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CHAPTER 28 REPRODUCTIVE SYSTEMS In the reproductive systems: Gonads--ovaries or testes--produce gametes (eggs [ova] or sperm)--also secrete hormones Accessory sex organs--produce materials that support gametes To understand the major function of the reproductive system, production of gametes, we must first understand a process of nuclear division that differs from the process of mitosis. In mitosis, the goal is to produce daughter cells identical to the parent cell, with exactly the same number and kind of chromosomes. This is not suitable for the production of gametes, since gametes need to have half the regular number of chromosomes, one of each of the usual 23 pairs. Only in this way can the 2 gametes, which unite to form a new individual both contribute half of the genetic material, but still produce an individual with a total of 23 pairs of chromosomes. Remember, human somatic cells each contain 23 pairs of chromosomes (46 total). One chromosome of each pair came from each parent. The 2 chromosomes of a pair are called homologous chromosomes. Homologous pairs 1 - 22 are called autosomes. The 2 members of all of these pairs look the same and have similar genes. / The 23rd pair are the sex chromosomes. Females have 2 X chromosomes for this pair; males have one X and one Y. Somatic cells are all cells of the body except those that produce eggs and sperm. Somatic cells with their 23 pairs of chromosomes are said to be diploid. Their number of chromosomes is represented as the 2n number. In sexual reproduction, 2 gametes come together to create a new individual. Each gamete contributes one- half of the genetic material. Since the total number of chromosomes must be 46, each gamete must carry only 23 total chromosomes. The process of nuclear division that reduces the chromosomes in a gamete to half the normal number, or one chromosome of each homologous pair, is called meiosis. Cells produced by meiosis are called haploid cells, with the n number of chromosomes. The process of meiosis is similar in some ways to mitosis. However, meiosis involves 2 divisions, instead of 1, and results in haploid cells whose genetic content is NOT identical to the parent cell. The process begins with replication of chromosomes (same as mitosis). An identical copy of each chromosome is made. The cell builds a mitotic apparatus to organize division of the chromosomes. In both meiotic divisions, the same 4 phases occur: Prophase
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Metaphase Anaphase Telophase Meiosis I---reduction division---begins after chromosomes are replicated. Prophase I Each chromosome consists of 2 chromatids connected by a centromere. Duplicated chromosomes shorten and thicken, nucleolus and nuclear membrane disappear, mitotic apparatus is built (same events as mitosis). However, a very important difference exists in meiosis. During prophase I, homologous pairs of chromosomes come into close association with each other
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