taxon_rank cercozoa
The Cercozoa are a group of protists . They are defined by molecular phylogenies of rRNA and actin or polyubiquitin . Microfilaments , also called actin filaments , are filamentous structures in the cytoplasm of eukaryotic cells and ___MASK61___ part of the cytoskeleton . They are primarily composed of polymers of actin , but in cells are modified by and interact with numerous other proteins . Microfilaments are usually about 7 nm in diameter and composed of two strands of actin . Microfilament functions include cytokinesis , amoeboid movement and cell motility in general , changes in cell shape , endocytosis and exocytosis , cell contractility and mechanical stability . Microfilaments are flexible and relatively strong , resisting buckling by multi - piconewton compressive forces and filament fracture by nanonewton tensile forces . In inducing cell motility , one end of the actin filament elongates while the other end contracts , presumably by myosin II molecular motors . Additionally , they function as part of actomyosin - driven contractile molecular motors , wherein the thin filaments serve as tensile platforms for myosin ' s ATP - dependent pulling action in muscle contraction and pseudopod advancement . Microfilaments have a tough , flexible framework which helps the cell in movement . The nematodes or roundworms constitute the ___MASK11___ Nematoda . They are a diverse animal ___MASK11___ inhabiting a broad range of environments . Nematode species can be difficult to distinguish , and although over 25 , 000 have been described , of which more than half are parasitic , the total number of nematode species has been estimated to be about 1 million . Nematodes are classified along with insects and other moulting animals in the clade Ecdysozoa , and , unlike flatworms , have tubular digestive systems with openings at both ends . A monomer (" mono -", " one " + "- mer ", " part ") is a molecule that may bind chemically or supramolecularly to other molecules to ___MASK61___ a ( supramolecular ) polymer . The process by which monomers combine to ___MASK61___ a polymer is called polymerization . Molecules made of a small number of monomer units ( up to a few dozen ) are called oligomers . The term " monomeric protein " may also be used to describe one of the proteins making up a multiprotein complex . Cell ___MASK53___ is the process by which a parent cell divides into two or more daughter cells . Cell ___MASK53___ usually occurs as part of a larger cell cycle . In eukaryotes , there are two distinct types of cell ___MASK53___ : a vegetative ___MASK53___ , whereby each daughter cell is genetically identical to the parent cell ( mitosis ), and a reproductive cell ___MASK53___ , whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes ( meiosis ). Meiosis results in twelve haploid daughter cells by undergoing one round of DNA replication followed by two divisions : homologous chromosomes are separated in the first ___MASK53___ , and sister chromatids are separated in the second ___MASK53___ . Both of these cell ___MASK53___ cycles are used in sexually reproducing organisms at some point in their life cycle , and both are believed to be present in the last eukaryotic common ancestor . Prokaryotes also undergo a vegetative cell ___MASK53___ known as binary fission , where their genetic material is segregated equally into two daughter cells . All cell divisions , regardless of organism , are preceded by a single round of DNA replication . Actin is a ___MASK10___ of globular multi - functional proteins that ___MASK61___ microfilaments . It is found in essentially all eukaryotic cells ( the only known exception being nematode sperm ), where it may be present at a concentration of over 100 M . An actin protein ' s mass is roughly 42 - kDa , with a diameter of 4 to 7 nm , and it is the monomeric subunit of two types of filaments in cells : microfilaments , one of the three major components of the cytoskeleton , and thin filaments , part of the contractile apparatus in muscle cells . It can be present as either a free monomer called G - actin ( globular ) or as part of a linear polymer microfilament called F - actin ( filamentous ), both of which are essential for such important cellular functions as the mobility and contraction of cells during cell division . A polymer ( Greek " poly -", " many " + "- mer ", " parts ") is a large molecule , or macromolecule , composed of many repeated subunits . Because of their broad range of properties , both synthetic and natural polymers play an essential and ubiquitous role in everyday life . Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function . Polymers , both natural and synthetic , are created via polymerization of many small molecules , known as monomers . Their consequently large molecular mass relative to small molecule compounds produces unique physical properties , including toughness , viscoelasticity , and a tendency to ___MASK61___ glasses and semicrystalline structures rather than crystals .