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Microbial Genetics Chapter 8 Structure and Function of Genetic Material DNA & RNA DNA • deoxyribonucleic acid RNA • ribonucleic acid Nucleotides • Phosphate group • Pentose sugar • Nitrogenous base Structure of DNA Double stranded (double helix) polymers of nucleotides 5’ to 3’ (strands are anti-parallel) Complimentary Base Pairing • A-T • G-C DNA Replication Bacteria have only 1 chromosome (ccDNA) E. coli • about 4 million base pairs • 1 mm long (1000 times longer than the cell) • DNA takes up only about 10 % of cells volume E. coli DNA 1u 1,360u 1ft. 1,360 ft. DNA Replication occurs at the Replication Fork (5’ to 3’) DNA Helicase 1. Leading Strand ( 5’ to 3’ ) • DNA Polymerase 2. Lagging Strand ( 5’ to 3’) • RNA Polymerase (RNA Primer) • DNA Polymerase (extends primer and digests RNA) • DNA Ligase • Okazaki Fragments Bidirectional replication Origin of replication Termination site Protein Synthesis DNA Transcription mRNA Protein Translation Central Dogma of Molecular Genetics Transcription One strand of DNA is used as a template to form a complimentary strand of mRNA mRNA ----> messenger RNA Transcription 3 ways RNA differs from DNA Promoter RNA Polymerase Termination Site 5’ to 3’ 3 Types of RNA 1. mRNA • Contains the codons 2. rRNA • Ribosomes (70S) 3. tRNA • Transfer amino acids to the ribosomes for protein synthesis • Anti-codon Genetic Code DNA Triplet code mRNA codons tRNA anticodons Genetic Code Codons code for a specific amino acid 20 amino acids 3 base code - 4 bases ( A,U,G,C ) 64 possible combinations ( 43) Amino acids are coded for by more than one codon Genetic Code is Degenerative Genetic Code is Universal Translation rRNA • ribosomal RNA (forms the ribosomes) (70S) tRNA • transfer RNA (transfers amino acids to the ribosomes for protein synthesis) • anticodon • 3 base sequence that is complimentary to the codon on mRNA Regulation of Gene Expression All Genes are not always being expressed Genes turned on all the time - Constitutive Other genes can be regulated: • Turned On • Turned Off lac operon Lac - Lactose (dissacharide) • Glucose and Galactose operon - series of structural genes all under the control of a Regulatory Gene lac operon is normally turned off lac operon is an Inducible operon Tryptophan operon Tryptophan - amino acid operon - series of structural genes all under the control of a Regulatory Gene Tryptophan operon is normally turned on Tryptophan operon is a repressible operon Mutations - a change in the base sequence of DNA Mutations can be: 1. Harmful 2. Lethal 3. Beneficial 4. Silent (neutral) • UUU - Phe • UUC - Phe Types of Mutations Normal DNA Base Substitution Mutation C Missense Mutation Base Substitution Mutation T Nonsense Mutation Frame Shift Mutation ATG CAT GCA TGC ATT TCC TGC TTA AAA 1. Addition Mutation AAT GCA TGC ATG CAT TTT CCT GCT TAA Reading Frame is Shifted 2. Deletion Mutation TGC ATG CAT GCA TTT CCT GCT TAA Reading Frame is Shifted Genetic Transfer in Bacteria Genetic Transfer - results in Genetic Variation Genetic Variation - is needed for Evolution 3 Ways Genetic Transfer can occur in Bacteria • 1. Transformation • 2. Cunjugation • 3. Transduction Transformation Genes are transferred from one bacterium to another as “naked” DNA Frederick Griffith (1928) Griffith’s Experiment 2 Strains of Streptococcus pneumoniae • 1. Virulent strain with a capsule - Pneumonia • 2. Avirulent strain without a capsule - no disease Griffith’s Experiment Griffith’s Experiment Griffith’s Experiment Griffith’s Experiment Conjugation One bacterium passes some DNA (in a plasmid) to another bacterium Transduction DNA is transferred from one bacterium to another by a virus Bacteriophage • virus that only infects bacteria