Cellular Reproduction Cellular Reproduction is the process by which all living things produce new organisms similar or identical to themselves. This is essential in that if a species were not able to reproduce, that species would quickly become extinct. Always, reproduction consists of a basic pattern: the conversion by a parent organism of raw materials into offspring or cells that will later develop into offspring. (Encarta, 2) In almost all animal organisms, reproduction occurs during or after the period of maximum growth. (Fichter, 16). But in Plants, which continue to grow through out their lifetime, therefore making the process more complex. Plants' reproduction is usually caused by a stimulant, mostly environmental or growth factors. The reproductive process, whether asexual or sexual always involves an exchange in hereditary material from the parent(s) so that the new organism may also be able to reproduce. Reproductive processes can be categorized in many diffrent ways although the most common is to put them into either asexual reproduction or sexual reproduction. Asexual reproduction is the process by which a single organism gives rise to two or more daughter cells. Most single celled organisms reproduce by the asexual process known as fission, which is commonly called mitosis. Fission (or Mitosis) is the division of one cell into two identical daughter cells. Interphase, the first phase of the cell cycle and also the phase before mitosis, starts as soon as the cell is born. Interphase is broken up into three phases, G1, S, and G2. During the G1 phase, the cell increases in mass except for the chromosomes, which stay the same, uncoiled. Protein synthesis is also occurring rapidly in this phase. If a cell doesn't divide further, it remains permanently in the G1 phase. Next is the S phase, in which the mass of the cell continues to increase, and DNA is duplicated, and then the chromosomes divide to form identical sister chromatids attached by a centromere. (Harold, 45). During the G2 phase of Interphase, the cell becomes double its mass at birth, the chromosomes begin to shorten and coil, and the centrioles appear, the cell is now ready to enter into mitosis. In the first and longest phase of mitosis, prophase, the chromosomes become visible and the centrioles split in half and then move to opposite sides of the cell. At this point chromosomes have formed into sister chromatids separated by a structure called a centromere. Spindle fibers are barely visible. This phase takes fifty to sixty percent of the time of mitosis. (Biggs, 214) During metaphase, the short second phase of mitosis, the sister chromatids line up along the equator of the cell. Each sister chromatid has its own spindle fiber. Next comes the third phase of mitosis, Anaphase, in which the centromeres break in half, causing each of the daughter chromatids to start to be pulled to different poles of the cell by means of spindle fibers. The spindle fibers pull the daughter chromatids until they reach the poles of the cell. Finally, the last phase of mitosis called Telophase in which the daughter chromatids are fully pulled toward the poles and the nuclear envelope begins to
reappear, the cell also begins to cleave, usually in the spot where the chromatids lined up in metaphase. After mitosis is complete, cytokinesis occurs, cytokinesis is the process in which a cell’s cytoplasm is split into two equal parts, therefore causing the mother cell to split into two daughter cells. In plants, however, cytokinesis is replaced by the placing of a cell plate on the metaphase “equator” Mitosis is a process that guarantees better cellular work. The two new daughter cells will carry out the same functions and cellular processes as the parent, ensuring that the two cells will carry ...
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reappear, the cell also begins to cleave, usually in the spot where the chromatids lined up in metaphase. After mitosis is complete, cytokinesis occurs, cytokinesis is the process in which a cell’s cytoplasm is split into two equal parts, therefore causing the mother cell to split into two daughter cells. In plants, however, cytokinesis is replaced by the placing of a cell plate on the metaphase “equator” Mitosis is a process that guarantees better cellular work. The two new daughter cells will carry out the same functions and cellular processes as the parent, ensuring that the two cells will carry out twice as much as the original cell. Mitosis often occurs in multicellular animals and plants, not making the organism reproduce, but rather increasing the cells in a certain tissue which indirectly helps the organism. Fission is not the only form of asexual reproduction, there are many others. Budding is one method of asexual reproduction in which a group of self-supporting cells sprout from, than detach from the parent organism. Unlike most other spores, eggs, etc. buds are usually multi-cellular and contain more than one cell layer. Hydras and sea squirts are some organisms that reproduce by budding. (Encarta, 1). Vegetative reproduction is much like budding, in that the certain parts of a plant eventually root and sprout (Example: tubers of potatoes). Parthenogenesis is a form of asexual reproduction in which new individual organisms are formed from unfertilized eggs. This normally occurs in some insects, amphibians, reptiles, and birds. Sporulation is another way some organisms asexually reproduce. A spore is a specially modified cell, similar to a small seed. Spores are usually produced by the division of cells within a structure called a sporophagnum. (Harold, 42). Regeneration is a specialized form of reproduction. By regeneration some organisms have the ability to replace an injured or lost appendage. Many plants can grow another entire individual from a small part of itself, such as a root, stem, leaf, or even something as small as a piece of an organ. Using this knowledge, in 1958 F.C. Steward founded the science of horticulture. Regeneration can only be a form of reproduction if it is full reproduction, meaning that that organism can produce another organism with a single body part. No vertebrates are capable of this, although it can be done synthetically by cloning. Asexual reproduction is often a second means of multiplying in organisms that ordinarily reproduce sexually. Sometimes that is the only way for that organism to reproduce. Asexual reproduction demands less energy and time then would sexual reproduction and is better suited to a harsh environment. This is why bacteria grow so easily and so numerous. Some species reproduce both asexually and sexually depending on which one the conditions are more suitable for. (Encarta, 1) Organisms that reproduce asexually are particularly well suited for establishing entire populations. This is the case for many weeds and pests. The offspring of organisms that reproduce asexually are identical to their parents and to each other. That means they cannot mix genes, and therefore don’t adapt or evolve as quick as other organisms do. (Encarta, 1). Sexual Reproduction is the primary means of reproduction for most multicellular animals and plants. For higher invertebrates and all vertebrates, except the few that reproduce by Parthenogenesis, sexual reproduction is the way they must reproduce. Sexual reproduction is essentially cellular in nature, meaning that it involves the fertilization of one sex cell by another, producing a newly fertilized cell called a zygote, which then develops into a new organism. In some lower forms, the union of two isogametes (gametes that are the same structurally but not physiologically) occurs, and is called isogamy or conjuction. The fusion of two clearly different kind of gametes is called heterogamy.(Encarta 4) Sexual reproduction is very significant, in that because of the fusion of two entirely different nuclei, the offspring could inherit an endless varied combination of characteristics, which may help improve the species. This is why organisms which reproduce sexually adapt quicker than do asexually reproducers. Multicellular plants alternate sexually and asexually reproducing generations. First, the plant starts as a gametophyte, which produces gametes. Then, it undergoes fertilization, and then mitosis, forming a sporophyte, which produces spores. When the sporophyte is ready to reproduce, it undergoes meiosis and produces spores. These form a gametophyte, and the cycle starts again and again until the plant dies. Pollination is the transfer of pollen from the male reproductive organ called the stamen, to the female reproductive organ, called the pistil. Pollination is not fertilization, which comes much later. The most common “helpers” with pollination are flying insects, which carry the pollen on their underside to the pistil, and the wind, which blows the pollen around in hopes that it might reach another plant. Since most ofd these plants have both male and female parts, the season in which the stamens release pollen is earlier or later to when the pistil is open to fertilization, preventing the plant from fertilizing itself. These plants have many ways to attract pollinators, some, while the pollinator (an insect) is landing, shoot up their stamens so some of the pollen could get onto the insect and hopefully pollinate another plant. Another strategy for pollinating is to have the flower in such a way that the insect goes in one way, and has to brush up against the pistil and the stamens before getting out. Some plants release their pollen by the wind. An allergy sufferers nightmare. Gregor Mendel was the first scientist who studied heredity, the passing on of characteristics from parents to offspring. This was the first major study of genetics- the branch of biology that studies heredity. (Biggs, 259) Mendel was the first in predicting how traits would be passed on through generations. Traits are characteristics that are inherited from one’s parents. Mendel chose to do his experiment with peas, because they have distinct sex cells, called gametes. The unison of the male and female gametes is called fertilization. Mendel’s first study was to breed tall and short pea plants and to study the hybrids. Hybrids are offspring of parents that have different forms of a trait, such as in Mendel’s experiment: height. His results for this experiment were that all the plants grew as tall as the first. This was in the first generation or F1. The parents of the F1’s are called P1. And F2’s come from F1’s. Mendel let the second generation (F1) pollinate normally, and then studied the third generation (F2). He noticed that one in every four plants was as short as the short P1 plant. Mendel did more tests with six more pairs of traits, and got the same result. These were Mendel’s monohybrid crosses. Based on these observations, Mendel made three rules. The rule of unit factors, the rule of dominance, and the law of segregation. The rule of unit factors states that each organism has two factors that control each of its traits. Genes exist in different forms, these forms, called alleles determine different things. In Mendel’s case, the height of the pea plant. The rule of dominance states that some traits are dominant, while others are recessive. As in the case with the height, the tall allele was obviously dominant, because if you mixed a tall allele with a short one, the plant would always come out tall. The only way for a recessive trait to affect the organism would be to have two recessive alleles. Meaning, a recessive trait could only be brought out by having two of the same alleles. The law of segregation states that because an organism has two different alleles, it can produce two different gametes. During fertilization, these gametes randomly pair up to produce four different zygotes, each with its own set of alleles. Mendel then made up the idea for phenotypes and genotypes. A phenotype is how an organism looks and behaves. A genotype is an organism’s gene combination. If an organism has two alleles that are the same, it is said to be homozygous. Mendel performed another set of crosses known as dihybrid crosses. In this set of experiments he tested the seed of the pea plant. He took round, yellow (both dominant) pea plants and breeded them with green, wrinkled (both recessive) pea plants. Not surprisingly, the F1 offspring were round and yellow, but the F2 plants showed something new: there was one of every kind. They was a round yellow, round green, wrinkled yellow and wrinkled green pea. This formed Mendel’s second law: the law of independent assortment. This law states that genes for different traits and inherited independently of each other (Biggs, 256-266) Meiosis is the process by which haploid gametes are formed. Haploid means half the diploid number of chromosomes. The diploid number is the number of chromosomes found in each cell of an organisms body. Gametes must be haploid number or else the organism would be born with twice as many chromosomes. If that happened something bad would happen, probably a mutation. Homologous chromosomes are the pair of chromosomes that shape an organisms appearance. They are in no way the same, but must share the same characteristics When cells divide by fission (mitosis) the new cells have the same number as the first cell. If this happened with meiosis, we would have twice as many chromosomes as our parents. Meiosis only occurs in the specialized reproductive cells of each parent. Meiosis consists of two separate divisions, known as meiosis I and meiosis II. Meiosis I begins with one normaly a centromere. During prophase I the chromosomes coil up and the spindle forms. But then each pair of homologous comes together, forming a four part structure called a tetrad. A tetrad is made of two homologous chromosomes, which are each made up of two sister chromatids. During this phase, crossing over, also known as genetic recombination occurs. When this happens, genetic material is exchanged between chromosomes. Kind of like switching the same shape piece to a different puzzle. As metaphase I begins, the centromere of each sister chromatid attaches to a spindle fiber. The spindle fibers pull the chromatids to the center of the spindle, and the homologous chromosomes line up side by side as tetrads. (Biggs, 275) Anaphase I begins with the homologous chromosomes separate and are pulled to opposite sides of the cell. However, the centromeres do not split, ensuring that the gamete will get only chromosome from each homologous pair. Telophase I breaks down everything in the reverse order that it happened,first the spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells. Even though, each cell has the diploid amount of genetic information, another division is needed because each chromosome is still doubled. In meiosis II, some cells have a short interphase in which the chromosomes do not replicate, in some other organisms, the cells go from late anaphase of meiosis I, to metaphase II, skipping telophase I, the short interphase, and prophase II. During Prophase II, the spindle forms in both of the newly born cells and the spindle fibers attach to the chromosomes. During metaphase II the chromosomes are yet again pulled to the “equator” of the cell. Anaphase II begins as soon as the centromere of each sister chromatid splits allowing them to move towards their poles. In telophase II, the nuclei reform, the spindles break down, and cytokinesis occurs. The end product of meiosis is very similar to Mendel’s experiment in that there are sixteen different combinations for a zygote. Although meiosis usually goes ok, sometimes there is a mistake and chromosomes don’t seperate correctly. This is called nondisjunction. There are three types of nondisjunction, Trisomy- when a gamete with an extra chromosome is fertiized with a normal gamete. Monosomy- when a gamete with one chromosome is missing and is then fertilized by normal gamete. And Trioloidy- where both zygotes have an extra chromosome. Reproduction is as essential to a species as food, water, or shelter. If a species cannot reproduce anymore, that species will eventually become extinct. There are many means of reproduction but primarily only two: asexual and sexual. In asexual reproduction, One organism gives a part or its whole self, in order to give rise to two or more new organisms. During sexual reproduction, two parents each form sex cells, which unite, and eventually form a new individual. Bibliography Works cited Biggs, Alton & others. Biology: The Dynamics of Life. Glencoe McGraw-Hill.: New York, NY, 2000 Encarta Encyclopedia. CD-ROM. 1998 Fichter, Sheila. Reproduction. Universal Press: Columbus, OH, 1994 Harold, Christopher. Cells: A Guided Tour. Simon and Schuster: New York, NY, 1987 O’Neil, Franklin. Cells. McMeel Publishing, Inc.: Atlanta, GA, 1990 Word Count: 2589
