Cells are divided into two main classes, initially defined by whether they contain a nucleus. Prokaryotic cells lack a nuclear envelope and are generally smaller and simpler than eukaryotic cells. The same basic molecular mechanisms govern the lives of both prokaryotes and eukaryotes, indicated that all present day cells are descended from a single primordial ancestor.
Life first emerged at least 3.8 billion years ago. The critical characteristic of the macromolecule from which life evolved must have been the ability to replicate itself. RNA is generally believed to have been the initial genetic system, and an early stage of chemical evolution is thought of have been based on self replicating RNA molecules – a period of evolution known as the RNA world. Ordered interactions between RNA and amino acids then evolved into the present day genetic code, and DNA eventually replaced RNA as the genetic material.
All present day cells use DNA as the genetic material and employ the same basic mechanisms for DNA replication and expression of the genetic information. Genes are the functional units of inheritance. The nucleotide sequence of a gene is copied into RNA by a process called transcription. For RNAs that encode proteins, their nucleotide sequence is then used to specify the order of amino acids in a protein by a process called translation. The first cell is presumed to have arisen by the enclosure of self replicating RNA in a membrane composed of phospholipids.
All cells use ATP as their source of metabolic energy to drive the synthesis of cell constituents and carry out activities that require energy. The first cells obtained energy by glycolysis, the anaerobic breakdown of glucose to lactic acid. The development of photosynthesis is thought to have been the next major evolutionary step. With few exceptions, prsent day cells use oxidative metabolism as their principal source of energy.
Both prokaryotic cells and eukaryotic cells are surrounded by a plasma membrane and contain ribosomes (the sites of protein synthesis). However, eukaryotic cells are much more complex and contain a nucleus and a variety of organelles within their cytoplasm. Two of these organelles, mitochondria (the sites of oxidative metabolism) and chloroplasts (the sites of photosynthesis), play critical roles in energy metabolism. Lysosomes and peroxisomes provide specialized compartments for digestion of macromolecules and various oxidative reactions, respectively. The endoplasmic reticulum and the Golgi apparatus are specifically devoted to sorting and transport of proteins destined for secretion, incorporation into the plasma membrane, and incorporation into lysosomes and peroxisomes. The internal organization of eukaryotic cells is maintained by the cytoskeleton. Most plant cells contain large vacuoles that perform a variety of functions. Plant cells are also surrounded by a cell wall.
The Eukarya arose as a branch from the Archaea. A critical step in the evolution of eukaryotic cells was the acquisition of membrane enclosed subcellular organelles. At least two of these organelles, mitochondria and chloroplasts, arose by endosymbiosis – one cell living inside another. Both are thought to have evolved from bacteria and both contain their own DNA (distinct from the nuclear DNA of the cell). Most of the genes originally present in these bacteria apparently became incorporated into the nuclear genome of the cell. The genomes of eukaryotes are mosaics, with some eukaryotic genes more similar to bacterial genes and others more similar to archaeal genes.
The evolution of multicellular organisms from unicellular eukaryotes occurred multiple times, independently for plants and animals. Increasing specialization and division of labor among the cells of simple multicellular organisms then led to the complexity and diversity observed in the many types of cells that make up present day plants and animals.
Organisms that serve as model systems for understanding the molecular biology of cells include Escherichia coli (a bacterium); yeasts (such as Saccharomyces cerevisiae); Caenorhabditis elegans (a nematode); Drosophila melanogaster (a fruit fly); Arabidopsis thaliana (a flowering plant); the zebrafish; the mouse; and viruses.
—August 2020
Lance Jones 