Typed lecture notes covering chapter 7 of Microbiology: An Evolving Science, the textbook used in the "General Microbiology" course (BioM122) at UCI. Aligns with lecture 10.
Bacterial Genetics I (Ch. 7, Lec. 10)
Monday, October 26, 2020 12:10 PM Active learning:
Mutations occur randomly, regardless of what is exposed to the population.
Drugs are tested using Ames Test for mutagenicity. Mutagenic if # of his+ revertant
• Genome: all the genetic info of an organism. colonies INC. possible conversion products may INC/DEC the # of colonies.
• Microbial genomes are encoded by DNA, (w/ exception of RNA viruses) b/c it is more
stable, mutable, and replicable.
7.1 DNA: The Genetic Material
• DNA is the genetic material within chromosomes, not protein.
7.2: Genome Organization
• Microbes have diverse lifestyles that require diverse genomes. -> From species to
species genomes vary in size, configuration, and content.
• bacterial/archaeal chromosomes range in size 106 - 16,000 kilobase (kb) pairs.
• The simpler the organism, the smaller the genome-- viruses have evolved into smaller
genomes as they become dependent on it host.
• Most sequenced bacterial genomes consist of a single circular chromosome; some
have multiple circular chromosomes, linear chromosomes, or contain a mix of both.
• Genes: the functional units that make up the coding content of a chromosome; code for
proteins.
• Promoters and enhancer: regulatory sequences that partially make up the noncoding
content for the chromosome; control gene expression.
○ Noncoding content makes up <15% of sequences in prokaryotic genomes. (>90%
in eukaryotes!)
• Nucleotides: subunits of nucleic acid polymers--DNA. Consists of…
1. Nitrogenous base: purines (A,G) or pyrimidines (T,C).
2. Deoxyribose sugar* (RNA will have an extra H here)
3. Phosphate group
• Phosphodiester bonds: covalent bonding b/w nucleotides; form b/w the 5' phosphate
group and the 3' OH group of another.
• Hydrogen bonds: how nitrogenous bases associate w/ one another:
○ A pairs w/ T (2H bonds)
○ G pairs w/ C (3H bonds)
• Phosphodiester backbones of 2 complimentary molecules orient themselves
antiparallel, or in the opp direction.
• At high temp, H bonds break and DNA denatures into 2 single strands.
• The major/minor grooves of DNA give access to DNA-binding proteins buried in the
center of the molecule-- strands do not have to be separated in order for proteins to
interact w/ specific base sequences.
• RNA vs DNA:
○ Contains a ribose sugar.
○ Uracil replaces Thymine.
○ Usually single stranded, however it often folds back on itself to form complex
hairpin structures.
• Bacterial chromosomes are compacted into a nucleoid. Ex. E coli has a huge bp
chromosome that's packed carefully into the cell so it so that it remains available to
proteins, but only takes up 3-4% of bacteria's mass.
• Nucleoid: loops/domains that bacterial DNA pack themselves into.
○ Loops are supercoiled, and anchored by histone-like proteins.
○ Positive supercoils: DNA is OVERwound.
○ Negative supercoils: DNA is UNDERwound.
• Most cells, including eukaryotes, possess negatively supercoiled DNA. -> 2 DNA
strands are easier to separate when negative.
• Some archaea that live in acidic, high temp protect their DNA by maintaining a
positively supercoiled state.
• Supercoils are introduced: when a double-strand break is produced in the supercoil, an
intact region of DNA is passed thru the break, and the broken ends are sealed to twist
the chromosome.
○ Topoisomerases: enzymes that change DNA supercoiling.
• Type I: single proteins that cleave one strand of DNA;, and relieve
supercoiling.
• Type II: a multi-subunit complex that cleaves both strands of DNA, and uses
ATP to introduce supercoils. Ex. DNA gyrase, targeted by quinolone
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