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Foundations in Microbiology, 4/e
Kathleen Park Talaro, Pasadena City College
Arthur Talaro

Microbial Genetics

Chapter Capsule

I. Genes and the Genetic Material
A. Genetics is the study of heredity, and the genome is the sum total of genetic material of a cell.

B. A chromosome is composed of DNA in all organisms; genes are specific segments of the elongate DNA molecule. Genes code for polypeptides and proteins that become enzymes, antibodies, or structures in the cell.
II. Gene Structure and Replication
A. A gene consists of DNA, a double helix formed from linked nucleotides composed of a phosphate, deoxyribose sugar, and a nitrogen base—purine or pyrimidine.

B. The backbone of the molecule is formed of antiparallel strands of repeating deoxyribose sugar-phosphate units that are linked together by the base-pairing of adenine with thymine and cytosine with guanine. The order of base pairs in DNA constitutes the genetic code. The very long DNA molecule must be highly coiled to fit into the cell.

C. Pairing ensures the accuracy of the copying of DNA synthesis or replication.
1. Replication is semiconservative and requires enzymes such as helicase, DNA polymerase, ligase, and gyrase.

2. These components in conjunction with the chromosome being duplicated constitute a replicon. The unzipped strands of DNA function as templates. Synthesis proceeds along two replication forks, each with a leading strand and a lagging strand.
III. Gene Function
A. Processing of genetic information proceeds from DNA to RNA and to protein. RNA synthesis is called transcription, and protein synthesis is called translation.

B. An organism’s genetic makeup, or genotype, provides information for the expression of traits, called the phenotype. The genetic code is organized into triplets of nitrogen bases, and each triplet (codon) corresponds to a particular amino acid in a protein.

C. Types of RNA: Unlike DNA, RNA is single-stranded and contains uracil instead of thymine and ribose instead of deoxyribose.
1. Chief forms are messenger, transfer, and ribosomal RNA (mRNA, tRNA, and rRNA).

2. Triplets of bases on mRNA called codons convey genetic information for protein structure. The corresponding anticodon on tRNA ensures delivery of the appropriate amino acid. Ribosomes combine rRNA and protein to form small and large subunits that provide staging sites for translation.
D. Transcription and Translation:
1. Transcription begins when RNA polymerase recognizes a promoter region on DNA and elongation of the new strand proceeds in the 59 to 39 direction. Only one DNA strand, the template, is copied. The mRNA strand goes to a ribosome for translation.

2. Translation begins at the AUG or start codon. Protein assembly proceeds as tRNAs enter the ribosome with anticodons complementary to the codons of the mRNA. Peptide bonds are formed between amino acids on adjacent tRNAs. This process continues until the nonsense (stop) codon on mRNA is reached.
E. Eucaryotic Gene Expression: Eucaryotes have split genes interrupted by noncoding regions called introns that separate coding segments called exons. The synthesis of the final mRNA transcript requires splicing to delete stretches that correspond to introns.
IV. The Genetics of Animal Viruses
A. Genomes of viruses can be linear or circular; segmented or not; made of double-stranded (ds) DNA, single-stranded (ss) DNA, ssRNA, or dsRNA.

B. In general, DNA viruses replicate in the nucleus, RNA viruses in the cytoplasm.

C. Retroviruses synthesize dsDNA from ssRNA.

D. The DNA of some viruses can be silently integrated into the host’s genome. Integration by oncogenic viruses can lead to transformation of the host cell into an immortal cancerous cell.

E. RNA viruses have strand polarity (positive- or negative-sense genome) and double-strandedness.
V. Regulation of Genetic Function
A. Protein synthesis and metabolism are regulated by gene induction or repression, as controlled by an operon.

B. An operon is a DNA unit of regulatory genes (made up of regulators, promoters, and operators) that controls the expression of structural genes (which code for enzymes and structural peptides).
1. Inducible operons such as the lactose operon are normally off but are turned on by a lactose inducer.

2. Repressible operons govern anabolism and are usually on, but can be shut off when the end product is no longer needed.

3. Certain antibiotics affect transcription and translation. Adverse modes of action include interference with RNA polymerase, RNA elongation, and ribosomal activity.
VI. Gene Mutation
A. Genome changes in microbes come from mutations and intermicrobial genetic exchanges.

B. The term wild type, or strain, denotes the original form; mutant strain refers to the altered version.

C. Mutations benefit organisms through an adaptive advantage, but some are lethal.

D. Types of Mutations:
1. Mutations are spontaneous if they occur randomly and induced if they are due to directed chemical or physical agents called mutagens.

2. Point mutations entail addition, removal, or substitution of a few bases.
a. A missense mutation leads to amino acid substitution.

b. A nonsense mutation arrests peptide synthesis without amino acid insertion.

c. A silent mutation causes base substitution without amino acid substitution.

d. A back-mutation is a reversion to the original base composition.
E. Repair of Mutations: Enzymes can identify and repair mutations. Some enzymes locate mismatched bases and engage in repairs. DNA damaged by ultraviolet light can be corrected by photoactivation or light repair.

F. The Ames test is a mutation-screening method based on the susceptibility of mutant Salmonella typhimurium to a back-mutation. It is used to measure the mutagenicity of chemicals.
VII. DNA Recombination
A. Intermicrobial transfer and genetic recombination permit gene sharing between bacteria.
1. In conjugation, one bacterium donates a plasmid (fertility, or F factor) to a compatible recipient via a conjugative pilus. Conjugation resulting in transfer of resistance, or R, plasmids is biomedically important.

2. In bacterial transformation, naked DNA is transferred without special carriers. It was first demonstrated in smooth and rough pneumococcal strains.

3. Transduction is transfer of host DNA by bacteriophages. Drug resistance and virulence genes can be transferred by this route.
B. Transposons are large DNA sequences that regularly depart and reinsert into chromosomal and plasmid sites within the same cell or between cells. These rearrangements in DNA generate mutations and variations in chromosome structure.