant: any organism that contains and expresses genes that originated in another organism Conjugation: Bacterial “Sex” Transformation – capturing DNA from the environment Transformation discovered through an elegant experiment conducted in the 1920s by Frederick Griffith. – Worked with encapsulated and non-encapsulated Streptococcus pneumoniae and laboratory mice – Encapsulated strains have a smooth (S) colony appearance – Strains lacking a capsule have a rough (R) appearance Transformation – capturing DNA from the environment Transformation: – The acceptance by a bacterial cell of small fragments of soluble DNA from the surrounding environment – Competent: cells that are capable of accepting genetic material through transformation When might transformation be advantageous? When might it be detrimental? Transformation Transduction Transduction: – The process by which bacteriophage serve as a carrier of DNA from a donor cell to a recipient cell – DNA is packaged into viral particles during assembly – Occurs in a broad spectrum of bacteria The participating bacteria in a single transduction event must be the same species or closely related – Why? Transposable Elements “Jumping genes” shift from one part of the genome to another Also called Transposons (Tn) First proposed by Barbara McClintock in 1951 Jumping genes are widespread among cells and viruses Can carry antibiotic resistance genes or virulence factors Often carry transposase enzyme that facilitates moving around genome  Mutations: Changes in the Genetic Code Mutation: – Any change to the nucleotide sequence in the genome – The driving force of evolution – In microorganisms, mutations become evident in altered gene expression Altered pigment production Development of resistance to a drug Gain/loss of virulence, etc. Wild type – unmutated organism present in the highest numbers in a population Different types of point mutations Causes of Mutations Spontaneous mutation: – A random change in the DNA arising from errors in replication that occur randomly Induced mutations: – Result from exposure to known mutagens Repair of Mutations Mismatch repair: – A repair system can locate mismatched bases that were missed during proofreading – The base must be replaced soon after the mismatch is made, or it will not be recognized by the repair enzymes What happens if a mutation is not fixed? Pathogenicity Islands Have the ability to make their hosts pathogenic. – Contain multiple genes that are coordinated to create a new trait on the bacterium – Often flanked by insertion sequences, and carry mobility genes – Different G+C content from rest of chromosome Comparative genomics Many mutations are not repaired Mutations are permanent and heritable A small number of mutations contribute to the success of the individual Large accumulations of mutations over time lead to new strains with Inner ring = E. coli O157:H7 strain EC4115 vastly different niches Dark blue = Shigella spp. and pathogenic Outermost three ring = E. fergusonii, E. albertii, and Salmonella enterica potentials All other tings = other E. coli strains