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Reproduction Reproduction is the process by which organisms produce offspring of their own kind. Organisms can reproduce asexually or sexually. In asexual reproduction, only one parent is required. Sexual reproduction involves the union of two reproductive cells to form a new organism. A special kind of cell division produces these reproductive cells. Asexual Reproduction The production of offspring without the union of special sex cells is known as asexual reproduction. There is only one parent. Each of the offspring is an exact copy of its parents because it receives copies of the parent’s chromosomes. An organism that comes from one parent and is genetically identical to it is called a clone. Many organisms reproduce asexually. The process can occur in a number of different ways. The result, however, is always the same—one or more offspring genetically identical to the parent. The simplest form of asexual reproduction occurs when a unicellular organism splits into two and forms two new organisms. Bacteria, algae, and protozoa commonly reproduce this way. This type of reproduction is called fission. If the two resulting cells are of equal size, the process is called binary fission. The picture to the right shows a bacterium in undergoing binary fission. The growth of a small reproductive fragment called a bud, from a larger parent organism is called budding. In unicellular organism budding occurs when a parent cell forms a small bud or knob by means of unequal cell division. In this kind of fission, the nuclear material is evenly divided, but the cytoplasm is not. Thus, the bud has a complete set of genetic instructions but only a small amount of cytoplasm. The bud may separate from the parent cell, synthesize cytoplasm, and grow to be an exact copy of the parent cell. Budding in yeast cells is shown to the left. The arrows are pointing to just some of the budding yeast cells in the picture. In multicellular animals, buds form as outgrowths from the parent’s body. The cells in these buds divide, and the buds increase in size while still attached to the parent. Eventually they fall off the parent organism. They then develop into full-sized organism identical to their parents. The picture to the right shows a hydra budding. Multicellular organism may also reproduce by regeneration of body parts. This is commonly seen in the little worm called planaria. Planaria, as seen to the left, if cut into two, each of the two pieces has the ability to regenerate the missing parts. It will even work if you cut it into more than two pieces. The only requirement is that some of the worm’s primitive nervous system is contained in the piece and it will regenerate. Many organisms such as molds reproduce asexually by means of special reproductive cells called spores. A hard coat often protects spores. This enables them to survive unfavorable environmental conditions. Spores are set free from the parent and can form new organisms when conditions are right. The picture to the left shows the fungus that causes athlete’s foot. The spores are being pointed to. Asexual reproduction in plants is called vegetative propagation. In nature many plants reproduce themselves. Some plants, like strawberries pictured below and ferns have stems that run along the ground. These runners can take root and produce new plants. The eyes of potatoes will grow new potato plants when planted. Leaves and stems of many plants will form roots when placed in water or a rooting medium. Vegetative propagation is an important technique used in agriculture to make sure that the offspring of a desired plant will be exactly like the parent. Conjugation Sometimes, simple organisms undergo a mating process called conjugation. Organisms that undergo conjugation are bacteria, some protists, and some fungi. This process is not like mating in higher organisms that reproduce sexually. During conjugation, DNA is passed from one strain of organism to another strain of the same species. In bacteria, DNA is passed through pili that grow out of the donor bacterium and attach to the recipient bacteria, as seen in the picture to the right. In protists such as paramecia, conjugation occurs when two paramecia become joined at their oral grooves (where they take in food) by a bridge-like structure. DNA material is exchanged after a series of changes. In fungi such as bread mold, conjugation occurs when two different mating strains that look alike are close together. Their “rootlike” structures of attachment form short club shaped structures that grow toward each other. When they touch, the nuclei from each type fuse in pairs making a new organism. This can be seen in the picture to the left. Conjugation often results in the transfer of operating genes from one bacterium to another. Traits such as drug resistance in bacteria are transferred in this way. Conjugation provides genetic variations that could lead to an increased chance for species’ survival since it’s mixing up the genes in the population. Sexual Reproduction Sexual reproduction involves the fusion or joining of two special cells. These sexual reproductive cells are called gametes. The fusion of two gametes is a process known as fertilization. The cell produced by fertilization is called a zygote. The zygote develops into the new organism by means of mitotic cell divisions. Gametes, the special reproductive cells, are produced by a different kind of cell division. A gamete from the female parent is known as an ova or egg. The male parent produces gametes called sperm. Eggs and sperm differ from body cells in the number of chromosomes they contain. Gametes have only half the normal number of chromosomes. Cells with half the number of chromosomes are called haploid cells. All other cells that have the full number of chromosomes are said to be diploid. Body cells are all diploid. Diploid cells have to have an even number of chromosomes. Meiosis Not all cells reproduce by mitosis. A different type of cell reproduction is important to the passing on of traits to offspring. An Introduction to Meiosis Besides body cells, most living things also have sex cells. Sex cells, or to use their fancy name gametes, are special reproductive cells produced in sex organs. Sperm are sex cells made by the male. An egg is a sex cell made by the female. Another name for egg is ovum. Sex cells are made during meiosis. Meiosis is a kind of cell reproduction that forms eggs and sperm that have half of the chromosomes that the body cells do. To explain how a cell goes through meiosis, let’s look at a cell with four chromosomes. These four chromosomes make up 2 matching pairs as seen in the cell at the top of the picture below. In meiosis, a cell divides twice. When the cell divides the first time, each chromosome in a pair moves away from its partner. Each chromosome of a pair goes to a different cell. The sister chromatids stay joined together. The two cells then divide again. Look at the picture above. How many chromosomes are in each of the final four cells? The number of chromosomes in each cell is one-half of the original number. The original cell started with four chromosomes. Each chromosome had doubled to make the X looking thing. Each new cell has only two chromosomes, or exactly half of the original number. Lets look at the process of meiosis in more detail to see how the chromosome number was halved. The Steps of Meiosis The steps of meiosis have very familiar names since they are the same as in mitosis. The things that happen to the chromosomes are very similar as well. Because there are two cell divisions in meiosis, all of the steps that happen in mitosis happen then they happen again. It could get kind of confusing as to which step you are talking about so the first set of steps are called interphase I, prophase I, metaphase I, anaphase I, and telophase I. The second time the cell divides it uses the same steps but this time they are called interphase II, prophase II, metaphase II, anaphase II, and telophase II. Follow along in the diagram below as each step of meiosis is described. Just before meiosis begins, in interphase I, the chromosomes double just like they do in interphase of mitosis. Prophase I 1. Sister chromatids shorten and thicken. 2. The nuclear membrane begins to break down. 3. Centrioles begin to move away for one another and fiber form. 4. Matching chromosomes now come together to form pairs. 5. Matching pairs wrap around each other and trade pieces (crossing over). Note: steps 1-4 are the same as in prophase of mitosis. Step 5 is unique to meiosis. Metaphase I 1. The centrioles have moved to opposite ends of the cell. 2. Sister chromatids are lined up in the middle of the cell. 3. Sister chromatids become attached to the fibers. Note: Each chromosome is made up of sister chromatids, which are hooked together by a centromere. The pairs of chromosomes are not hooked together. Anaphase I 1. Fibers begin moving the matching chromosomes apart. The sister chromatids remain joined in this step. In mitosis they separate. Telophase I 1. The chromosomes are on opposite sides of the cell and the cell membrane pinches the cell into two. 2. The cytoplasm and organelles are divided. 3. Two new cells have formed. Each cell has two chromosomes (2 X’s). Interphase II 1. Usually very short. 2. Centrioles double. This is the beginning of the second division. The remaining steps happen in both of the new cells that were just made. The movement of the chromatids is the same as in mitosis. Prophase II 1. The centrioles move apart and fibers form between them. 2. Fibers attached to the middle of each of the sister chromatids. Metaphase II 1. The sister chromatids line up in the center of the cell. Anaphase II 1. Each chromatid in the chromosome begins to be pulled to the opposite side of the cell. Telophase II 1. Sister chromatids are on opposite sides of the cell. 2. Nuclear membrane forms around the chromosomes. 3. Cell membranes begin to pinch each cell in two along the center. The cytoplasm is divided between the cells. The four new cells are sex cells or gametes. Remember that in our cell we started with four chromosomes. We now have two chromosomes in each of four cells. In a normal cell, there may be many more chromosomes. But, at the end of meiosis, there will always be half the original number of chromosomes in each of four gametes. Sperm, Eggs and Fertilization The picture to the left shows a human egg and sperm cells. How are the cells alike? How are they different? The cells are alike in four ways. 1. Both are gametes. 2. Both formed during meiosis 3. Each as half the number of chromosomes found in body cells. 4. In humans, both cells begin to develop by meiosis at puberty. Puberty is the stage in life when a person begins to develop sex cells. It takes place between the ages of 10 and 15 most of the time. How are sperm and eggs different? As you can see from the picture on the previous page, eggs are much larger than sperm. Sperm have tails to help them swim, eggs do not. Sperm form in the testes, eggs form in the ovary. In animals, testes are male sex organs that produce sperm. In animals and some plants, ovaries are the female sex organs. During meiosis, each original male cell becomes four sperm just like in the diagram of meiosis. Meiosis in males occurs all the time form the beginning of puberty. Now look at how many eggs are formed in the diagram below. The main difference between male and female meiosis comes when the cells are dividing. The male cells divide equally. The female cells do not. Because the female cell has the responsibility for carrying all the food and organelles the new organism will need to live, the cell divisions cannot be equal. The female cell can’t keep the chromosomes so it has to throw them away but it doesn’t want to lose anything else. What happens is unequal cell division. One large cell is formed and one smaller cell. This smaller cell is called a polar body. Since the polar body doesn’t have much cytoplasm and no organelles, it soon dies. The large cell goes on to the second part of meiosis. When the large cell is ready to divide in telophase II, it divides unequally again. This makes a very nice large ovum and a very small polar body. The polar body does not develop and soon dies. The result is a nice big ovum that has almost all of the food and organelles it began meiosis with. An egg or ovum is formed once a month from the onset of puberty in females. In animals, when sperm and egg join, the chromosomes from each cell also come together. The new organism has a complete set of chromosomes in each one of its body cells. Half the chromosomes in the organism come from the father, and half come from the mother. Sexual reproduction in plants Flowering plants make our world beautiful to live in. Flowers come in a huge variety of colors, shapes and sizes. But, plants don’t make flowers for our pleasure. Flowers are important to the survival of the plant species. Without flowers, many plants wouldn’t be able to reproduce sexually. Flowers and sexual Reproduction In the diagram to the left, you can see a section through one kind of flower. Find the parts on the diagram as you read the statements describing them. In most flowers, some flower parts are showy. These parts are neither male nor female. In most flowers some parts are female and other parts are male. If you find male and female parts, the plant must reproduce sexually. Yes, plants have sex too. Flowers that have both male and female parts are called complete flowers. An example of a complete flower is a tulip. Some flowers have just female part or the male parts. These are called incomplete flowers. An example of an incomplete flower is a holly. There are male and female holly bushes. If you have just female bushes, you will never get any bright red holly berries on any of them. The gametes from the male bush’s flowers must fertilize the female gametes found in the female bush’s flowers. The sepals and petals are neither male nor female. The sepals are found around the base of the flower and are usually green, leaf like parts that protect the flower when it is a bud. The petals are more involved in the reproduction process. Petals are often brightly colored and scented parts of a flower. Their function is to attract something (an insect or bird) so it can help with the pollination. Both sepals and petals are special modified leaves. The male reproductive organ in the flower is called the stamen. Usually there are many stamens in each flower. Each stamen has a long stalk called a filament with a saclike part at its top. The saclike part is called the anther. The anther makes the pollen. Each grain of pollen contains male gametes (sperm cells). The female flower reproductive organ is called a pistil. Often there is only one pistil in the center of the flower. Each pistil has a large, round, ovary at its base. Ovules are small, round parts found in the ovary. The ovules contain the egg cells of a seed plant. Above the ovary is a stalk like structure called a style with a sticky end. The sticky end is called a stigma. The stigma is sticky so the pollen will stick to it when it get put there by the wind, an insect, or a bird. If you have ever looked closely at a flower, such as a dandelion, a rose, a carnation, or a daffodil, you may have noticed that none of these has the structure of our typical flower in the picture on the previous page. Some flowers don’t have sepals, some have petals that are fused to form a tube, some have many tiny flowers all clustered together in a head, and dome don’t have any stamens. Flowers, just like all other organisms, show an enormous range of variation. Pollination and Fertilization You know that in sexual reproduction, an egg and a sperm must join for fertilization. How does pollen get from the stamens of a plant to the pistil? Then how do sperm cells inside the pollen reach the eggs within the ovules? Pollen is transported from the stamens to pistils by a process called pollination. Pollination is the transfer of pollen from the male parts of a seed plant to the female plant. Sometime this is a relatively short trip, sometimes this is a greater distance. In flowering plants, the design of the flower is very important in pollination. Many flowers with colorful petals have a strong scent that attracts insects. Insects climb over the flowers and pick up the pollen on their bodies. Many insects, such as bees, collect pollen for food. Some flowers also make a sugary chemical called nectar that bees and birds also use as food. Pollen catches on the hairs and feathers of their bodies while they feed on the nectar. Some plants don’t have large, colorful petals or attractive scents. These plants usually rely on wind to move the pollen from one plant to another. Grasses and many trees have this type of flower. Many trees, such as maples and willows have long, hanging flowers that are yellow or green in color. Their flowers are not attractive to insects. The pollen formed in these flowers is blown around by the wind. So remember in the spring when your car is covered with yellow pollen, its really plant sperm. Pollen is often carried by insects, birds, or the wind between different plants of the same species. This is called crosspollination. Cross-pollination is when pollen from the stamen of one flower is carried to the pistil of another flower on a different plant. Self pollination is when pollen moves from the stamen of one flower to the pistil of the same flower or of another flower on the same plant. How does sperm inside a pollen grain reach the egg inside the ovary? Follow the steps in the figure to the left, as they are decribed. 1. A pollen grain lands on the sticky tip of the pistil. It must somehow reach the egg within the ovule. 2. The pollen grain begins to grow a tube into the stalk of the pistil. This pollen tube grows all the way down to the ovary. Inside each pollen grain are three nuclei. One nucleus leads the way down the tube. The other two nuclei are sperm nuclei and these follow the first nucleus. 3. The tube reaches the ovule. A sperm nucleus inside the tube can now pass into the ovule and join the ebb. Fertilization takes place. The other sperm from the pollen tube joins with another nucleus in the ovule to form a food supply for the new seed. 4. A new plant or embryo now grows inside the ovule. As the ovule grows, it becomes a seed.