In animal cells during prophase, microscopic bodies called centrioles begin to migrate to opposite sides of the cell. As prophase unfolds, the chromatids become visible in pairs (called sister chromatids), the spindle fibers form, the nucleoli disappear, and the nuclear envelope dissolves. Two chromatids are joined to one another at a region called the centromere. Two copies of each chromosome exist each one is a chromatid. ■ Prophase: Mitosis begins with the condensing of the chromatin to form chromosomes in the phase called prophase. Mitosis is a continuous process, but for convenience in denoting which portion of the process is taking place, scientists divide mitosis into a series of phases: prophase, metaphase, anaphase, and telophase (see Figure 7-1):įigure 7-1 The process of mitosis, in which the chromosomes of a cell duplicate and pass into two daughter cells. The chromosomes organize in the center of the cell, and then they separate, and 46 chromosomes move into each new cell that forms. The term mitosis is derived from the Latin stem mito, meaning “threads.” When mitosis was first described a century ago, scientists had seen “threads” within cells, so they gave the name “mitosis” to the process of “thread movement.” During mitosis, the nuclear material becomes visible as threadlike chromosomes. Spindle fibers are composed of microtubules. The spindle is constructed from amino acids for each mitosis, and then taken apart at the conclusion of the process. Proteins organize themselves to form a series of fibers called the spindle, which is involved in chromosome movement during mitosis. In the G 2 phase, the cell prepares for mitosis. Human cells contain 92 chromosomes per cell in the S phase. During this process, each chromosome is faithfully copied, so by the end of the S phase, two DNA molecules exist for each one formerly present in the G 1 phase. In normal human cells, there are 46 chromosomes per cell (except in sex cells with 23 chromosomes and red blood cells with no nucleus and, hence, no chromosomes).ĭuring the S phase of the cell cycle, the DNA within the nucleus replicates. During the G 1 phase, each chromosome consists of a single molecule of DNA and its associated histone protein. For example, a pancreas cell in the G 1 phase will produce and secrete insulin, a muscle cell will undergo the contractions that permit movement, and a salivary gland cell will secrete salivary enzymes to assist digestion. The G 1 phase follows mitosis and is the period in which the cell is synthesizing its structural proteins and enzymes to perform its functions. The interphase stage of the cell cycle includes three distinctive parts: the G 1 phase, the S phase, and the G 2 phase. Mitosis is the phase of the cell cycle during which the cell divides into two daughter cells. During interphase, the cell spends most of its time performing the functions that make it unique. The cell cycle is generally divided into two phases: interphase and mitosis. With few exceptions (for example, red blood cells), all the cells of living things undergo a cell cycle. The cell cycle involves many repetitions of cellular growth and reproduction.
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