Chapter 9: Microbial Genetics: Structure, Function, and Regulation of Genetic Material

Introduction to Microbial Genetics

Overview of Genetics

Genetics is the scientific study of heredity, focusing on how traits are transmitted from parents to offspring, how these traits are expressed and vary, the structure and function of genetic material, and the mechanisms by which genetic material changes.

• Genetics: The study of heredity and variation in organisms.

• Explores transmission, expression, structure, and change of biological traits.

Structure and Function of the Genome

Levels of Genetic Organization

Genetic material is organized at multiple biological levels, from the organism to the molecular scale. The genome represents the complete set of genetic material in a cell or virus.

• Genome: The sum total of genetic material (DNA) in a cell.

• Most genetic material exists as chromosomes, but some is found in non-chromosomal sites such as mitochondria, chloroplasts, and plasmids.

• Cellular genomes are composed of DNA; viral genomes may be DNA or RNA.

Chromosomes

Chromosomes are discrete cellular structures composed of tightly packaged DNA molecules. Their organization differs between eukaryotes and prokaryotes.

• Eukaryotic chromosomes: Located in the nucleus, multiple and linear.

• Bacterial chromosomes: Single, circular loop.

Genes: Types and Functions

Genes are the fundamental units of heredity, each responsible for a specific trait or function. They are segments of DNA that contain the necessary code to make proteins or RNA molecules.

• Structural genes: Code for proteins.

• RNA genes: Code for RNA molecules.

• Regulatory genes: Control gene expression.

Genotype and Phenotype

The genetic makeup of an organism is its genotype, while the observable traits resulting from gene expression constitute the phenotype.

• Genotype: The complete set of genes in an organism.

• Phenotype: Observable traits produced by gene expression.

Organization and Packaging of Genomes

Genome Size and Complexity

Genome size varies widely among organisms, from viruses to humans.

• Smallest virus: 4-5 genes.

• Escherichia coli: Single chromosome with 4,288 genes; DNA is 1,000 times longer than the cell.

• Human cell: 46 chromosomes with 31,000 genes; DNA is 180,000 times longer than the cell.

DNA Packaging

DNA is compacted in cells through supercoiling and association with proteins.

• Prokaryotes: DNA gyrase introduces supercoils, tightly packing the chromosome.

• Eukaryotes: DNA is wrapped around histone proteins forming nucleosomes, with multiple levels of coiling for further compaction.

Additional info:

• Further topics in microbial genetics include DNA replication, transcription, translation, gene regulation, mutation, and genetic recombination, which are essential for understanding microbial function and adaptation.