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CHAPTER 12 THE CELL CYCLE The ability to reproduce distinguishes living organisms from nonliving objects; this ability has a cellular basis. All cells arise from preexisting cells. This fundamental principle, known as the cell doctrine, was originally postulated by Rudolf Virchow in 1858, and it provides the basis for the continuity of life. A cell reproduces by undergoing a coordinated sequence of events in which it duplicates its contents and then divides in two. This cycle of duplication and division, known as the cell cycle, is the means by which all living things reproduce. I. The Key Roles of Cell Division A. Cell division functions in reproduction, growth, and repair Cells reproduce for many reasons. - In unicellular organisms, the division of one cell to form two reproduces an entire organism (e.g., bacteria, yeast, Amoeba). - In multicellular organisms, cell division allows: - Growth and development from the fertilized egg - Replacement of damaged or dead cells Cell division is a finely controlled process that results in the distribution of identical hereditary material-DNA-to two daughter cells. A dividing cell: - Precisely replicates its DNA - Allocates the two copies of DNA to opposite ends of the cell - Separates into two daughter cells containing identical hereditary information B. Cell division distributes identical sets of chromosomes to daughter cells The total hereditary endowment of a cell of a particular species is called its genome . The genomes of some species are quite small (e.g., prokaryotes), while the genomes of other species are quite large (e.g., eukaryotes). The replication, division, and distribution of the large genomes of eukaryotes is possible because the genomes are organized into multiple functional units called chromosomes .
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Eukaryotic chromosomes have the following characteristics: - They are supercoils of a DNA-protein complex called chromatin . Each chromosome consists of the following: - A single, long, double-stranded molecule of DNA, segments of which are called genes - Various proteins which serve to maintain the structure of the chromosome or are involved with the expression of genes, DNA replication, and DNA repair - They exist in a characteristic number in different species (e.g., human somatic cells have 46); gamete cells (sperm or ova) possess half the number of chromosomes of somatic cells (e.g., human gametes have 23) - They exist in different states at different stages of the cell cycle. - During interphase, the chromosomes are loosely folded; cannot be seen with a light microscope - During the mitotic phase, chromosomes are highly folded and condensed; can be seen with a light microscope In preparation for eukaryotic cell division, the complete genome is duplicated. As a result of this duplication, each chromosome consists of two sister chromatids .
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