Sexual reproduction is a form of reproduction in which two distinct morphologically different types of reproductive cells called gametes join together, involving a large female ovum (or egg) and a smaller male sperm. Each gamete contains half the number of normal cell chromosomes. They are created by a special type of cell division, which occurs only in eukaryotic cells, known as meiosis. Both gametes fuse during conception to produce DNA replication and the creation of a single-celled zygote that includes the genetic material of both gametes. In a process called genetic recombination, the genetic material (DNA) joins so that the homologous chromosome sequence is parallel to one another, and this is followed by the exchange of genetic information. Two rounds of cell division then produce four child cells with half the number of chromosomes from each original stem cell, and the same number of chromosomes as both parents. For example, in human reproduction every human cell contains 46 chromosomes in 23 pairs. Meiosis in parental gonads produces gamete cells containing only 23 chromosomes. When the gametes are combined through sexual intercourse to form a fertilized egg, the resulting child will have 23 chromosomes from each of the genetically recombined parents to 23 pairs of chromosomes or 46 total.
The division of mitotic cells then begins the development of new individual organisms in multicellular organisms, including animals and plants, as most of these are the main methods of reproduction.
The evolution of sexual reproduction is a major puzzle because asexual reproduction must be able to defeat it because any young organism created can bear children of its own. This implies that the asexual population has an intrinsic capacity to grow faster with each generation. This 50% fee is a loss of sexual reproductive fitness. The double cost of sex includes this cost and the fact that any organism can only inherit 50% of its genes to its offspring. One of the definite advantages of sexual reproduction is to prevent the accumulation of genetic mutations.
Sexual selection is a mode of natural selection in which some individuals out-reproduce others from the population because they are better at securing a partner for sexual reproduction. It has been described as "a powerful evolutionary force that is absent in the asexual population."
Prokaryotes, whose cells initially have additional or altered genetic material, reproduce through asexual reproduction but may, in lateral gene transfer, display processes such as bacterial conjugation, transformation and transduction, which are similar to sexual reproduction even though they do not lead to reproduction.
Video Sexual reproduction
Evolution
The first fossil evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1 to 1.2 billion years ago.
Biologists who study evolution offer some explanations as to why sexual reproduction develops and why it is preserved. These reasons include reducing the likelihood of accumulation of damaging mutations, increasing the level of adaptation to environmental change, handling competition, and covering damaging mutations. All ideas on why sexual reproduction is maintained are generally supported, but ultimately population size determines whether sexual reproduction is entirely beneficial. Larger populations seem to respond more quickly to the benefits gained through sexual reproduction than smaller population sizes.
Maintenance of sexual reproduction has been explained by theories that work at some level of selection, although some of these models are still controversial. However, the new models presented in recent years indicate a basic advantage for sexual reproduction in slowly reproducing complex organisms.
Sexual reproduction allows these species to exhibit characteristics that depend on the specific environment they live in, and the particular survival strategies they apply.
Maps Sexual reproduction
Sex span id = "Sexual_selection"> Sexual selection
To reproduce sexually, both men and women must find a partner. Generally the preferred breeding animals are made by women while men compete to be chosen. This can cause the organism to make extreme attempts to reproduce, such as combat and display, or produce extreme features caused by positive feedbacks known as Fishermen escape. So sexual reproduction, as a form of natural selection, has an effect on evolution. Sexual dimorphism is where the basic phenotypic properties vary between males and females of the same species. Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other body traits. Nevertheless, sexual selection is only implied in the long term that leads to sexual dimorphism.
Sex ratio
Apart from some eusocial bees, sexually reproducing organisms have a 1: 1 sex ratio between men and women. British statistician and biologist Ronald Fisher explains why this happens in what came to be known as the Fisher principle. It basically says the following:
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- Suppose that men's births are less common than women.
- A newborn man has a better marriage prospect than a newborn woman, and therefore can expect to have more offspring.
- Therefore, parents who genetically tend to produce men tend to have more than the average number of grandchildren born to them.
- Therefore the genes for male-producing tendencies spread, and male births are becoming more common.
- Since the 1: 1 sex ratio is approached, the profits associated with producing men will die.
- The same reason applies if women are changed for men all over. Therefore 1: 1 is the equilibrium ratio.
Animal
Insects
Insect species make up more than two thirds of all existing animal species. Most species of insects reproduce sexually, although some species are facultatively parthenogenetic. Many species of insects have sexual dimorphism, while in others the sexes look almost identical. Usually they have two sexes with men producing spermatozoa and female ovum. Ova develops into eggs that have a cover called chorion, which is formed before internal fertilization. Insects have very diverse marriage and reproductive strategies that most often produce male spermatophore in the female, which he keeps until he is ready for egg fertilization. After fertilization, and the formation of zygote, and various levels of development, in many species of eggs are stored outside the female; while in others, they develop further in females and are born alive.
Bird
Mammals
There are three types of mammals that still exist: monotremes, placentas and marsupials, all with internal fertilization. In placental mammals, the offspring are born as teenagers: complete animals with sex organs are present although not functioning reproductively. After a few months or years, depending on the species, the sex organs develop further until maturity and the animal becomes sexually mature. Most female mammals are only fertile during certain periods during their estrus cycle, where they are ready to mate. Male and female mammals meet and intercourse. For most mammals, men and women exchange sexual partners throughout their adult life.
Fish
Most species lay eggs which are then fertilized by males, some species lay eggs on substrates such as rocks or plants, while others scatter eggs and fertilized eggs as they drift or drown in the water column..
Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fish that have live descendants include guppy and mollies or Poecilia . Fish that give birth to young life can be ovoviviparous, where the egg is fertilized inside the female and the egg just hatches inside the woman's body, or on the sea horse, the males carrying young children who are developing in the pouch, and giving birth to live young. Fish can also be vivipar, in which women supply food to the growing offspring internally. Some fish are hermaphrodites, in which a single fish is male and female and can produce eggs and sperm. In hermaphroditic fish, some male and female fish at the same time while in other fish they are serialized hermaphrodites; started as one sex and changed to another. At least one hermaphroditic species, self-fertilization occurs when eggs and sperm are released together. Internal self-fertilization can occur in several other species. One species of fish does not reproduce with sexual reproduction but uses sex to produce offspring; Poecilia formosa is a unisex species that uses a form of parthenogenesis called gynogenesis, in which unfertilized eggs develop into embryos that produce female offspring. Poecilia formosa mates with males of other fish species that use internal fertilization, the sperm does not fertilize the egg but stimulates the growth of the egg that develops into the embryo.
Reptile
Amphibians
Molluscs
Plants
Animals usually produce gametes directly with meiosis. Male gametes are called sperm, and female gametes are called eggs or eggs. In animals, fertilization follows immediately after meiosis. Plants on the other hand have a mitosis that occurs in spores, which are produced by meiosis. Spores germinate into gametophyte phases. Gametophytes of various plant groups vary in size; angiosperms have at least three cells in the pollen, and other so-called primitive moss and plants may have several million cells. Plants have a generation shift where the sporophyte phase is replaced by the gametophyte phase. Sporophyte phases produce spores in sporangium by meiosis.
Flowering plants
Flowering plants are the dominant plant form on land and they reproduce sexually or asexually. Often the most distinguishing feature of them is their reproductive organs, commonly called flowers. Anther produces pollen containing male gametophytes (sperm). In order for pollination, the pollen must be attached to the stigma of the female reproductive structure (carpel), where the female gametophyte (ovula) is located inside the ovary. Once the pollen tube grows through the carpel style, the sex cell nucleus of the pollen grains migrates into the ovul to fertilize the egg and endosperm nucleus in the female gametophyte in a process called double fertilization. The resulting zygote develops into an embryo, whereas the triploid endosperm (one sperm cell plus two female cells) and the female tissue of the ovula cause surrounding tissue in the developing seed. The ovaries, which produce the female gametophyte (s), then grow into the fruit, which surrounds the seed (s). Plants can do self-pollination or cross-pollination.
Cutting plants such as ferns, mosses and liverworts using other sexual reproductive devices.
In 2013, flowers derived from Lime (100 million years before now) are found wrapped in amber, proof of the oldest sexual reproduction in a flowering plant. Microscopic images show tubes growing from pollen and penetrating the stigma of flowers. The pollen was sticky, indicating it was carried by an insect.
Fern
Ferns generally produce large diploid sporophytes with rhizomes, roots, and leaves; and on the fertile leaves called sporangium, the spores are produced. Spores are released and germinate to produce thin and thin gametophytes that are usually heart-shaped, small and green in color. Gametophytes or thallus, producing motile sperm in antheridia and eggs in separate archegonia. After rain or when the dew deposits the water film, the motile sperm is scattered from antheridia, which is usually produced on the upper side of the talus, and swims in the water film to the archegonia where they fertilize the egg. To promote cross or sperm cross fertilization is released before the egg receives sperm, so it is more likely sperm will fertilize a different thallus egg. The zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporephyte and gametophyte plants is called generational turnover. Other plants with the same reproductive means include Psilotum, Lycopodium, Selaginella and Equisetum.
Bryophytes
The bryophytes, which include liverworts, horn and moss lichens, reproduce both sexually and vegetatively. They are small plants that are found to grow in damp locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. This plant begins as a haploid spore that grows into a dominating form, which is a multicellular haploid body with a leaf-like structure that photosynthesizes. Haploid gametes are produced in antherides and archegonia with mitosis. Sperms released from the antheridal response to chemicals released by mature archegonia and swim into them in water film and fertilize the egg to produce a zygote. The zygote divides by mitotic division and grows into a diploid sporophyte. Multicellular diploid sporophyte produces a structure called spore capsule, which is connected by seta to archegonia. Spore capsules produce spores with meiosis, when mature open capsules and spores are released. Bryophytes show variations in their breeding structures and above are the baseline. Also in some species each plant is one sex while the other species produce both sexes on the same plant.
Mushroom
Fungi are classified according to the sexual reproduction method they use. The result of sexual reproduction most often is the production of resting spores used to survive fold and spread. There are usually three phases in the sexual reproduction of fungi: plasmogami, kariogami and meiosis. The cytoplasm of the two stem cells fuses during plasmogamy and the nucleus fuses during karyogami. New haploid gametes are formed during meiosis and develop into spores.
Bacteria and archaea
Three different processes in prokaryotes are considered to be the same as eukaryotic sex: bacterial transformation, which involves the incorporation of foreign DNA into the bacterial chromosome; bacterial conjugation, which is the transfer of plasmid DNA between bacteria, but plasmids are rarely inserted into the bacterial chromosome; and gene transfer and genetic exchange in archaea.
Bacterial transformation involves the recombination of genetic material and its function mainly related to DNA repair. Bacterial transformation is a complex process encoded by many bacterial genes, and is a bacterial adaptation for DNA transfer. This process occurs naturally in at least 40 species of bacteria. In order for the bacteria to bind, take, and combine exogenous DNA into its chromosomes, it must enter a special physiological state called competence (see Natural Competency). Sexual reproduction in early single cell eukaryotes may have evolved from bacterial transformation, or from similar processes in archaea (see below).
On the other hand, bacterial conjugation is a type of direct transfer of DNA between two bacteria through external frills called conjugate pilus. Bacterial conjugation is controlled by a plasmid gene that is adapted to propagate a copy of the plasmid between bacteria. The sparse integration of the plasmid becomes the host bacterial chromosome, and subsequent transfer from the host portion of the host chromosome to the other does not look like a bacterial adaptation.
Exposure of hypophophophyte archaea species Sulfolobus to damaging DNA conditions induces cell aggregation accompanied by the exchange of high frequency genetic markers. Ajon et al. hypothesized that this cell aggregation enhances species-specific DNA repair by homologous recombination. DNA transfer in Sulfolobus may be a preliminary form of sexual interaction similar to a better-studied bacterial transformation system that also involves the transfer of species-specific DNA that leads to improved homologous recombination of DNA damage.
See also
References
Further reading
- Pang, K. "Biology Certificate: New Mastery Basic Concepts", Hong Kong, 2004
- Journal of Reproductive Biology, accessed in August 2005.
- "Sperm Using Heat Sensor To Find Eggs; Research Research Weizmann Institute To Understand Human Fertilization", Science Daily , 3 February 2003
- Michod, RE; Levin, BE, eds. (1987). Sex evolution: Examination of current ideas . Sunderland, Massachusetts: Sinauer Associates. ISBN 978-0878934584.
- Michod, RE (1994). Eros and Evolution: Natural Sex Philosophy . Perseus Books. ISBN 978-0201407549.
External links
- Khan Academy, video lecture
Source of the article : Wikipedia
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