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Sexual reproduction

Sexual reproduction is a process that creates a new organism by combining the genetic material of two organisms; it occurs both in eukaryotes[1][2] and in prokaryotes.[3] A key similarity between bacterial sex and eukaryotic sex is that DNA originating from two different individuals (parents) join up so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information (a process called genetic recombination). After the new recombinant chromosome is formed it is passed on to progeny).

On the other hand, bacterial conjugation, a type of transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction,[4] because the mechanics are similar. However, bacterial conjugation is controlled by plasmid genes that are adapted for spreading copies of the plasmid between bacteria. The infrequent integration of a plasmid into a host bacterial chromosome, and the subsequent transfer of a part of the host chromosome to another cell do not appear to be bacterial adaptations.[3][5]

In contrast, bacterial transformation can be regarded as a form of sex in bacteria.[3][6] Bacterial transformation is a complex process encoded by numerous bacterial genes, and is clearly a bacterial adaptation for DNA transfer. This process occurs naturally in at least 40 bacterial species.[7] For a bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as competence (see Natural competence). Sexual reproduction in early single-celled eukaryotes may have evolved from bacterial transformation.[8]

There are two main processes during sexual reproduction in eukaryotes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination.

Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. The evolution of sexual reproduction is a major puzzle (see Evolution of sexual reproduction). The first fossilized evidence of sexual reproduction in organisms such as eukaryotes is in the Stenian period, about 1 to 1.2 billion years ago.[9]

Evolutionary thought proposes several explanations for why sexual reproduction developed and why it is maintained. These reasons include fighting the accumulation of deleterious mutations, increasing rate of adaptation to changing environments[10] (see the red queen hypothesis), dealing with competition (see the tangled bank hypothesis) or as an adaptation for repairing DNA damage (see Evolution of sexual reproduction).[3][6][8] The maintenance of sexual reproduction has been explained by theories that work at several different levels of selection, though some of these models remain controversial. New models presented in recent years, however, suggest a basic advantage for sexual reproduction in slowly reproducing, complex organisms, exhibiting characteristics that depend on the specific environment that the given species inhabit, and the particular survival strategies that they employ.
Animals typically produce male gametes called sperm, and female gametes called eggs and ova, following immediately after meiosis, with the gametes produced directly by meiosis. Plants on the other hand have mitosis occurring in spores, which are produced by meiosis. The spores germinate into the gametophyte phase. The gametophytes of different groups of plants vary in size; angiosperms have as few as three cells in pollen, and mosses and other so called primitive plants may have several million cells. Plants have an alternation of generations where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores within the sporangium by meiosis.
Flowering plants
Flowers are the sexual organs of flowering plants.

Flowering plants are the dominant plant form on land and they reproduce by sexual and asexual means. Often their most distinguishing feature is their reproductive organs, commonly called flowers. The anther produces male gametophytes, the sperm is produced in pollen grains, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate. Nonflowering plants like ferns, moss and liverworts use other means of sexual reproduction.

Ferns mostly produce large diploid sporophytes with rhizomes, roots and leaves; and on fertile leaves called sporangium, spores are produced. The spores are released and germinate to produce short, thin gametophytes that are typically heart shaped, small and green in color. The gametophytes or thallus, produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the archegonia where they fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of different thallus. A zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporephyte and gametophyte plants is called alternation of generations. Other plants with similar reproductive means include the Psilotum, Lycopodium, Selaginella and Equisetum.

The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominate form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antherida and archegonia by mitosis. The sperm released from the antherida respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. The zygote divides by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules, which are connected by seta to the archegonia. The spore capsules produce spores by meiosis, when ripe the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. Also in some species each plant is one sex while other species produce both sexes on the same plant.

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Anika Devi received her Bachelor’s degree in Media, Culture and Communication from New York University in 2012. She began freelancing for Business Solutions BD in 2010 and joined the team as a staff writer three years later. She currently serves as the assistant editor.
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