A sperm and ovum fuse at fertilization to produce a diploid zygote.
Meiosis in animals The production of gametes in a male animal (spermatogenesis) takes place in the testes. There, diploid primordial germ cells divide mitotically to produce diploid cells called spermatogonia ( FIGURE 2.22a). Each spermatogonium can undergo repeated rounds of mitosis, giving rise to numerous additional spermatogo-nia. Alternatively, a spermatogonium can initiate meiosis and enter into prophase I. Now called a primary sperma-tocyte, the cell is still diploid because the homologous chromosomes have not yet separated. Each primary sper-matocyte completes meiosis I, giving rise to two haploid secondary spermatocytes that then undergo meiosis II, with each producing two haploid spermatids. Thus, each primary spermatocyte produces a total of four haploid spermatids, which mature and develop into sperm.
The production of gametes in the female (oogenesis) begins much like spermatogenesis. Diploid primordial germ cells within the ovary divide mitotically to produce oogonia
(FIGURE 2.22b). Like spermatogonia, oogonia can undergo repeated rounds of mitosis or they can enter into meiosis. Once in prophase I, these still-diploid cells are called primary oocytes. Each primary oocyte completes meiosis I and divides.
Here the process of oogenesis begins to differ from that of spermatogenesis. In oogenesis, cytokinesis is unequal: most of the cytoplasm is allocated to one of the two haploid cells, the secondary oocyte. The smaller cell, which contains half of the chromosomes but only a small part of the cytoplasm, is called the first polar body; it may or may not divide further. The secondary oocyte completes meiosis II, and again cytokinesis is unequal — most of the cytoplasm passes into one of the cells. The larger cell, which acquires most of the cytoplasm, is the ovum, the mature female gamete. The smaller cell is the second polar body. Only the ovum is capable of being fertilized, and the polar bodies usually disintegrate. Oogenesis, then, produces a single mature gamete from each primary oocyte.
in prokaryotic and eukaryotic cells; (2) the cell cycle and its genetic results; (3) meiosis, its genetic results, and how it differs from mitosis of the cell cycle; and (4) how meiosis fits into the reproductive cycles of plants and animals.
Several of the concepts presented in this chapter serve as an important foundation for topics in other chapters of this book. The fundamental differences in the organization of genetic material of prokaryotes and eukaryotes are important to keep in mind as we explore the molecular functioning of DNA. The presence of histone proteins in eukaryotes affects the way that DNA is copied (Chapter 12) and read (Chapter 13). The direct contact between DNA and cytoplasmic organelles in prokaryotes and the separation of DNA by the nuclear envelope in eukaryotes have important implications for gene regulation (Chapter 16) and the way that gene products are modified before they are translated into proteins (Chapter 14). The smaller amount of DNA per cell in prokaryotes also affects the organization of genes on chromosomes (Chapter 11).
A critical concept in this chapter is meiosis, which serves as the cellular basis of genetic crosses in most eu-karyotic organisms. It is the basis for the rules of inheritance presented in Chapters 3 through 6 and provides a foundation for almost all of the remaining chapters of this book.
• A prokaryotic cell possesses a simple structure, with no nuclear envelope and usually a single, circular chromosome. A eukaryotic cell possesses a more complex structure, with a nucleus and multiple linear chromosomes consisting of DNA complexed to histone proteins.
• Cell reproduction requires the copying of the genetic material, separation of the copies, and cell division.
• In a prokaryotic cell, the single chromosome replicates, and each copy attaches to the plasma membrane; growth of the plasma membrane separates the two copies, which is followed by cell division.
• In eukaryotic cells, reproduction is more complex than in prokaryotic cells, requiring mitosis and meiosis to ensure that a complete set of genetic information is transferred to each new cell.
• In eukaryotic cells, chromosomes are typically found in homologous pairs.
• Each functional chromosome consists of a centromere, a telomere, and multiple origins of replication. Centromeres are the points at which kinetochores assemble and to which microtubules attach. Telomeres are the stable ends of chromosomes. After a chromosome is copied, the two copies remain attached at the centromere, forming sister chromatids.
We have now examined the place of meiosis in the sexual cycle of two organisms, a flowering plant and a typical multicellular animal. These cycles are just two of the many variations found among eukaryotic organisms. Although the cellular events that produce reproductive cells in plants and animals differ in the number of cell divisions, the number of haploid gametes produced, and the relative size of the final products, the overall result is the same: meiosis gives rise to haploid, genetically variable cells that then fuse during fertilization to produce diploid progeny. __
In the testes, a diploid spermatogonium undergoes meiosis, producing a total of four haploid sperm cells. In the ovary, a diploid oogonium undergoes meiosis to produce a single large ovum and smaller polar bodies that normally disintegrate.
This chapter focused on the processes that bring about cell reproduction, the starting point of all genetics. We have examined four major concepts: (1) the differences that exist in the organization and packaging of genetic material
The cell cycle consists of the stages through which a eukaryotic cell passes between cell divisions. It consists of: (1) interphase, in which the cell grows and prepares for division and (2) M phase, in which nuclear and cell division take place. M phase consists of mitosis, the process of nuclear division, and cytokinesis, the division of the cytoplasm.
Interphase begins with G1, in which the cell grows and synthesizes proteins necessary for cell division, followed by S phase, during which the cell's DNA is replicated. The cell then enters G2, in which additional biochemical events necessary for cell division take place. Some cells exit G1 and enter a nondividing state called G0. M phase consists of prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. In these stages, the chromosomes contract, the nuclear membrane breaks down, and the spindle forms. The chromosomes line up in the center of the cell. Sister chromatids separate and become independent chromosomes, which then migrate to opposite ends of the cell. The nuclear membrane reforms around chromosomes at each end of the cell, and the cytoplasm divides.
The usual result of mitosis is the production of two genetically identical cells.
Progression through the cell cycle is controlled by interactions between cyclins and cyclin-dependent kinases.
Sexual reproduction produces genetically variable progeny and allows for accelerated evolution. It includes meiosis, in which haploid sex cells are produced, and fertilization, the fusion of sex cells. Meiosis includes two cell divisions. In meiosis I, crossing over occurs and homologous chromosomes separate. In meiosis II, chromatids separate.
The usual result of meiosis is the production of four haploid cells that are genetically variable.
Genetic variation in meiosis is produced by crossing over and by the random distribution of maternal and paternal chromosomes.
In plants, diploid microsporocytes in the stamens undergo meiosis, each microsporocyte producing four haploid microspores. Each microspore divides mitotically to produce two haploid sperm cells. In the ovary, diploid megasporocytes undergo meiosis, each megasporocyte producing four haploid macrospores, only one of which survives. The surviving megaspore divides mitotically three times to produce eight haploid nuclei, one of which forms the egg. During pollination, one sperm fertilizes the egg cell and the other fuses with two haploid nuclei to form a 3n endosperm. In animals, diploid spermatogonia initiate meiosis and become diploid primary spermatocytes, which then complete meiosis I, producing two haploid secondary spermatocytes. Each secondary spermatocyte undergoes meiosis II, producing a total of four haploid sperm cells from each primary spermatocyte. Diploid oogonia in the ovary enter meiosis and become diploid primary oocytes, each of which then completes meiosis I, producing one large haploid secondary oocyte and one small haploid polar body. The secondary oocyte completes meiosis II to produce a large haploid ovum and a smaller second polar body.
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