A&p 1 Lab Test

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Interphase

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Interphase is the period of a cell's life when it is carrying out its normal metabolic activities and growing. During interphase, the chromosomal material is seen in the form of extended and condensed chromatin, and the nuclear membrane and nucleolus are intact and visible. Microtubule arrays (asters) are seen extending from the centrosomes. During various periods of this phase, the centrioles begin replicating (g1 through g2), DNA is replicated (s), and the final preparations for mitosis are completed (g2). The centriole pair finishes replicating into two pairs during g2.

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Early Prophase

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As mitosis begins, microtubule arrays (asters) are seen extending from the centromere matrix around the centrioles. Early in prophase, the first and longest phase of mitosis, the chromatin threads coil and condense, forming barlike chromosomes that are visible with a light microscope. Since DNA replication has ocurred during interphase, each chromosome is actually made up of two identical chromatin threads, now called chromatids. The chromatids of each chromosome are held together by a small, buttonlike body called a centromere and a protein complex called cohesin. After the chromatids separate, each is considered a new chromosome.

As the chromosomes appear, the nucleoli disappear, and the cytoskeletal microtubules disassemble. The centriole pairs separate from one another. The centrioles act as focal points for growth of a new assembly of microtubules called the mitotic spindle. As these microtubules lengthen, they push the centrioles farther and farther apart, propelling them toward opposite ends (p

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Late Prophase

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While the centrioles are still moving away from each other, the nuclear envelope fragments, allowing the spindle to occupy the center of the cell and to interact with the chromosomes. Meanwhile, some of the growing spindle microtubules attach to special protein-DNA complexes, called kinetochores, on each chromosomes centromere. Such microtubules, which do not attach to any chromosomes, are called polar microtubules. The tips of the polar microtubules are linked near the center; these push against each other, forcing the poles apart. Meanwhile, the kinetochore microtubules pull on each chromosome from both poles, resulting in a tug-of-war that ultimately draws the chromosomes in a jerky march to the middle of the cell.