common gene families and housekeeping genes

• Advances in DNA sequencing have enabled a more comprehensive understanding of Earth's biodiversity. Sequencing allows classification and evolutionary analysis based on genetic relatedness.
• Disparities between organisms pose challenges for classification, especially microbes. Sequencing microbial populations expands knowledge of their diversity.
•  Ribosomal RNA sequences are fundamental for estimating evolutionary distances and constructing the tree of life, which has 3 domains - eukaryotes, bacteria, archaea.
• Eukaryotes have nucleus-enclosed DNA and larger, more complex cells compared to prokaryotes like bacteria and archaea.
• Bacteria exhibit great diversity, simplicity, miniature size, varied biochemistries and ecologies. Many affect human health positively and negatively.
• Archaea remain poorly understood but have distinct differences from bacteria despite similarities. Closer to eukaryotes based on genomes.
• Life inhabits all regions of Earth. Plants dominate terrestrial biomass, bacteria and archaea dominate soil and crust, animals dominate ocean biomass.
• Cells require matter and energy. Nitrogen and carbon are often fixed from inert atmospheric forms by certain cells and shared. Symbioses evolve to exchange needed compounds.
•  Organisms obtain energy through organotrophic, phototrophic or lithotrophic means. Aerobic and anaerobic lithotrophs thrive on geochemical energy.
• Evolution works on existing DNA through mutation, duplication, recombination and horizontal transfer. Gene families result.
• Homologs retain ancestry and sometimes function. Orthologs have common function across species, paralogs have diverged within a species. Homologs aid functional deductions.
• Conserved homologs across life reveal common inheritance and core functions like translation, metabolism, transport.
• 1. Intragenic mutation: Existing genes can be randomly modified by changes in their DNA sequence during DNA replication and repair processes.
• 2. Gene duplication: Existing genes can be accidentally duplicated, resulting in a pair of initially identical genes within a single cell, which may then diverge during evolution.
• • 3. DNA segment shuffling: Two or more existing genes can break and rejoin, creating a hybrid gene composed of DNA segments from separate genes.
• 4. Horizontal (intercellular) DNA transfer: Pieces of DNA can be transferred from one cell's genome to another, including between different species, contrasting with the usual vertical transfer of genetic information from parent to progeny.