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Eukaryotic cells reproduce through mitosis, and sex cells are produced through meiosis. This lesson will feature reproduction of organisms and how you developed from a single cell into the complex organism you are today. All living things, from the smallest bacteria to the biggest and most complex animals, reproduce. All pass genetic information in the form of DNA. Simple prokaryotic organisms only reproduce asexually. They do so by a process called binary fission . Most animals reproduce sexually, but there are exceptions. Among eukaryotes, there are many modes of reproduction, both asexual and sexual. Asexual Reproduction Asexual reproduction usually involves production of genetically identical offspring from a single parent. Meiosis and fusion of two genetically unique gametes to form a unique offspring characterize sexual reproduction. There are many different forms of asexual reproduction. For example, taking a cutting from a plant and rooting it is a form of asexual reproduction, because a single parent is used to produce a clonal offspring. A clone is simply a genetically identical copy of an organism and can be anything from a single cell to a giant tree. It all comes down to asexual reproduction. Usually asexual reproduction involves an exact copy of DNA and production of clonal offspring, but asexual reproduction does not always produce Many plants can reproduce both sexually and asexually. When a plant is produced asexually from a cutting, the offspring is clonal and is produced from a single parent. The same plant might also reproduce sexually. Sexual reproduction in most plants occurs through pollination. Self-pollination of a flower is a form of sexual reproduction, because the flower has both male and female reproductive structures. Each structure produces unique gametes that fuse via fertilization to produce a unique offspring. Click on theimage toenlarge. However, because those offspring are produced via fusion of gametes and are unique, it is still sexual reproduction. Some animals can do the same thing. Some species of lizards are all females and have eggs that divide by mitosis. Bees also commonly reproduce asexually. Let's look at the most common forms of asexual reproduction. Forms of asexual reproduction include: • • • • • mitosis and cytokinesis binary fission budding fragmentation and regeneration parthenogenesis and apomixis Mitosis, followed by cytokinesis, occurs in many single celled eukaryotes. This is the way human cells reproduce to allow growth and repairs. In this type of cell division, DNA replication is followed by mitosis, which is nuclear division. Cytokinesis is division of cytoplasm and its organelles. The nuclear membrane reforms in the clonal daughter cells. In the case of unicellular eukaryotes, the daughter cells are new organisms. Binary Fission Binary fission is simpler than mitosis and cytokinesis. As in all forms of reproduction, DNA must first be replicated. Bacteria have a single ring chromosome, and their DNA is in a ring form. Most unicellular eukaryotes are protists. Protists are a very diverse group. While many reproduce asexually via mitosis, others reproduce the same way as prokaryotes. Prokaryotes reproduce asexually by a process called binary fission . Following replication, the two rings of DNA attach to adjacent sites on the cell membrane. The cell stretches between the bound rings of DNA by adding phospholipids. Eventually, enough membrane is produced to divide the cell in half. This is a form of cytokinesis. Two clonal daughter cells Budding Budding is a form of asexual reproduction that occurs in eukaryotes. Bacteria reproduce very quickly. The process of binary fission takes only a few hours. Mutation often occurs during DNA replication, so offspring are not always clonal. Mutation is the main source of genetic variation in organisms that reproduce asexually. In yeast, a kind of unicellular fungus, DNA replication and mitosis occur, dividing the nucleus. After that, the new nucleus begins to bulge from the cell membrane. Eventually, the bulged portion which contains the nucleus and other necessary cellular elements buds off from the original cell. In hydra (a microscopic marine animal) cell division and differentiation produce a new, tiny copy of the parent organism. The offspring actually grows on the side of the parent, eventually buds off, and floats away. Even though the offspring is a multicellular organism, this is still a form of asexual reproduction because the offspring is genetically identical to the parent that produced it. Regeneration and Fragmentation Most plants and some animals are capable of regeneration. This is due to totipotency of cells, the ability of some cells to develop into any type of cell or tissue in the organism's body. Totipotent cells can also develop into a Fragmentation often comes before regeneration. It involves the parent organism breaking into pieces. Afterward, each fragment grows into a new individual. Many plants such as cacti break apart easily. In cacti, spines (hard and solid leaves) serve as a method of transporting fragments far away from the parent body. This is common in animals called planarians. Planarians are simple flatworms that live in wet environments. Each piece can produce roots and grow into a whole new plant. Organisms such as starfish can also regenerate broken segments. This process usually does not create a whole new organism, but reproduces parts of an organism by mitosis. Although not many plants fragment spontaneously, most are capable of regeneration. Asexual propagation (another word for reproduction) is a common way of growing plants. In fact, most of the plants that can be bought at a local nursery or garden center are propagated asexually and not from seeds. Seeds are the product of sexual reproduction. Most organisms that reproduce sexually must reduce their number of chromosomes by half via meiosis. In general, meiosis and genetically unique offspring characterize sexual reproduction. Asexual reproduction does not involve meiosis and almost always produces genetically identical offspring. However, there are some organisms that don't fit any clear definitions of sexual versus asexual reproduction. Parthenogenesis and Apomixis Sometimes two haploid cells fuse to form a unique diploid organism, while at other times, the whole organism is always haploid or diploid. In some cases, there are no males, only females, which leaves an unclear line between sexual and asexual reproduction. Parthenogenesis and apomixis involve production of offspring from gametes, but not through regular fertilization. Sometimes the process may be: • asexual • sexual • clonal In bees, fertilized eggs become females. Unfertilized eggs become males. Most of the females are sterile. The queen bee is fertile and produces haploid gametes by meiosis. The males produce haploid Clearly the female offspring are produced by sexual reproduction, but there are many questions regarding the process by which male bees are There are lizards that are all female. They are also all diploid and do not produce unique haploid cells. Their young come from eggs, but the offspring are genetically identical to the parents. This is clearly asexual reproduction and is called parthenogenesis. A similar process called apomixis occurs in plants. Haploid eggs develop into haploid seeds and haploid adults. In other cases, two haploid cells produced by the same parent cell fuse to produce a diploid offspring. The offspring are not clonal. As you can see, there are exceptions to every rule. Life Cycles For organisms that definitely reproduce sexually, meiosis occurs to produce haploid gametes at some point during their life cycle. Sexual reproducers have one of three life distinct life Click on the image to enlarge. Zygotic Life Cycle: The zygote is the only diploid form of the organism. A zygote is the cell produced when two haploid gametes fuse to form a diploid nucleus. In fungi, the haploid cells are not male or female. Sometimes they are labeled + and -. In some, the diploid cells are called (n + n) cells. The main thing to remember is this: The zygote never reproduces via mitosis. It quickly undergoes meiosis to produce haploid cells. The haploid cells then undergo mitosis to produce a dominant haploid form. All sexually reproducing fungi and some algae have zygotic life Sporic Life Cycle: This is a true alternation of generations, which means that both the diploid and haploid forms undergo mitosis to produce multicellular forms. Some algae and all plants have sporic life cycles. In algae and primitive non-vascular plants such as mosses, the haploid form is dominant. That means that the biggest and longest-lived form has half the number of chromosomes. The smaller and short-lived reproductive Click on the image toenlarge. In seed plants, the diploid form is dominant. The plants that we are most familiar with are seed plants. In fact, most of the plants in the world today are seed plants. The diploid form cannot live independently of the haploid form. It is simply a reproductive structure. In ferns, both the diploid and haploid forms are free living. That means they do not have to be attached to a more dominant form to survive. The diploid form is dominant, but it actually grows out of the haploid form. The diploid form produces haploid spores via meiosis. The spores are released and undergo mitosis to produce a free-living haploid form. The haploid form produces sperm and eggs, which unite to produce a zygote. The zygote undergoes mitosis on the haploid form In these plants, the diploid form is dominant. The haploid form cannot live on its own. Usually the haploid forms contain only a few cells. In flowering plants, both male and female gametes are produced in structures on flowers. The sperm and egg are single cells, but they are produced by tiny multicellular structures that are haploid. When a sperm fertilizes an egg, a zygote is produced. It will develop into a diploid seed. The diploid seed will grow into a diploid plant. Click on the image to enlarge. Gametic Life Cycle: The gamete is the only haploid form of the organism, and it is always unicellular. All animals that reproduce sexually, including humans, have gametic life cycles. Click on the image to enlarge. The main difference between sexual and asexual reproduction concerns whether or not genetic variation is introduced. The diversity of life on earth is thought to be the result of billions of years of change in the genetic code of organisms acted on by natural selection. In prokaryotes, almost all genetic variation is the result of mutation, conjugation, transformation, or transduction. You are a diploid organism. Shortly after fertilization, the zygote produced began to reproduce via mitosis. Never again would you be haploid. You do produce haploid sex cells by meiosis. There are no multicellular haploid forms in humans or other animals. Human gametes cannot survive for long outside the body. Overall, you can tell what kind of life cycle a sexually reproducing organism has by knowing when mitosis occurs. In zygotic life cycles, only haploid cells undergo mitosis. In gametic life cycles, only diploid cells do. In sporic life cycles, both do. Conjugation Bacteria often contain a bit of extra DNA called a plasmid, a small ring that carries extra genetic information that is not vital to life. It helps bacteria perform functions such as resisting antibiotics. Antibiotics are the drugs doctors often prescribe to stop bacterial infections. Some bacteria have learned how to fight those drugs. The information for how to do that is carried in plasmids. Bacteria can actually exchange plasmids by producing a conjugation tube. Even though bacteria always reproduce asexually, there is sometimes a way they join together to Conjugation is not really a form of reproduction, because no new organism is produced. It is simply a Some simple eukaryotes such as protists also exchange genetic information via conjugation. No new organisms are formed, only new combinations of genetic information. Even more are present in environments such as soil. Exchange of genetic information by transformation is thought to occur often. Transduction Transformation A living bacterium can actually get genetic information from a dead one. Transformation is the process by which bacteria can take up fragments of DNA from a surrounding environment. Many different species of bacteria usually live together. For instance, there are trillions of different bacterial species living in your Mutation As with all organisms, mutation can occur during DNA replication. Because DNA must always replicate before cells divide, this is the ultimate source of genetic variation in living things. For two billion years, when only prokaryotes existed, this was the way that evolution happened. Once sexual reproduction evolved, diversity of life increased dramatically. Sexual reproduction remarkably speeds up genetic variation. Transduction occurs when a virus called a bacteriophage puts DNA into a bacterial cell. Remember that viruses are not living things and are simply DNA or RNA surrounded by protein. The genetic material that viruses put into hosts is often taken into the host's DNA. This is why viruses cause disease. When gametes are produced via meiosis in sexually reproducing organisms, genetic variation is introduced in several ways. Crossing over of homologous chromosomes during prophase I leads to genetic recombination. This produces new chromosomes that have never existed before. Independent assortment of homologous chromosomes during anaphase I produces many unique haploid gametes. Fusion of gametes via fertilization creates even more variety. Asexual vs. Sexual Asexual reproduction is favorable for some organisms in several ways. It allows very fast production of many genetically identical offspring and it does not require a lot of energy. These are excellent characteristics to possess if the organisms in question are well adapted to a relatively stable environment. There is little need to introduce variation if a current system works. Asexual reproduction is also helpful for some plants and animals under stressful conditions, when mates or Think about it. Sometimes a deadly disease will be introduced into a population or organisms. If all members are genetically identical, the disease will affect all of them equally. With genetic variety, some members are more likely to survive. Genetic variation is the core of evolution by natural selection. It is thought to have produced the marvelously adapted organisms on earth today. Sexual reproduction introduces genetic diversity, a characteristic that is usually better for a species. Diverse populations are better able to handle environmental changes, and it is less likely that all members of a population will die when changes do occur. Embryonic Development - Differentiation Earlier you learned about totipotent cells called stem cells. You also learned that five to seven days later a process called differentiation begins. Let's take a closer look at what happens once stem cells begin to differentiate in vertebrate animals (animals with backbones) like humans. The mass of cells produced after fertilization and several days of division is called a morula. It contains about thirty-two cells and is actually no bigger than the zygote. The morula contains small cells tightly packed together. These cells continue to divide, but at this point, different genes get switched on and off. This leads to secretion of fluids and formation of a hollow space in the midst of the cell mass. Eventually, a hollow ball of around five hundred to two thousand cells is formed. This happens around ten days after fertilization. Inside, a mass of cells at one end called a blastocyst forms. It is destined to produce the new offspring. The rest of the mass will form the protective sac in which the offspring will develop. Next a process called gastrulation occurs. Certain groups of cells move inward within the hollow ball of cells. In this process, the blastocyst will differentiate into three distinctive germ (seed) layers. They are called the endoderm, the mesoderm, and the ectoderm. Remember thatendo means in. Ecto or exo means out. Meso means middle. The germ layers are named for the kinds of tissue offspring will develop into. After about three weeks, a process called neurulation begins. During this stage, the rest of the body plan is determined. From this point on, the offspring is destined for its final form. Eventually, the endoderm will develop into the linings of the digestive and respiratory systems. The ectoderm will develop into the outer skin and the nervous system. The mesoderm will develop into the bones, muscles, and outer parts of most of the body's Further development and growth occur until a new offspring is born. In most animals, development continues long after birth. You are very different from the baby you once were. As you grow older, you will be very different from what you are now. These changes are called development, and they occur throughout life. View the fetal development chart on As you can see, very early on some important things happen. This is why stem cells are so versatile. Once the process of differentiation begins, there is no turning back. Only stem cells have the ability to produce any part of the body. Once they change, a one-way process is set into motion. A single cell develops into an embryo, a fetus, a newborn, a child, and an adult. Even adults change. An old person is different from a young one. This is true for almost all living things. This lesson began with dividing cells and ends with old age, an interesting journey.