taxon_rank nanobacterium
Biology is a natural science concerned with the study of life and living organisms , including their structure , function , growth , evolution , distribution , identification and taxonomy . Modern biology is a vast and eclectic field , composed of many branches and subdisciplines . However , despite the broad scope of biology , there are certain general and unifying concepts within it that govern all study and research , consolidating it into single , coherent field . In general , biology recognizes the cell as the basic unit of life , genes as the basic unit of heredity , and evolution as the engine that propels the synthesis and creation of new ___MASK43___ . It is also understood today that all the organisms survive by consuming and transforming energy and by regulating their internal environment to maintain a stable and vital condition known as homeostasis . Uracil ( U ) is one of the four nucleobases in the nucleic acid of RNA that are represented by the letters A , G , C and U . The others are adenine ( A ), cytosine ( C ), and guanine ( G ). In RNA , uracil binds to adenine via two hydrogen bonds . In DNA , the uracil nucleobase is replaced by thymine . Uracil is a demethylated ___MASK61___ of thymine . A unicellular organism , also known as a single - celled organism , is an organism that consists of only one cell , unlike a multicellular organism that consists of more than one cell . Historically , simple unicellular organisms have been referred to as monads , though this term is also used more specifically to describe organisms of the ___MASK59___ " Monas " and similar flagellate ameboids . The main groups of unicellular organisms are bacteria , archaea , protozoa , unicellular algae , and unicellular fungi . Unicellular organisms fall into two general categories : prokaryotic organisms and eukaryotic organisms . Unicellular organisms are thought to be the oldest form of life , with early protocells possibly emerging 3 . 8  4 billion years ago . A fungus ( plural : fungi or funguses ) is any member of the ___MASK48___ of eukaryotic organisms that includes unicellular microorganisms such as yeasts and molds , as well as multicellular fungi that produce familiar fruiting forms such as mushrooms and bracket fungi . These organisms are classified as a ___MASK100___ , Fungi , which is separate from the other eukaryotic life kingdoms of plants and animals . Animals are multicellular , eukaryotic organisms of the ___MASK100___ Animalia ( also called Metazoa ). The animal ___MASK100___ emerged as a basal clade within Apoikozoa as a sister of the choanoflagellates . Sponges are the most basal clade of animals . Animals are motile , meaning they can move spontaneously and independently at some point in their lives . Their body plan eventually becomes fixed as they develop , although some undergo a process of metamorphosis later in their lives . All animals are heterotrophs : they must ingest other organisms or their products for sustenance . Plants are mainly multicellular , predominantly photosynthetic eukaryotes of the ___MASK100___ Plantae . Nanobacterium ( NAH - no - bak - TEER - ee - m , pl . nanobacteria NAH - no - bak - TEER - ee - uh ) is the unit or member name of a proposed class of living organisms , specifically cell - walled microorganisms with a size much smaller than the generally accepted lower limit for life ( about 200 nanometres for bacteria ) . Originally based on observed nano - scale structures in geological formations ( including one meteorite ) , the status of nanobacteria has been controversial , with some researchers suggesting they are a new class of living organism capable of incorporating radiolabeled uridine , and others attributing to them a simpler , abiotic nature . One skeptic dubbed them `` the cold fusion of microbiology '' , in reference to a notorious episode of supposed erroneous science . The term `` calcifying nanoparticles '' ( CNPs ) has also been used as a conservative name regarding their possible status as a life ___MASK61___ . Research tends to agree that these structures exist , and appear to replicate in some way . However , the idea that they are living entities has now largely been discarded , and the particles are instead thought to be nonliving crystallizations of minerals and organic molecules . Ribose is a carbohydrate with the formula CHO ; specifically , it is a pentose monosaccharide ( simple sugar ) with linear ___MASK61___ H ( C = O )( CHOH ) H , which has all the hydroxyl groups on the same side in the Fischer projection . Uridine is a glycosylated pyrimidine - analog containing uracil attached to a ribose ring ( or more specifically , a ribofuranose ) via a  - N - glycosidic bond . In chemistry , a glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate ( sugar ) molecule to another ___MASK48___ , which may or may not be another carbohydrate . In biology , an organism is any contiguous living system , such as an animal , plant , fungus , archaeon , or bacterium . All known types of organisms are capable of some degree of response to stimuli , reproduction , growth and development and homeostasis . An organism consists of one or more cells ; when it has one cell it is known as a unicellular organism ; and when it has more than one it is known as a multicellular organism . Most unicellular organisms are of microscopic scale and are thus loosely described as microorganisms . Humans are multicellular organisms composed of many trillions of cells grouped into specialized tissues and organs .