Reproduction is the biological process A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation by which new individual organisms The term "organism" first appeared in the English language in 1701 and took on its current definition by 1834 (Oxford English Dictionary) are produced. Reproduction is a fundamental feature of all known life Life is a characteristic that distinguishes objects that have self-sustaining biological processes from those that do not–either because such functions have ceased (death), or else because they lack such functions and are classified as "inanimate."; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms.

In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial The bacteria ( [bækˈtɪərɪə] ; singular: bacterium)[α] are a large group of unicellular, prokaryote, microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to single-celled organisms A microorganism or microbe is an organism that is microscopic (usually too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design. Most plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, have the ability to reproduce asexually.

Sexual reproduction requires the involvement of two individuals, typically one of each sex.

Contents

Asexual reproduction

Main article: Asexual reproduction Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms

Asexual reproduction is the process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. Bacteria The bacteria ( [bækˈtɪərɪə] ; singular: bacterium)[α] are a large group of unicellular, prokaryote, microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, divide asexually via binary fission Binary fission, or prokaryotic fission, is the form of asexual reproduction and cell division used by all prokaryotic and some organelles within eukaryotic organisms. This process results in the reproduction of a living prokaryotic cell by division into two parts which each have the potential to grow to the size of the original cell; viruses A virus is a small infectious agent that can replicate only inside the cells of other organisms. Viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 take control of host cells to produce more viruses; Hydras Hydra is a genus of simple freshwater animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted freshwater ponds, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas. They are usually a few (invertebrates An invertebrate is an animal without a spine. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata of the order The Latin suffix -formes meaning "having the form of" is used for the scientific name of orders of birds and fishes, but not for those of mammals and invertebrates Hydroidea) and yeasts Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with about 1,500 species currently described; they dominate fungal diversity in the oceans. Most reproduce asexually by budding, although a few do so by binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation are able to reproduce by budding Budding is a form of asexual reproduction. In single-celled organisms, such as budding yeast, it differs from binary fission in that the two resulting cells are not of equal size. Budding is also seen in multicellular organisms, including hydras and sponges. These organisms do not have different sexes, and they are capable of "splitting" themselves into two or more individuals. Some 'asexual' species, like hydra Hydra is a genus of simple freshwater animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted freshwater ponds, lakes and streams in the temperate and tropical regions by gently sweeping a collecting net through weedy areas. They are usually a few and jellyfish Jellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000-1500 species that make jellyfish and many more that do not). The, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction Vegetative reproduction is a type of asexual reproduction for plants, and is also called vegetative propagation, vegetative multiplication, or vegetative cloning. It is a process by which new plant "individuals" arise or are obtained without production of seeds or spores. It is both a natural process in many plant species and one used or—reproduction without seeds or spores—but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation 'Bacterial conjugation' is the transfer of genetic material between bacteria through direct cell to cell contact, or through a bridge-like connection between the two cells. Discovered in 1946 by Joshua Lederberg and Edward Tatum, conjugation is a mechanism of horizontal gene transfer—as are transformation and transduction—although these. Other ways of asexual reproduction include parthogenesis Parthenogenesis is an asexual form of reproduction found in females where growth and development of embryos occurs without fertilization by a male. In plants, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of apomixis. The offspring produced by parthenogenesis are always female in species, fragmentation Fragmentation or Clonal Fragmentation is a form of asexual reproduction or cloning where an organism is split into fragments. The splitting may or may not be intentional. Each of these fragments develop into mature, fully grown individuals that are a clone of the original organism. If the organism is split any further the process is repeated and spore formation Sporogenesis is the production of spores in biology. The term is also used to refer to the process of reproduction via spores. Reproductive spores are formed in many eukaryotic organisms, such as plants, algae and fungi, during their normal reproductive life cycle. Dormant spores are formed, for example by certain fungi and algae, primarily in that involves only mitosis Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and. Parthenogenesis Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occurs without fertilization by a male. In plants, parthenogenesis means development of an embryo from an unfertilized egg cell, and is a component process of apomixis. The offspring produced by parthenogenesis are always female in species (from the Greek παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is the growth and development of embryo An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus or seed A seed ( /ˈsiːd/ ) is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant. The formation of the seed completes the process of reproduction in seed without fertilization Fertilization , is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation by a male Male refers to the sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization. A male cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually. Parthenogenesis occurs naturally in some species, including lower plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, (where it is called apomixis In botany, apomixis is replacement of the normal sexual reproduction by asexual reproduction, without fertilization. (The original definition did not mention meiosis, although it is sometimes misquoted.) Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis,), invertebrates An invertebrate is an animal without a spine. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata (e.g. water fleas Water flea is a generic term for a number of small aquatic crustacea characterised by their jumping or jerky mode of swimming. The characteristic of the water flea is that the main part of the body is enclosed by a kind of shell. Most are between 0.1 mm and 3 mm in length. Most commonly, they will be species of Daphnia but a number of other genera, aphids Aphids, also known as plant lice , are small plant-eating insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions. The damage they do to plants has made them enemies of farmers and gardeners the world over, but from a purely zoological standpoint they are a, some bees Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila. There are nearly 20,000 known species of bees in seven to nine recognized families, and parasitic wasps The term Parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. They are primarily parasitoids of other animals, mostly other arthropods. Many of them, such as the family Braconidae, are considered beneficial to humans because they control populations of agricultural pests. Some of these wasps), and vertebrates Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony fish, sharks and rays, amphibians, reptiles, (e.g. some reptiles Reptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" (poikilothermic) amniotes whose skin is usually covered in scales or scutes. They are tetrapods (either having four limbs or being descended from four-limbed ancestors) and lay amniotic eggs, in which the embryo is surrounded by a membrane called the,[1] fish A fish is any aquatic vertebrate animal that is covered with scales, and equipped with two sets of paired fins and several unpaired fins. Most fish are ectothermic . Fish are abundant in most bodies of water. Fish can be found in high mountain streams (e.g., char and gudgeon) and in the deepest ocean depths (e.g., gulpers and anglerfish), and, very rarely, birds Birds are winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 3 m (10 ft) Ostrich[2] and sharks Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs[3]). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.

Sexual reproduction

Main article: Sexual reproduction Further information: Human reproduction Human reproduction is any form of sexual reproduction resulting in the conception of a child, typically involving sexual intercourse between a man and a woman. During sexual intercourse, the interaction between the male reproductive system and the female reproductive system results in fertilization of the woman's ovum by the man's sperm, which Hoverflies As their common names suggest, they are often seen hovering or nectaring at flowers; the adults of many species feed mainly on nectar and pollen, while the larvae eat a wide range of foods. In some species, the larvae are saprotrophs, eating decaying plant and animal matter in the soil or in ponds and streams. In other species, the larvae are mating in midair flight

Sexual reproduction is a biological process A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation by which organisms create descendants that have a combination of genetic Genetics , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of material contributed from two (usually) different members of the species. Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid Ploidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid , but sex cells (sperm and egg) are haploid. In contrast, tetraploidy (four sets of chromosomes) is a type of polyploidy and is common in plants, and not uncommon in amphibians, reptiles, and various species of gametes A gamete is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum (or egg)—and a. Most organisms The term "organism" first appeared in the English language in 1701 and took on its current definition by 1834 (Oxford English Dictionary) form two different types of gametes. In these anisogamous species, the two sexes are referred to as male Male refers to the sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization. A male cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually (producing sperm or microspores) and female Female is the sex of an organism, or a part of an organism, which produces non-mobile ova (egg cells) (producing ova or megaspores). In isogamous species the gametes are similar or identical in form, but may have separable properties and then may be given other different names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as ciliates, have more than two kinds of gametes.

Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have two sets of genes for every trait (called alleles). Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. Having two copies of every gene, only one of which is expressed, allows deleterious alleles to be masked, an advantage believed to have led to the evolutionary development of diploidy (Otto and Goldstein).

Allogamy

Main article: Allogamy

Allogamy is a term used in the field of biological reproduction describing the fertilization of an ovum from one individual with the spermatozoa of another.

Autogamy

Self-fertilization (also known as autogamy) occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual. They are bound and all the cells merge to form one new gamete.

Mitosis and meiosis

Mitosis and meiosis are an integral part of cell division. Mitosis occurs in somatic cells, while meiosis occurs in gametes.

Mitosis The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell. Meiosis The resultant number of cells is four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell. A diploid cell duplicates itself, then undergoes two divisions (tetraploid to diploid to haploid), in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.

Same-sex reproduction

In recent decades, developmental biologists have been researching and developing techniques to facilitate same-sex reproduction [4]. The obvious approaches, subject to a growing amount of activity, are female sperm and male eggs, with female sperm closer to being a reality for humans, given that Japanese scientists have already created female sperm for chickens.[citation needed] More recently, by altering the function of a few genes involved with imprinting, other Japanese scientists combined two mouse eggs to produce daughter mice.[citation needed]

Reproductive strategies

There are a wide range of reproductive strategies employed by different species. Some animals, such as the human and Northern Gannet, do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood. For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and a fruit fly (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population.

Other types of reproductive strategies

Main article: Semelparity and Iteroparity

Polycyclic animals reproduce intermittently throughout their lives.

Semelparous organisms reproduce only once in their lifetime, such as annual plants (including all grain crops), and certain species of salmon, spiders, bamboos and centru plants. Often, they die shortly after reproduction. This is often associated with r-strategists.

Iteroparous organisms produce offspring in successive (e.g. annual or seasonal) cycles, such as perennial plants. Iteroparous animals survive over multiple seasons (or periodic condition changes). This is more associated with K-strategists.

Asexual vs. sexual reproduction

Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease.

Many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, some species of starfish (by fragmentation), and many plants are examples. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources.

When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs.

Life without reproduction

The existence of life without reproduction is the subject of some speculation. The biological study of how the origin of life led from non-reproducing elements to reproducing organisms is called abiogenesis. Whether or not there were several independent abiogenetic events, biologists believe that the last universal ancestor to all present life on earth lived about 3.5 billion years ago.

Today, some scientists have speculated about the possibility of creating life non-reproductively in the laboratory. Several scientists have succeeded in producing simple viruses from entirely non-living materials[5]. The virus is often regarded as not alive. Being nothing more than a bit of RNA or DNA in a protein capsule, they have no metabolism and can only replicate with the assistance of a hijacked cell's metabolic machinery.

The production of a truly living organism (e.g., a simple bacterium) with no ancestors would be a much more complex task, but may well be possible according to current biological knowledge.

Lottery principle

Sexual reproduction has many drawbacks, since it requires far more energy than asexual reproduction and diverts the organisms from other pursuits, and there is some argument about why so many species use it.

George C. Williams used lottery tickets as an analogy in one explanation for the widespread use of sexual reproduction[6]. He argued that asexual reproduction, which produces little or no genetic variety in offspring, was like buying many tickets that all have the same number, limiting the chance of "winning" - that is, producing surviving offspring. Sexual reproduction, he argued, was like purchasing fewer tickets but with a greater variety of numbers and therefore a greater chance of success.

The point of this analogy is that since asexual reproduction does not produce genetic variations, there is little ability to quickly adapt to a changing environment. The lottery principle is less accepted these days because of evidence that asexual reproduction is more prevalent in unstable environments, the opposite of what it predicts.

See also

Notes

  1. ^ Halliday, Tim R.; Kraig Adler (eds.) (1986). Reptiles & Amphibians. Torstar Books. pp. 101. ISBN 0-920269-81-8.
  2. ^ Savage, Thomas F. (September 12, 2005). "A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs". Oregon State University. http://oregonstate.edu/Dept/animal-sciences/poultry/index.html. Retrieved 2006-10-11.
  3. ^ "Female Sharks Can Reproduce Alone, Researchers Find", Washington Post, Wednesday, May 23, 2007; Page A02
  4. ^ [1] (timeline of research in the area of same-sex reproduction)
  5. ^ Chemical synthesis of poliovirus cDNA: generation of infectious virus in the absence of natural template Scientists Create Artificial Virus
  6. ^ Williams G C. 1975. Sex and Evolution. Princeton (NJ): Princeton University Press.

References

External links

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