Microbial Genetics: Structure, Replication, and the Central Dogma

Microbial Genetics: Chapter 7

Learning Objectives

• Describe DNA structure and replication.

• Explain the central dogma of biology.

• Understand how bacterial and eukaryotic genomes differ.

Introduction to Microbial Genetics

Definition and Importance

Genetics is the study of inheritance, focusing on the transmission of traits from parent to offspring, and the variation and function of genetic material. Microbial genetics is crucial for understanding how genetic material changes (mutation or recombination) and how these changes affect organism phenotype, such as fitness or virulence.

• Inheritance: Passing of genetic traits from one generation to the next.

• Variation: Differences in genetic traits among individuals.

• Mutation: Heritable change in DNA sequence.

• Recombination: Exchange of genetic material between organisms.

Applications: Microbial genetics enables the engineering of bacteria to produce useful products (e.g., insulin, Hepatitis B vaccine) and helps explain the genetic basis of diseases and traits.

The Language of Genetics

Key Terms and Concepts

• Gene: A sequence of nucleotides that encodes the assembly of specific amino acids into a protein.

• Locus: The particular location of a gene on a chromosome.

• Genome: The complete set of DNA from a species.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable physical or biochemical characteristics of an organism.

The basic language of protein synthesis involves nucleotide triplets in DNA (codons), which are translated into amino acids, forming proteins that determine phenotype.

Genome Size and Complexity

Gene Number vs. Organism Complexity

The number of genes does not necessarily correlate with organism complexity. Genome sizes vary widely among organisms:

Organism	Number of Genes
Mycoplasma	517
E. coli	4,288
Yeast	6,340
Fruit fly	13,600
Roundworm	19,000
Human	25,000

Additional info: Some plants and amphibians have even larger genomes than humans.

Genetic Material Packaging in Cells and Viruses

Prokaryotes, Eukaryotes, and Viruses

• Prokaryotes: Genetic material is typically packaged as a single circular chromosome and may include plasmids (small, circular DNA molecules).

• Eukaryotes: Genetic material is organized into multiple linear chromosomes within a nucleus. Additional DNA may be found in mitochondria and chloroplasts.

• Viruses: Genetic material can be DNA or RNA, packaged within a protein coat.

Plasmids are extra-chromosomal DNA elements found in prokaryotes and some eukaryotes, often carrying genes for antibiotic resistance or other survival advantages.

Central Dogma of Biology

Flow of Genetic Information

The central dogma describes the flow of genetic information from DNA to RNA to protein, which determines phenotype.

• DNA (genotype) → RNA (transcription) → Protein (translation) → Phenotype

Transcription: The process by which DNA is copied into messenger RNA (mRNA).

Translation: The process by which mRNA is decoded to synthesize proteins.

Comparison of Bacterial and Eukaryotic Genomes

Structural and Functional Differences

• Bacterial Genomes: Usually circular, located in the cytoplasm (nucleoid region), and may contain plasmids.

• Eukaryotic Genomes: Linear chromosomes, contained within a membrane-bound nucleus, and may have additional DNA in organelles (mitochondria, chloroplasts).

Example: E. coli has a single circular chromosome and several plasmids, while human cells have 46 linear chromosomes in the nucleus.

Summary Table: Prokaryotic vs. Eukaryotic Genomes

Feature	Prokaryotes	Eukaryotes
Chromosome Structure	Circular	Linear
Location	Cytoplasm (nucleoid)	Nucleus
Plasmids	Common	Rare
Genome Size	Smaller	Larger

Key Takeaways

• Microbial genetics is foundational for understanding inheritance, genetic variation, and the molecular basis of phenotype.

• Gene number does not equate to organism complexity.

• Prokaryotic and eukaryotic genomes differ in structure, packaging, and location.

• The central dogma explains how genetic information is expressed as phenotype.