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At the interphase, before prophase, the centrioles and genetic material have already duplicated in the cell. In the G2 phase, the cytoskeleton has also been reorganized.

The following processes occur in prophase:

  • The chromatin condenses, so that the chromosomes are becoming visible. Since DNA has already been replicated in S phase, chromosomes are made up of two identical sister chromatids  joined at the centromere. In the centromeres, the kinetochore are formed, from which the kinetochoric microtubules originate and act as organizing centers for microtubules.   
  • Movement of chromosomes towards the nuclear membrane , leaving the center of the nucleus free.
  • Progressive disappearance of the nucleolus.
  • Formation of the mitotic spindle. Near each pair of centrioles, microtubules are formed that are arranged radially, forming the fibers of the aster. The centrioles are separating towards the opposite poles, growing continuous fibers that give rise to the mitotic spindle. The chromosomes are oriented with the kinetochores towards each pole. As the microtubules lengthen , the chromosomes get closer to the cell equator.
  • Disappearance of the nuclear membrane  ( open mitosis ), except in closed mitoses (in these cases the spindle can be intranuclear or foreign).

In prophase, chromosomes are made up of two identical sister chromatids, since they come from replication, in the S phase of the interphase of a DNA molecule. They could only be different if there had been some error in DNA replication.