Hello friends of steemit in this publication will talk about the reproduction of life, one of the basic characteristics of every living being is the ability to reproduce. Reproduction is the process by which new individuals are generated. It allows the continuity of life; without reproduction, life would come to an end. In this topic of study we will investigate the types of reproduction experienced by living beings and we will mention some examples.
Two types of reproduction are recognized: the asexual and the sexual. All living beings reproduce by one, another or both types of reproduction.
The Asexual Reproduction, "speed and simplicity" ...
It is the generation of new organisms without the participation of sex cells or gametes. In some cases, but not always, it is also called vegetative reproduction. There is a single parent organism involved, there are no special reproductive cells or organs and offspring are produced equal to the parent and equal to each other. As a result, what are called "clones" are produced, descendants that are genetically identical to the parent. In comparison with sexual reproduction, which we will explain later, you will see that it has the disadvantage that it does not provide genetic variability. If there is any variation among offspring, it is likely due to mutations or particular changes that may have occurred during DNA replication or by other means. Asexual reproduction is a quick and effective means of generating new individuals.
The basic forms of asexual reproduction are: fission (binary or multiple), budding, fragmentation and sporulation.
The binary and multiple fission
Binary fission is the only type of reproduction experienced by both prokaryotic and eukaryotic organisms. In this type of reproduction, the cell grows in size, duplicates its DNA and then divides into two daughter cells identical to each other and identical to the progenitor.
Prokaryotes (bacteria and archaea) are unicellular organisms that reproduce only asexually, by binary fission. In general, prokaryotes reproduce very fast. There are species that can be divided every 20 minutes, for example. Hence, the importance of combating bacterial infections in a timely manner.
The group of Protest Euglenoides and most of the Ciliates, of the genus Paramecium, experience this type of reproduction, as well as amoebas and several other protists.
In multiple fission, the nucleus divides repeatedly before dividing the cytoplasm, producing many daughter cells simultaneously. This form of reproduction is common among some protozoan parasites, such as, for example, the parasite protista Plasmodium, causal agent of malaria.
The budding
It can occur in unicellular and multicellular organisms. At the unicellular level, it is a process of asymmetric mitosis. A bulge is formed, which is called a bud, in a certain portion of the plasma membrane. The nucleus of the progenitor cell divides and one of the child nuclei passes to the yolk. It occurs in unicellular organisms such as yeasts (fungi).
At a multicellular level, a miniature version of the organism (a bud) grows by mitotic cell division, directly on the body of the adult, feeding on its parent. When it grows enough, the yolk comes off and becomes independent. The yolk is genetically identical to the parent. This type of reproduction is typical of marine animals such as sponges and cnidarians, such as hydra, and some anemones. It is also experienced by some plant species, this being a mechanism of vegetative reproduction. In this case, meristematic cells (cells responsible for plant growth, have capacity for division and from these cells arise other tissues) that appear on the adult individual, give rise to the new individual. As before, in higher plants, for example, buds are formed that can give rise to lateral branches.
Fragmentation
In a fragmentation or also called reproductive regeneration, the individual breaks into two or more parts and each fragment is capable of becoming a complete individual.
Many invertebrates can reproduce asexually by simply breaking into two parts and regenerating the lost parts of the fragments. This very interesting reproductive strategy is widely used as a fantasy strategy in many science fiction movies.
Echinoderms (starfish), for example, have remarkable abilities to regenerate. If the starfish are cut into pieces, each piece that includes a part of the central disk becomes a new animal. Regeneration can also take place when an animal is fragmented by an external force, such as a storm, for example. This can cause strong currents that fragment colonial cnidarians, such as corals. Through this mechanism, broken pieces of the colony can regenerate into new colonies.
Some species of segmented marine worms develop segments with rudimentary heads that carry sensory organs, and then separate from the rest of the body. Each fragmented segment forms a new worm.
Plants also experience this type of reproduction. It is common in some bryophytes.
Sporulation (formation of spores)
In animals, meiosis produces gametes, but in plants and fungi, on the other hand, it produces spores. A spore is a haploid reproductive cell (n) that, unlike a gamete, can produce a haploid organism without first fusing with another cell. The mechanism of reproduction by spores is common in fungi, plants, and algae.
Spores, in general, are resistant structures. They consist of a single cell protected by a thick envelope that allows them to withstand unfavorable environmental conditions. When the conditions become favorable, the envelope tears and the cell contained inside begins to germinate to give rise to the new organism.
Asexual reproduction, in all its versions, occurs in bacteria, in unicellular eukaryotes (protists or fungi), in multicellular fungi, in plants and in many groups of invertebrates (such as cnidarians that include jellyfish, annelid worms and the echinoderms that include starfish, among others).
The interesting thing is that this type of reproduction, apparently simple, simple and fast, does not occur in vertebrates.
Sexual Reproduction; genetic variability
This type of reproduction requires, in general, two parents, and each one of them contributes with their sexual cells, the gametes. The union of both gametes, during fertilization, gives rise to a new individual, called zygote.
The cells involved in fertilization, produced by meiosis, have a haploid number of chromosomes (n), and by fertilization the diploid number (2n) is restored.
Now, what adaptive advantages does this type of reproduction provide?
Sexual reproduction allows to redistribute the genes between the individuals to generate genetically unique descendants, different from each other and from the parents.
The wonderful thing about sexual reproduction is that the offspring will have a new genotype, different from either parent, and this occurs through recombination of parental characters, thus multiplying the variability and making possible a more diverse evolution.
This genetic variation among offspring means that some of them can adapt better than others to survive and reproduce in a particular environment. This genetic diversity provides the "raw material" for natural selection and evolution, topics that we will study later.
Sexual reproduction includes processes defined as bisexual (or biparental) reproduction, as the most common form, involving two separate individuals, and also to Hermaphroditism and parthenogenesis, less common forms.
Bisexual reproduction
The bisexual reproduction is the production of offspring formed by the union of gametes of two genetically different progenitors that are of different sexes (female or male). Each one provides its gametes or reproductive cells, previously produced by meiosis. Each gamete or reproductive cell contains a single set of chromosomes, it is haploid (n).
The distinction between both sexes is not based on any difference in size or appearance, but on the size and mobility of the gametes they produce. The egg (or egg) is produced by the female and are generally large, compared to the size of the male gamete. The sperm are small in comparison to the female gamete, mobile and in general, are produced in large quantities. They are like small "packages" of highly condensed genetic material designed for the sole purpose of reaching and fertilizing an egg.
Both gametes are generated in the reproductive organs, and fuse to produce a single cell, the zygote, also called a fertilized egg. The zygote, therefore, contains two sets of chromosomes, one from each parent. In multicellular organisms, from that single cell, the zygote, the whole organism will develop during embryonic development.
It should be noted that organisms that have separated sexes are defined as dioic organisms. Male and female gametes are found in separate individuals. Almost all vertebrates have separate sexes, that is, they are dioic.
This phenomenon also occurs in plants, although it is more characteristic of more primitive species, such as ginkgo. That is, there are plants that are female or male. In fungi there is no morphological distinction between structures of both sexes, or between female and male individuals. Instead, there is a genetically determined distinction between two or more types of mating. Specimens of different types of mating differ genetically from one another, but are often visually indistinguishable.
Hermaphroditism and parthenogenesis are less common forms of sexual reproduction that involve only one individual.
Hermaphroditism
In contrast to organisms called dioicos (separate sexed organisms), hermaphrodites are monocytes, which means that the same organism presents both sexes.
Many sessile endoparasite invertebrate animals, such as flat worms, hydroids, annelids, all lung snails and few vertebrates (some fish), are hermaphrodites. In turn, many plant species are also.
Some hermaphrodites fertilize themselves, but self-fertilization can be prevented by exchanging the germ cells with another member of the same species.
In some fish, called sequential hermaphrodites, the animal undergoes a genetically programmed sex change during its life, which depends on how the environmental factors in which they develop change and also on certain patterns of behavior and social organization.
Parthenogenesis
This type of reproduction consists of the development of an organism from a non-fertilized gamete or sex cell. The organisms generated are genetically identical and haploid. It is a common method of reproduction in arthropods, although it can also occur in some species of fish, amphibians and reptiles. The majority of the species that reproduce by this mechanism also reproduce by bisexual reproduction.
The following video summarizes all the aforementioned concepts schematically. Enjoy it!.
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