Sexual Reproduction - The most successful kinds of plants and animals are those that have developed a method of shuffling and exchanging genetic information. This usually involves organisms that have two sets of genetic data, one inherited from each parent. Sexual reproduction is the formation of a new individual by the union of two sex cells. Before sexual reproduction can occur, the two sets of genetic information must be reduced to one set. This is somewhat similar to shuffling a deck of cards and dealing out hands; the shuffling and dealing assure that each hand will be different. An organism with two sets of chromosomes can produce many combinations of chromosomes when it produces sex cells, just as many different hands can be dealt from one pack of cards. When one of these sex cells unites with another, a new organism containing two sets of genetic information is formed. This new organism’s genetic information might very well have survival advantages over the information found in either parent; this is the value of sexual reproduction.
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| Life Cycle |
In last study, we discussed the cell cycle and pointed out that it is a continuous process, without a beginning or an end. The process of mitosis followed by growth is important in the life cycle of any organism. Thus, the cell cycle is part of an organism’s life cycle.
The sex cells produced by male organisms are called sperm, and those produced by females are called eggs. A general term sometimes used to refer to either eggs or sperm is gamete (sex cell). The cellular process that is responsible for generating gametes is called gametogenesis. The uniting of an egg and sperm (gametes) is known as fertilization.
In many organisms the zygote, which results from the union of an egg and a sperm, divides repeatedly by mitosis to form the complete organism. Notice in figure 9.1 that the zygote and its descendants have two sets of chromosomes. However, the male gamete and the female gamete each contain only one set of chromosomes. These sex cells are said to be haploid. The haploid number of chromosomes is noted as n. A zygote contains two sets and is said to be diploid. The diploid number of chromosomes is noted as 2n (n + n = 2n). Diploid cells have two sets of chromosomes, one set from each parent. Remember, a chromosome is composed of two chromatids, each containing double-stranded DNA. These two chromatids are attached to each other at a point called the centromere. In a diploid nucleus, the chromosomes occur as homologous chromosomes a pair of chromosomes in a diploid cell that contain similar genes throughout their length. One of the chromosomes of a homologous pair was donated by the father, the other by the mother.
Different species of organisms vary in the number of chromosomes they contain. lists several different organisms and their haploid and diploid chromosome numbers. It is necessary for organisms that reproduce sexually to form gametes having only one set of chromosomes. If gametes contained two sets of chromosomes, the zygote resulting from their union would have four sets of chromosomes. The number of chromosomes would continue to double with each new generation, which could result in the extinction of the species. However, this does not usually happen; the number of chromosomes remains constant generation after generation. Because cell division by mitosis and cytokinesis results in cells that have the same number of chromosomes as the parent cell, two questions arise: how are sperm and egg cells formed, and how do they get only half the chromosomes of the diploid cell? The answers lie in the process of meiosis, the specialized pair of cell divisions that reduce the chromosome number from diploid (2n) to haploid (n). One of the major functions of meiosis is to produce cells that have one set of genetic information. Therefore, when fertilization occurs, the zygote will have two sets of chromosomes, as did each parent.
Not every cell goes through the process of meiosis. Only specialized organs are capable of producing haploid cells. In animals, the organs in which meiosis occurs are called gonads. The female gonads that produce eggs are called ovaries. The male gonads that produce sperm are called testes. Organs that produce gametes are also found in algae and plants. Some of these are very simple. In algae such as Spirogyra, individual cells become specialized for gamete production. In plants, the structures are very complex. In flowering plants, the pistil produces eggs or ova, and the anther produces pollen, which contains sperm.\
Source: Enger−Ross: Concepts in Biology, Tenth Edition The McGraw−Hill Companies, 2002
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