Microbial Genetics

Introduction to Genetics and Genomes

Genetics is the study of inheritance and inheritable traits as expressed in an organism’s genetic material. The genome is the entire genetic complement of an organism, including its genes and nucleotide sequences. Understanding microbial genetics is essential for comprehending how microorganisms function, adapt, and evolve.

The Structure and Replication of Genomes

Structure of Nucleic Acids

Nucleic acids are polymers of nucleotides, each composed of a phosphate group, a pentose sugar, and a nitrogenous base. The length of DNA is typically expressed in base pairs. The two main types of nucleic acids are DNA and RNA, which differ in their sugars and nitrogenous bases.

• DNA: Contains deoxyribose sugar; bases include adenine (A), thymine (T), guanine (G), and cytosine (C).

• RNA: Contains ribose sugar; bases include adenine (A), uracil (U), guanine (G), and cytosine (C).

Prokaryotic Genomes

Prokaryotic chromosomes are typically circular DNA molecules located in the nucleoid. Prokaryotic cells are haploid, meaning they have a single chromosome copy. In addition to chromosomes, prokaryotes may contain plasmids—small, independently replicating DNA molecules that can confer survival advantages such as antibiotic resistance.

• Types of plasmids: Fertility, resistance, bacteriocin, and virulence plasmids.

Eukaryotic Genomes

Eukaryotic cells typically have multiple, linear chromosomes sequestered within a nucleus and are often diploid. Eukaryotes also possess extranuclear DNA in mitochondria and chloroplasts, which resemble prokaryotic chromosomes. Some fungi, algae, and protozoa carry plasmids.

Comparison of Microbial Genomes

DNA Replication

DNA replication is semiconservative, meaning each new DNA molecule consists of one original and one daughter strand. The process requires monomers and energy, both provided by triphosphate deoxyribonucleotides.

Initial Processes in Bacterial DNA Replication

• Replication begins at the origin.

• DNA polymerase synthesizes DNA only in the 5′ to 3′ direction.

• Leading strand: synthesized continuously.

• Lagging strand: synthesized discontinuously (Okazaki fragments).

Other Characteristics of Bacterial DNA Replication

• Replication is bidirectional.

• Gyrases and topoisomerases remove supercoils.

• DNA methylation regulates gene expression, replication initiation, viral protection, and DNA repair.

Replication in Eukaryotes

• Uses four DNA polymerases.

• Thousands of replication origins.

• Shorter Okazaki fragments.

• Methylation occurs only on cytosine bases in plants and animals.

Gene Function

Genotype and Phenotype

The genotype is the set of genes in the genome, while the phenotype refers to the physical and functional traits of the organism. The flow of genetic information follows the central dogma: DNA is transcribed to RNA, which is then translated into polypeptides.

Transcription

Transcription is the process by which information in DNA is copied as RNA. In prokaryotes, transcription occurs in the nucleoid and involves three steps: initiation, elongation, and termination. Six types of RNA are transcribed: RNA primers, mRNA, rRNA, tRNA, regulatory RNA, and ribozymes.

Transcription in Eukaryotes

• Occurs in the nucleus, mitochondria, and chloroplasts.

• Involves three types of nuclear RNA polymerases and numerous transcription factors.

• mRNA is processed before translation (capping, polyadenylation, splicing).

Translation

Translation is the process in which ribosomes use the genetic information in mRNA to synthesize polypeptides. The genetic code is read in codons (triplets of nucleotides), each specifying an amino acid.

Participants in Translation

• Messenger RNA (mRNA)

• Transfer RNA (tRNA)

• Ribosomes and ribosomal RNA (rRNA)

Stages of Translation

• Initiation

• Elongation

• Termination

All stages require protein factors; initiation and elongation require GTP as an energy source.

Translation in Eukaryotes

• Initiation occurs when the ribosomal subunit binds to the 5′ guanine cap.

• The first amino acid is methionine (not formyl-methionine).

• Ribosomes can synthesize polypeptides into the rough endoplasmic reticulum.

Comparison of Genetic Processes

Regulation of Genetic Expression

Gene Regulation

Most genes are expressed at all times, but some are regulated to conserve energy. Regulation typically occurs at the transcriptional level but can also occur during translation. In prokaryotes, gene regulation often involves operons—clusters of genes under the control of a single promoter and operator.

• Inducible operons (e.g., lac operon): Activated by inducers, regulate catabolic pathways.

• Repressible operons (e.g., trp operon): Transcribed continually until deactivated by repressors, regulate anabolic pathways.

Regulatory RNAs (miRNAs, siRNAs, riboswitches) can also control translation by binding to mRNA and affecting its stability or translation efficiency.

Mutations of Genes

Types of Mutations

A mutation is a change in the nucleotide sequence of a genome. Mutations are rare and usually deleterious, but occasionally they can confer an advantage. Types include:

• Point mutations: Affect a single base pair (substitutions, insertions, deletions).

• Gross mutations: Involve larger changes such as inversions, duplications, and transpositions.

Mutagens

Mutagens are agents that increase the mutation rate. They include:

• Radiation: Ionizing (e.g., X-rays) and nonionizing (e.g., UV light).

• Chemical mutagens: Nucleotide analogs, nucleotide-altering chemicals, frameshift mutagens.

DNA Repair Mechanisms

Cells possess several DNA repair mechanisms to correct mutations:

• Direct repair

• Single-strand repair (e.g., excision repair, mismatch repair)

• Error-prone repair (e.g., SOS response in E. coli)

Identifying Mutants, Mutagens, and Carcinogens

• Mutants: Descendants of cells with uncorrected mutations.

• Wild-type: Cells normally found in nature.

• Methods: Positive selection, negative (indirect) selection, Ames test.

Genetic Recombination and Horizontal Gene Transfer

Horizontal Gene Transfer in Prokaryotes

Horizontal gene transfer involves the movement of genetic material between organisms other than by descent. Main mechanisms include:

• Transformation: Uptake of free DNA from the environment by competent cells.

• Transduction: Transfer of DNA via bacteriophages (generalized or specialized).

• Conjugation: Direct transfer of DNA between cells via physical contact (e.g., F plasmid, Hfr cells).

Transposons and Transposition

Transposons are DNA segments that can move from one location to another within a genome, causing frameshift mutations. Types include:

• Insertion sequences: Simplest transposons, containing only the gene for transposase and inverted repeats.

• Complex transposons: Carry additional genes not related to transposition.

Additional info: This guide covers the core concepts of microbial genetics, including genome structure, replication, gene expression, mutation, and genetic exchange, as relevant to a college-level microbiology course.