Prokaryotes: Eubacteria and Archaea

Introduction

Prokaryotes, which include the domains Bacteria and Archaea, are among the most ancient and diverse forms of life on Earth. They are characterized by their simple cell structure, lack of a nucleus, and remarkable adaptability to a wide range of environments. This section explores their structural features, diversity, evolutionary history, and ecological significance.

Learning Objectives

• Describe the structural and functional adaptations of prokaryotes.

• Identify sources of genetic diversity in prokaryotes.

• Give examples of nutritional and metabolic adaptations in prokaryotes.

• Identify major phylogenetic groups of prokaryotes.

• Describe ecological roles played by prokaryotes.

• Give examples of the beneficial and harmful effects that prokaryotes have on humans.

Unique Characteristics of Prokaryotes

Adaptations for Survival and Diversity

• Small Size and Rapid Reproduction: Prokaryotes are typically 1–10 μm in length. Their small size and rapid binary fission allow populations to grow quickly under favorable conditions.

• High Mutation Rates: Large population sizes and frequent cell divisions lead to high mutation rates, generating significant genetic diversity.

• Diverse Adaptations: Prokaryotes can survive in extreme environments (e.g., hot springs, acidic lakes) due to specialized adaptations such as protective coats and endospores.

• Rapid Evolution: High genetic diversity and short generation times enable prokaryotes to evolve rapidly, adapting to new environments and challenges.

An Overview of Prokaryotic Diversity

Phylogenetic Relationships

Prokaryotes are divided into two main domains: Bacteria and Archaea. Despite both being prokaryotic, Archaea are more closely related to Eukaryotes than to Bacteria, as shown by molecular phylogenies.

• Bacteria: Includes groups such as Proteobacteria, Cyanobacteria, Gram-positive bacteria, Chlamydias, and Spirochetes.

• Archaea: Includes Euryarchaeotes, Thaumarchaeotes, Aigarchaeotes, Crenarchaeotes, and Korarchaeotes.

• Eukaryota: Includes plants, animals, fungi, and various protists.

Example: The phylogenetic tree shows the evolutionary relationships among these groups, with a universal ancestor at the base.

Evolution of Prokaryotes

Timeline and Origin

• Ancient Origin: Prokaryotes are the earliest known life forms, appearing approximately 3.8 billion years ago.

• Evolutionary Clock: Diagrams illustrate that prokaryotes dominated Earth's biosphere for billions of years before the emergence of eukaryotes.

Additional info: The evolutionary clock diagram helps visualize the vast timescale over which prokaryotes have existed compared to eukaryotes.

Summary Table: Domains of Life

The following table summarizes the main domains of life and their relationships:

Key Terms

• Prokaryote: An organism whose cells lack a nucleus and membrane-bound organelles.

• Binary Fission: A form of asexual reproduction in prokaryotes where a cell divides into two genetically identical cells.

• Mutation: A change in the DNA sequence, contributing to genetic diversity.

• Phylogeny: The evolutionary history and relationships among species or groups of organisms.

• Endospore: A resistant, dormant structure formed by some bacteria to survive harsh conditions.

Viruses: Learning Objectives

After the completion of this lecture, you should be

able to do the following:

• Describe a virus.

• Explain how viruses replicate and evolve.

• Use examples to illustrate the effects of animal and

plant viruses, as well as prions, on their hosts.

Virus Structure

A. Basic Structure of

Viruses

• Acellular

• Tiny: 20-1500 nm

• Nucleic acid core:

DNA or RNA

• Protein coat: Capsid

made up of units called

capsomeres

• Envelope (some

viruses): derived from

the host cell membrane;

may have spike proteins

• Shape: varies