Overview
Reproduction is key to the survival of all species. Without reproduction, species would be unable to reproduce and create new offspring and thus the "cycle of life" would be terminated. Many scientists even claim that reproduction is the basis and purpose of all life.
There are two general types of reproduction, asexual and sexual reproduction. Asexual reproduction occurs mainly in prokaryotes (bacteria), sponges, starfish, hydras, fungi and some plants [1]. Asexual reproduction is centered around the process of mitosis, the division of one diploid cell into two identical daughter diploid cells. Thus, the offspring produced in asexual reproduction are clones and share the same genetic information as their “mother”. Furthermore, asexual reproduction does involve fertilization and thus only one parent is needed for asexual reproduction. This allows for an organism to reproduce in isolation and allows for organisms to reproduce rapidly (bacteria can double every 20 minutes while Pangolins can remain pregnant for up to 150 days) [2]. Yet, because there is no genetic variability betweens the parents and offspring, a single environmental factor can have a drastic effect on an entire asexually reproducing species, possibly wiping them out.
There are numerous different types of asexual reproduction. These include: binary fission, budding, fragmentation and vegetative propagation. Binary fission is the process of cell division (mitosis) which occurs within bacteria, allowing bacteria to double every twenty minutes. Budding is a unique type of asexual reproduction in which offspring are reproduced through the unequal division of genetic information. In budding, the offspring begins as a knob on its “mother” and develops into a bud via mitosis [3]. Once a bud, the offspring may branch off from its “mother”or simply breakaway. In fragmentation, a piece of an organism detaches itself or breaks off. This broken piece then develops into an organism identical to the one that it broke off from, as they each share identical genetic information [4]. Finally, in vegetative propagation, plants grow without seeds or spores. Often this occurs when the “mother’s” roots extend underground via mitosis, which creates a cloned plant.
Sexual reproduction is very common among mammals, amphibians, birds and numerous other species of animals. Sexual reproduction, unlike asexual reproduction, allows for the creation of genetically varied offspring. These genetic variations within a species are caused by both meiosis and fertilization. During meiosis, four haploid cells (gametes/sex cells) are created, each carrying different genetic information. Subsequently, during fertilization, two of these haploid cells (one an egg and the other a sperm) and their chromosomes are combined to create a diploid cell. Because two haploid cells are required for fertilization, sexual reproduction requires two parents, one a father with male sex organs and the other a mother with female sex organs, to be present during sexual reproduction. Furthermore, the genetic variation, which is caused by fertilization and meiosis, is necessary for natural selection, as by being so genetically diverse, organisms that are produced sexually can easily adapt to their environment while organisms produced asexually can not.
There are two general types of reproduction, asexual and sexual reproduction. Asexual reproduction occurs mainly in prokaryotes (bacteria), sponges, starfish, hydras, fungi and some plants [1]. Asexual reproduction is centered around the process of mitosis, the division of one diploid cell into two identical daughter diploid cells. Thus, the offspring produced in asexual reproduction are clones and share the same genetic information as their “mother”. Furthermore, asexual reproduction does involve fertilization and thus only one parent is needed for asexual reproduction. This allows for an organism to reproduce in isolation and allows for organisms to reproduce rapidly (bacteria can double every 20 minutes while Pangolins can remain pregnant for up to 150 days) [2]. Yet, because there is no genetic variability betweens the parents and offspring, a single environmental factor can have a drastic effect on an entire asexually reproducing species, possibly wiping them out.
There are numerous different types of asexual reproduction. These include: binary fission, budding, fragmentation and vegetative propagation. Binary fission is the process of cell division (mitosis) which occurs within bacteria, allowing bacteria to double every twenty minutes. Budding is a unique type of asexual reproduction in which offspring are reproduced through the unequal division of genetic information. In budding, the offspring begins as a knob on its “mother” and develops into a bud via mitosis [3]. Once a bud, the offspring may branch off from its “mother”or simply breakaway. In fragmentation, a piece of an organism detaches itself or breaks off. This broken piece then develops into an organism identical to the one that it broke off from, as they each share identical genetic information [4]. Finally, in vegetative propagation, plants grow without seeds or spores. Often this occurs when the “mother’s” roots extend underground via mitosis, which creates a cloned plant.
Sexual reproduction is very common among mammals, amphibians, birds and numerous other species of animals. Sexual reproduction, unlike asexual reproduction, allows for the creation of genetically varied offspring. These genetic variations within a species are caused by both meiosis and fertilization. During meiosis, four haploid cells (gametes/sex cells) are created, each carrying different genetic information. Subsequently, during fertilization, two of these haploid cells (one an egg and the other a sperm) and their chromosomes are combined to create a diploid cell. Because two haploid cells are required for fertilization, sexual reproduction requires two parents, one a father with male sex organs and the other a mother with female sex organs, to be present during sexual reproduction. Furthermore, the genetic variation, which is caused by fertilization and meiosis, is necessary for natural selection, as by being so genetically diverse, organisms that are produced sexually can easily adapt to their environment while organisms produced asexually can not.
MitosisMitosis is the duplication of a cell, a process which allows for all asexual reproduction. Within the life cycle of a cell, ninety percent is spent in interphase ( in which DNA inside of the nucleus is duplicated ) while the other ten percent is spent performing mitosis. During mitosis, the organelles and chromosomes of a diploid cell are split, creating two identical diploid daughter cells.
When describing mitosis, scientists often break up the process into four phases. These are prophase, metaphase, anaphase and telophase. During prophase, duplicated DNA which is in the form of chromatin rearranges itself into sister chromatid (chromosomes). Additionally, the nucleus of the cell dissolves and spindle fibers (special micro-tubules) are formed by centrioles found within the cell. In metaphase, the newly formed chromosomes align themselves along the equatorial plane. This allows for an even distribution of genetic material among the daughter cells. In the following phase, anaphase, the spindle fibers pull the chromosomes apart and the sister chromatids are divided into single chromatid. Each half of the sister chromatid travels to the opposite poles of the cell. Then, in telophase, the last phase of mitosis, the cell pinches in half creating cleavage and the cytoplasm divides. Finally, the nucleus once again reappears, the chromatids return to the form of chromatin and the cell splits in to two uniform cells. |
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MeiosisMeiosis is the process of cellular division in which gametes (sex cells) are formed and which thus enables sexual reproduction. These gametes are haploid cells, cells with half the number of typical chromosomes. During meiosis, one diploid cell (a cell with a complete set of chromosomes) becomes divided into four genetically varied daughter haploid cells. During fertilization, two of these haploid cells (one from either parent) are combined to create a diploid cell, which ultimately develops into the offspring.
Numerous phases of cell division occur throughout the process of meiosis. These phases of meiosis are divided into two stages, meiosis I and meiosis II. Prophase I, crossing over, metaphase I, anaphase I and telophase I, each occur during meiosis I. The purpose of meiosis I is to create two haploid daughter cells with varied genetic information. During the first phase of meiosis I, prophase I, the nucleus dissolves, chromatin becomes rearranged into sister chromatids (chromosomes) and centrioles within the cell begin to form spindle fibers. Additionally, the sister chromatids form tetrads with their homologous pairs [5]. These homologous pairs are sets of sister chromatids, one from the mother and one from the father, whose genetic coding for certain alleles is located in the same places [6]. As the homologous pairs have the genetic information for alleles in the same places, crossing over, the next stage of meiosis I is able to occur. During crossing over, genetic information is exchanged between homologous pairs [7]. This allows for a greater genetic variation between the haploid cells created during meiosis and thus a greater genetic variation between the offspring and parents. In metaphase I, these homologous chromosomal pairs line up along the equatorial plane. Because of the random way in which the pairs line up, metaphase I, like crossing over, also allows for an increase in the genetic variation. Thereafter, in anaphase I, spindle fibers pull the homologous pairs apart and each sister chromatid is pulled towards opposite poles of the cell. Finally, in telophase I, the cytoplasm of the cell begins to pinch creating cleavage and the cell organelles move to either pole of the cell. Simultaneously, the chromosomes arrange themselves into chromatin and once again a nucleus is created. Once this has occurred, the cell completely splits apart into two daughter haploid cells. Following the creation of the two haploid cells, the second stage of meiosis, meiosis II, occurs. Meiosis II is almost identical to mitosis, and in fact they each share similar phases. The phases of meiosis II are prophase II, metaphase II, anaphase II and telophase II. During prophase II of meiosis II, chromatin within each of the two haploid cells rearranges into sister chromatids, the nucleus dissolves, and spindle fibers are formed. Then during metaphase II, the sister chromatids line up along the equatorial plane. Once again, their random placement along the equatorial line creates a greater genetic variability. In anaphase II, fully formed spindle fibers pull sister chromatids apart, pulling each chromatid towards either end of the cell’s poles. Finally, in telophase II, the last phase of meiosis, the two daughter haploid cells begin to divide. Chromatids within each of the two cells are rearranged into chromatin, a nucleus is formed and cell organelles equally split themselves among the final four cells, moving towards each of the poles. Then, the two haploid cells split, creating four daughter haploid cells, each carrying diverse genetic information. |
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Reproduction in PangolinsTree Pangolins are dioecious organisms, and similarly to all other mammals they reproduce sexually [8]. This signifies that male pangolins and female pangolins must each have diverse sexual reproduction organs, as one must produce sperm and the other eggs. It also requires for both a female and male pangolin to be present during reproduction, as the male pangolin’s sperm must fertilize the female pangolin’s egg.
Like humans, both the male and female pangolins have both internal and external sexual reproductive organs. The male pangolins internal sex organs include the testes and seminal vesicle. The testes are gonads, as they produce sperm, while the seminal vesicle is a gland which produces semen, a liquid which serves as a medium of transportation for sperm. The external sexual reproduction organ of male pangolins is the penis. However, the pangolin’s penis is stored internally and is only erect during copulation. Little else is known though about the male pangolins reproductive systems, as pangolins, since they are nocturnal, elusive, and difficult to capture (because of numerous government laws which prohibit the capturing of pangolins), are unfrequently studied. Yet, as both humans and pangolins are mammals, one can assume that male pangolins share many of the similar sex organs as humans, including the vas deferens (which carries sperm to the urethra to be ejected), the epiditymus ( which stores sperm), the prostate (which secretes an alkaline solution which protects the sperm from the acidity of the vagina), and the bulbonretheral gland (which secretes a fluid to clean the urethra). Likewise, there is not much research about the female reproduction organs. It is believed though that female pangolins have two ovaries, an oviduct, and a uterus as their internal reproductive organs. Scientists studying the anatomy of pangolins have also identified that the ovary, the gonad of the female pangolin, produces eggs in a manner similar to that of humans. When born, a female pangolin has thousands off eggs already produced. When she reaches sexual maturity at age two, follicles develop the eggs in the ovaries until they are ready for release. If copulation is successful, this egg is fertilized in the oviduct and then attaches to the uterus. During pregnancy, an embryo develops within the female pangolins uterus. The embryo lies in an amniotic sac and receives nutrition from the placenta. The external reproduction organ for female pangolins is the vagina, a thin chamber which serves as the location of fertilization. During copulation, the male pangolins penis is inserted into the vagina and internal fertilization occurs. Pangolin reproduction occurs once a year during mating season, either in the fall and summer months [9]. As pangolins are a very isolated and territorial species, mating season is the only time of interaction between pangolins. Male pangolins attract female pangolins by urinating, an action which produces a strong smell which female pangolins can then identify and locate using their extraordinary sense of smell [10]. Though it does not occur often within the wild, two male pangolins within the same area may fight over a female, using their tails as clubs to knock the other out [11]. Once copulation has occurred and the female pangolin is impregnated, gestation, or their time of pregnancy, can last from 120 to 150 days [12]. During this time, the female pangolin does not undergo any large or notable changes, except for the minimal swelling of her belly. As pangolins are viviparous organisms, their young are born live [13][14]. When pangolins are born, they are 80-450 g and approximately 6 inches long [15]. Likewise, their scales are soft, white, and do not overlap or harden until two days later [16]. At approximately two weeks of age, a young pangolin is finally able to walk. Despite this, they travel on their mothers tail and back and if a mother pangolin believes her offspring to be in danger or threatened, she will curl her tail around her young, creating a protective shell. Mother pangolins nurse their offspring from the mammary glands until they are three to four months old. Once they have reached this age, they begin to eat ants, termites and all other insects found within the pangolin diet. At two years old, when the pangolins have reached sexual maturity, the mother pangolins will finally abandon their young [17]. |
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