Milk is a nutrient-rich, white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for infant mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to its young and can reduce the risk of many diseases, it contains many other nutrients including lactose. Interspecies consumption of milk is not uncommon among humans, many of whom consume the milk of other mammals; as an agricultural product, milk called dairy milk, is extracted from farm animals during or soon after pregnancy. Dairy farms produced about 730 million tonnes of milk from 260 million dairy cows. India is the world's largest producer of milk, is the leading exporter of skimmed milk powder, yet it exports few other milk products; the increasing rise in domestic demand for dairy products and a large demand-supply gap could lead to India being a net importer of dairy products in the future. The United States, India and Brazil are the world's largest exporters of milk and milk products.
China and Russia were the world's largest importers of milk and milk products until 2016 when both countries became self-sufficient, contributing to a worldwide glut of milk. Throughout the world, more than six billion people consume milk products. Over 750 million people live in dairy farming households; the term "milk" comes from "Old English meoluc, from Proto-Germanic *meluks "milk"". Milk consumption occurs in two distinct overall types: a natural source of nutrition for all infant mammals and a food product obtained from other mammals for consumption by humans of all ages. In all mammals, milk is fed to infants through breastfeeding, either directly or by expressing the milk to be stored and consumed later; the early milk from mammals is called colostrum. Colostrum contains antibodies that provide protection to the newborn baby as well as nutrients and growth factors; the makeup of the colostrum and the period of secretion varies from species to species. For humans, the World Health Organization recommends exclusive breastfeeding for six months and breastfeeding in addition to other food for up to two years of age or more.
In some cultures it is common to breastfeed children for three to five years, the period may be longer. Fresh goats' milk is sometimes substituted for breast milk, which introduces the risk of the child developing electrolyte imbalances, metabolic acidosis, megaloblastic anemia, a host of allergic reactions. In many cultures in the West, humans continue to consume milk beyond infancy, using the milk of other mammals as a food product; the ability to digest milk was limited to children as adults did not produce lactase, an enzyme necessary for digesting the lactose in milk. People therefore converted milk to curd and other products to reduce the levels of lactose. Thousands of years ago, a chance mutation spread in human populations in Europe that enabled the production of lactase in adulthood; this mutation allowed milk to be used as a new source of nutrition which could sustain populations when other food sources failed. Milk is processed into a variety of products such as cream, yogurt, ice cream, cheese.
Modern industrial processes use milk to produce casein, whey protein, condensed milk, powdered milk, many other food-additives and industrial products. Whole milk and cream have high levels of saturated fat; the sugar lactose is found only in milk, forsythia flowers, a few tropical shrubs. The enzyme needed to digest lactose, reaches its highest levels in the human small intestine after birth and begins a slow decline unless milk is consumed regularly; those groups who do continue to tolerate milk, however have exercised great creativity in using the milk of domesticated ungulates, not only of cattle, but sheep, yaks, water buffalo, horses and camels. India is buffalo milk in the world. In food use, from 1961, the term milk has been defined under Codex Alimentarius standards as: "the normal mammary secretion of milking animals obtained from one or more milkings without either addition to it or extraction from it, intended for consumption as liquid milk or for further processing." The term dairy relates to animal milk production.
A substance secreted by pigeons to feed their young is called "crop milk" and bears some resemblance to mammalian milk, although it is not consumed as a milk substitute. The definition above precludes non-animal products which resemble dairy milk in color and texture, such as almond milk, coconut milk, rice milk, soy milk. In English, the word "milk" has been used to refer to "milk-like plant juices" since 1200 AD. In the USA, milk alternatives now command 13% of the "milk" market, leading the US dairy industry to attempt, multiple times, to sue producers of dairy milk alternatives, to have the name "milk" limited to animal milk, so far without success; the mammary gland is thought to have derived from apocrine skin glands. It has been suggested. Much of the argument is based on monotremes; the original adaptive significance of milk secretions may have been nutrition or immunological protection. This secretion became more copious and accrued nutritional complexity over evolutionary time. Tritylodontid cynodonts seem to have displayed lactation, based on
The placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, gas exchange via the mother's blood supply. Placentas are a defining characteristic of placental mammals, but are found in marsupials and some non-mammals with varying levels of development; the placenta functions as a fetomaternal organ with two components: the fetal placenta, which develops from the same blastocyst that forms the fetus, the maternal placenta, which develops from the maternal uterine tissue. It metabolizes a number of substances and can release metabolic products into maternal or fetal circulations; the placenta is expelled from the body upon birth of the fetus. The word placenta comes from the Latin word for a type of cake, from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like", in reference to its round, flat appearance in humans; the classical plural is placentae, but the form placentas is common in modern English and has the wider currency at present.
Placental mammals, such as humans, have a chorioallantoic placenta that forms from the chorion and allantois. In humans, the placenta averages 22 cm in length and 2–2.5 cm in thickness, with the center being the thickest, the edges being the thinnest. It weighs 500 grams, it has crimson color. It connects to the fetus by an umbilical cord of 55–60 cm in length, which contains two umbilical arteries and one umbilical vein; the umbilical cord inserts into the chorionic plate. Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells; this results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta has a disc shape, but size varies vastly between different mammalian species; the placenta takes a form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, four, or more.
Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, so on. If there is a discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during labor, called vasa previa. About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta; some 350 of these genes are more expressed in the placenta and fewer than 100 genes are placenta specific. The corresponding specific proteins are expressed in trophoblasts and have functions related to female pregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the cancer testis antigen PAGE4 expressed in cytotrophoblasts, CSH1and KISS1 expressed in syncytiotrophoblasts, PAPPA2 and PRG2 expressed in extravillous trophoblasts; the placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.
The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer; the syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development; the syncytiotrophoblast, thereby contributes to the barrier function of the placenta. The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14. In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased; the increased diameter and straighter flow path both act to increase maternal blood flow to the placenta.
There is high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal chorion, though no fluid is exchanged; as the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins. Maternal blood flow is 600–700 ml/min at term; this begins at day 5 - day 12 Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels branch to form an extensive arterio-capillary-venous system, bringing the fetal blood close to the maternal blood. Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation.
On the other hand, there is no neural vascular regulation, catecholamines have only little effect. The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and
Mollusca is the second largest phylum of invertebrate animals. The members are known as mollusks. Around 85,000 extant species of molluscs are recognized; the number of fossil species is estimated between 100,000 additional species. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs live in freshwater and terrestrial habitats, they are diverse, not just in size and in anatomical structure, but in behaviour and in habitat. The phylum is divided into 8 or 9 taxonomic classes, of which two are extinct. Cephalopod molluscs, such as squid and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species; the gastropods are by far the most numerous molluscs and account for 80% of the total classified species. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, the structure of the nervous system.
Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc". This has a single, "limpet-like" shell on top, made of proteins and chitin reinforced with calcium carbonate, is secreted by a mantle covering the whole upper surface; the underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small; the main body cavity is a hemocoel. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles; the generalized mollusc has three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, all have sensors to detect chemicals and touch; the simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur.
All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced, they have kidney-like organs for excretion. Good evidence exists for the appearance of gastropods and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. Molluscs still are an important food source for anatomically modern humans. There is a risk of food poisoning from toxins which can accumulate in certain molluscs under specific conditions and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, their shells have been used as money in some preindustrial societies. Mollusc species can represent hazards or pests for human activities.
The bite of the blue-ringed octopus is fatal, that of Octopus apollyon causes inflammation that can last for over a month. Stings from a few species of large tropical cone shells can kill, but their sophisticated, though produced, venoms have become important tools in neurological research. Schistosomiasis is transmitted to humans via water snail hosts, affects about 200 million people. Snails and slugs can be serious agricultural pests, accidental or deliberate introduction of some snail species into new environments has damaged some ecosystems; the words mollusc and mollusk are both derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia, which he applied inter alia to cuttlefish; the scientific study of molluscs is accordingly called malacology. The name Molluscoida was used to denote a division of the animal kingdom containing the brachiopods and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs.
As it is now known these groups have no relation to molluscs, little to one another, the name Molluscoida has been abandoned. The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, the organization of the nervous system. Many have a calcareous shell. Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies to apply to all modern groups; the most general characteristic of molluscs is they are bilaterally symmetrical. The following are present in all modern molluscs: The dorsal part of the body wall is a mantle which secretes calcareous spicules, plates or shells, it overlaps the body with enough spare room to form a mantle cavity. The anus and genitals open into the mantle cavity. There are two pairs of main nerve cords. Other characteristics that appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.
In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were ab
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell; the zygote possesses half the DNA from each of its two parents. In plants and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism; the result of this process is an embryo. In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus. First attested in English in the mid-14c; the word embryon itself from Greek ἔμβρυον, lit. "young one", the neuter of ἔμβρυος, lit. "growing in", from ἐν, "in" and βρύω, "swell, be full". In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula and organogenesis; the blastula stage features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells called blastomeres.
In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at a more advanced stage of development and up until hatching. In humans, this is from the eleventh week of gestation. However, animals which develop in eggs outside the mother's body, are referred to as embryos throughout development. During gastrulation the cells of the blastula undergo coordinated processes of cell division, and/or migration to form two or three tissue layers. In triploblastic organisms, the three germ layers are called endoderm and mesoderm; the position and arrangement of the germ layers are species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", is thought to have been an important novelty in the evolution of head structures. During organogenesis and cellular interactions between germ layers, combined with the cells' developmental potential, or competence to respond, prompt the further differentiation of organ-specific cell types.
For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord, peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis, myogenesis and many others. In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves and root, as well as one or more cotyledons. Once the embryo begins to germinate—grow out from the seed—it is called a seedling. Bryophytes and ferns produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated; the inner wall of the archegonium lies in close contact with the "foot" of the developing embryo. The structure and development of the rest of the embryo varies by group of plants; as the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo.
Embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the resulting embryo may be frozen for use; the potential of embryonic stem cell research, reproductive cloning, germline engineering are being explored. Prenatal diagnosis or preimplantation diagnosis enables testing embryos for conditions. Cryoconservation of animal genetic resources is a practice in which animal germplasms, such as embryos are collected and stored at low temperatures with the intent of conserving the genetic material; the embryos of Arabidopsis thaliana have been used as a model to understand gene activation and organogenesis of seed plants. In regards to research using human embryos, the ethics and legalities of this application continue to be debated. Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryos, combining certain types of stem cells; this method will help scientists to more study the first moments of the process of the birth of a new life, which, in turn, can lead to the emergence of new effective methods to combat infertility and other genetic diseases.
Fossilized animal embryos are known from the Precambrian, are found in great numbers during the Cambrian period. Fossilized dinosaur embryos have been discovered; some embryos do not survive to the next stage of development. When this happens it is called spontaneous abortion or miscarriage. There are many reasons; the most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants. In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources; this can disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedl
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
The annelids known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant species including ragworms and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, yet others in moist terrestrial environments; the annelids are bilaterally symmetrical, coelomate, invertebrate organisms. They have parapodia for locomotion. Most textbooks still use the traditional division into polychaetes and leech-like species. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora and Sipuncula regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that includes molluscs, brachiopods and nemerteans; the basic annelid form consists of multiple segments.
Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in others, Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates within blood vessels, the vessels in segments near the front ends of these species are built up with muscles that act as hearts; the septa of such species enable them to change the shapes of individual segments, which facilitates movement by peristalsis or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment. Earthworms are oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil.
The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as bait. Scientists observe annelids to monitor the quality of fresh water. Although blood-letting is used less by doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength. Since annelids are soft-bodied, their fossils are rare – jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil, identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago.
Palaeontologists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, the earliest indisputable fossils of the group appear in the Tertiary period, which began 66 million years ago. There are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus, which can both grow up to 3 metres long. Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids, most textbooks use the traditional classification into the following sub-groups: Polychaetes; as their name suggests, they have multiple chetae per segment. Polychaetes have parapodia that function as limbs, nuchal organs that are thought to be chemosensors. Most are marine animals, although a few species live in fresh water and fewer on land. Clitellates; these have few or no chetae per segment, no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch or, in moniligastrids, yolky eggs that provide nutrition for the embyros.
The clitellates are sub-divided into: Oligochaetes. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or decomposed organic materials. Hirudinea, whose name means "leech-shaped" and whose best known members are leeches. Marine species are blood-sucking parasites on fish, while most freshwater species are predators, they have suckers at both ends of their bodies, use these to move rather like inchworms. The Archiannelida, minute annelids that live in the spaces between grains of marine sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes; some other groups of animals have been classified in various ways, but are now regarded as annelids: Pogonophora / Siboglinidae were first discovered in 1914, their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate phylum, Pogonophora, or as two phyla and Vestimentifera. More they have been re-classified as a family, Siboglinidae
Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division: a vegetative division, whereby each daughter cell is genetically identical to the parent cell, a reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes. Meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated in the first division, sister chromatids are separated in the second division. Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor. Prokaryotes undergo a vegetative cell division known as binary fission, where their genetic material is segregated into two daughter cells. All cell divisions, regardless of organism, are preceded by a single round of DNA replication.
For simple unicellular microorganisms such as the amoeba, one cell division is equivalent to reproduction – an entire new organism is created. On a larger scale, mitotic cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Mitotic cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by meiotic cell division from gametes. After growth, cell division by mitosis allows for continual repair of the organism; the human body experiences about 10 quadrillion cell divisions in a lifetime. The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information, stored in chromosomes must be replicated, the duplicated genome must be separated cleanly between cells. A great deal of cellular infrastructure is involved in keeping genomic information consistent between generations. Interphase is the process a cell must go through before mitosis and cytokinesis.
Interphase consists of three main stages: G1, S, G2. G1 is a time of growth for the cell where specialized cellular functions occur in order to prepare the cell for DNA Replication. There are checkpoints during interphase that allow the cell to be either progressed or denied further development. In S phase, the chromosomes are replicated in order for the genetic content to be maintained. During G2, the cell undergoes the final stages of growth before it enters the M phase, where spindles are synthesized; the M phase, can be either meiosis depending on the type of cell. Germ cells, or gametes, undergo meiosis. After the cell proceeds through the M phase, it may undergo cell division through cytokinesis; the control of each checkpoint is controlled by cyclin and cyclin dependent kinases. The progression of interphase is the result of the increased amount of cyclin; as the amount of cyclin increases and more cyclin dependent kinases attach to cyclin signaling the cell further into interphase. The peak of the cyclin attached to the cyclin dependent kinases this system pushes the cell out of interphase and into the M phase, where mitosis and cytokinesis occur.
There are three transition checkpoints. The most important being the G1-S transition checkpoint. If the cell does not pass this phase the cell will most not go through the rest of the cell division cycle. Prophase is the first stage of division; the nuclear envelope is broken down, long strands of chromatin condense to form shorter more visible strands called chromosomes, the nucleolus disappears, microtubules attach to the chromosomes at the kinetochores present in the centromere. Microtubules associated with the alignment and separation of chromosomes are referred to as the spindle and spindle fibers. Chromosomes will be visible under a microscope and will be connected at the centromere. During this condensation and alignment period, homologous over. In metaphase, the centromeres of the chromosomes convene themselves on the metaphase plate, an imaginary line, equidistant from the two centrosome poles. Chromosomes line up in the middle of the cell by MTOCs by pushing and pulling on centromeres of both chromatids which causes the chromosome to move to the center.
The chromosomes are still condensing and are at one step away from being the most coiled and condensed they will be. Spindle fibres have connected to the kinetochores. At this point, the chromosomes are ready to split into opposite poles of the cell towards the spindle to which they are connected. Anaphase is a short stage of the cell cycle and occurs after the chromosomes align at the mitotic plate. After the chromosomes line up in the middle of the cell, the spindle fibers will pull them apart; the chromosomes are split apart as the sister chromatids move to opposite sides of the cell. While the sister chromatids are being pulled apart and plasma gets elongated from non-kinetochore microtubules Telophase is the last stage of the cell cycle. A cleavage furrow splits the cell in two; these two cells form around the chromatin at the two poles of the cell. Two nuclear membranes begin to reform and the chromatin begin to unwind. Cells are broadly classified into two main categories: simple, non-nucleated prokaryotic cells, complex, nucleated eukaryotic cells.
Owing to their structural differences and prokaryotic cells do not divide in the same way. The pattern of cell division that tr