中期染色体(chromosomes at metaphase).ppt

* * * * * * * * * G2 checkpoint: Monitors DNA replication and damage repairing and blocks mitosis entry which allows cells more time to repair DNA damage or complete DNA synthesis. Spindle checkpoint: Monitors attachment tension of kinetochore and blocks transition from metaphase to anaphase which allows cells more time to correct non-bipolar attachment of kinetiochores. * * During late interphase, before the start of mitosis, duplicated chromatin is condensing into chromosomes, and centrosomes have duplicated in preparation for mitosis. * During early prophase, mitosis has begun. Duplicated chromosomes are now visible. Each chromosome is duplicated and composed of sister chromatids held together at a centromere. Centrosomes begin moving apart, the nuclear envelope is fragmenting, and the nucleolus will disappear. * During late prophase, the spindle is forming and spindle fibers appear between the separating chromosomes. Centromeres attach to spindle fibers called centromeric (or kinteochore) fibers. The chromosomes have no particular orientation as yet. * By the time of metaphase, the fully formed spindle consists of poles, asters, and fibers. The metaphase plate is a plane perpendicular to the axis of the spindle and equidistant from the poles. The chromosomes attached to the centromeric spindle fibers line up at the metaphase plate during metaphase. Polar spindle fibers reach beyond the metaphase plate and overlap. * During anaphase, daughter chromosomes (each consisting of one chromatid) are moving toward the poles of the spindle. The daughter chromosomes have a centromere and a single chromatid. The movement of the daughter chromosomes occurs because the centromeric spindle fibers disassemble at the region of the centromere, pulling the daughter chromosomes toward the poles, and because the polar spindle fibers push the poles apart as they lengthen and slide past one another. * During telophase, the spindle disappears, and the nuclear envelopes begin to r