Biology: Definition and Origins of Life

Definition of Life

• Cells and organization are essential characteristics of life
• Response to environmental changes, growth, and development are key features of life
• Biological evolution, energy use, and metabolism are fundamental aspects of life
• Regulation and homeostasis are critical for maintaining life

Origins of Life

• Microbial fossils, such as Swartkoppie chert, date back to 3.5 billion years ago
• Fossil record is sparse, and indirect evidence is used to study origins of life
• Scientific method is employed to understand the origins of life

Earliest Molecules - RNA

• RNA was the original molecule that fulfilled protein and hereditary functions
• Ribozymes are RNA molecules that form peptide bonds and perform cellular work and replication
• Earliest cells may have been RNA surrounded by liposomes

Earliest Metabolism

• Early energy sources were inorganic, such as FeS, under harsh conditions
• Photosynthesis evolved 2.5 billion years ago, with cyanobacteria playing a crucial role
• Stromatolites are mineralized layers of microorganisms

Evolution of 3 Domains of Life

• Universal phylogenetic tree is based on comparisons of small subunit rRNA (SSU rRNA)
• Aligned rRNA sequences from diverse organisms are compared to derive evolutionary distance
• Relatedness, but not time of divergence, is determined through this method

Last Universal Common Ancestor (LUCA)

• The root or origin of modern life is on the bacterial branch, but nature is still controversial
• Archaea and Eukarya evolved independently of Bacteria
• Archaea and Eukarya diverged from a common ancestry

Endosymbiotic Hypothesis

• Origin of mitochondria, chloroplasts, and hydrogenosomes from endosymbionts
• Mitochondria and chloroplasts have SSU rRNA genes showing bacterial lineage
• Genome sequences of mitochondria and chloroplasts are closely related to Rickettsia and Prochloron, respectively

Evolution of Cellular Microbes

• Mutation of genetic material led to selected traits
• New genes and genotypes evolved through horizontal gene transfer within the same generation

Microbial Species

• Eukaryotic microbes fit the definition of reproducing isolated populations
• Bacteria and Archaea do not reproduce sexually and are referred to as strains
• A strain consists of descendants of a single, pure microbial culture

Microbiology and Its Origins

• Microbiology is the study of microorganisms
• Tools used for the study include microscopes, culture techniques, molecular genetics, and genomics

Classification Schemes

• Three-domain system divides microorganisms into Bacteria, Archaea, and Eukarya
• This system is based on a comparison of ribosomal RNA genes

Domain Bacteria

• Usually single-celled, with a majority having cell walls with peptidoglycan
• Most lack a membrane-bound nucleus
• Ubiquitous, with some living in extreme environments
• Cyanobacteria produce significant amounts of oxygen

Domain Archaea

• Distinguished from Bacteria by unique rRNA gene sequences
• Lack peptidoglycan in cell walls
• Have unique membrane lipids
• Some have unusual metabolic characteristics
• Many live in extreme environments

Domain Eukarya - Eukaryotic

• Protists are generally larger than Bacteria and Archaea
• Algae are photosynthetic
• Protozoa may be motile, "hunters, grazers"
• Slime molds have two life cycle stages
• Water molds cause devastating diseases in plants
• Fungi include yeast (unicellular) and mold (multicellular)

Acellular Infectious Agents

• Viruses are the smallest of all microbes, requiring host cells to replicate
• Viruses cause a range of diseases, including some cancers
• Viroids and virusoids are infectious agents composed of RNA
• Prions are infectious proteins