Asexual and Sexual Reproduction Study Guide
Any living organism has a limited life span, but life itself goes on because organisms can create offspring through reproduction. Some organisms can do this without exchanging or combining genetic information with a partner. Thus, all of their offspring will be genetically identical to the parent or clones. Asexual reproduction occurs in all the Kingdoms (Bacteria, Protists, Fungi, Plants, and Animals). Sexual reproduction is when genetic material from one parent is combined with the genetic information from another to yield offspring. These offspring are not identical to either parent. Each parent produces a specialized cell called a gamete that contains half of his or her genetic information. The union of these gametes is called fertilization.
All Kingdoms of organisms have representatives that engage in asexual reproduction. Asexual reproduction is an advantage because you do not need to find a partner, and usually, you can reproduce in large numbers very quickly. Sometimes, asexual reproduction is known as vegetative reproduction. Organisms that engage in asexual reproduction also usually engage in sexual reproduction with a partner, at least part of the time.
In single-celled organisms such as bacteria and protists, asexual reproduction happens through a process known as binary fission (or bipartition) in which the cell duplicates certain parts of itself and then splits into two separate but identical cells. Bacteria can do this very fast, even as fast as just 15 or 20 minutes, which is why their populations can increase very quickly. Fungi and some animals, such as coral, reproduce asexually by a process of budding in which an offshoot of their body develops into a complete individual. Multicellular invertebrates such as sea stars can also reproduce asexually by fragmentation in which a portion of the organism's body is separated (as in when a sea star loses a limb) and then grows into a whole organism while the original body repairs itself as well. Plants can reproduce asexually by budding or fragmentation when they form tubers, rhizomes, bulbs, and other extensions of their bodies. Plants (and some animals such as jellyfish) also have a major sexual phase of their life cycle, which is part of a process called alternation of generations.
Alternation of Generations and Reproduction in Plants
Although asexual reproduction allows plants to reproduce quickly and colonize a large area, most plants still engage in sexual reproduction at least part of the time. A sexually reproducing plant cycles between two distinctly different body types. The first is called the sporophyte and the second is called the gametophyte. The word "phyte" refers to plant, whereas the prefix "sporo" refers to spores, and the prefix "gameto" refers to gametes (the eggs and sperm).
An adult sporophyte (the form of the plant you are most familiar with), such as a fern, will produce spores (the brown spots on the underside of a fern leaf, for instance). These spores are not gametes because they do not join like an egg and sperm do. Instead, they will be ejected and carried by wind or water to suitable habitats where they will sprout into a gametophyte form of the plant (usually small and not familiar). This gametophyte produces the eggs and sperm that can then join to form a new sporophyte.
Switching from the sporophyte form to the gametophyte form represents the concept of the alternation of generations. This happens in nonflowering plants (like the fern) and also flowering plants, although here it is less noticeable, and the gametophyte generation is very small and dependent on the sporophyte generation. An oak tree, for instance, is really the sporophyte generation of the plant, while the gametophyte generation is contained within the flowers, which are themselves inconspicuous.