The three main phases of a single cell cycle are: interphase, nuclear division and cytoplasmic division.
With the right techniques, the next stage in the cell cycle, mitosis (M), can be observed using a good light microscope. Early microscopists found it convenient to subdivide the nuclear division of cells into stages that were easily seen under the microscope using colored dyes that stained the chromosomes and some of the other participants.
Nuclear division or karyokinesis is a continuous process, however, and there are no artificial divisions in actively growing cells.
As the cell moves out of G2 and into M the granular nature of the nucleus begins to change and condense into a series of fine threads. The molecules of DNA become associated with more and more histone proteins and package themselves into higher and higher degrees of structure. This is generally taken as the beginning of prophase
early prophase - the nuclear membrane becomes more and more indistinct and the chromatin fibers become more and more packaged and condensed. It is usually not possible to follow individual threads, but the condensation of the material into individual units is becoming obvious.
The nucleolus also becomes indistinct and begins to vanish.
In animal cells (mostly), a double pair of short, rod-like structures called centrioles, appear, separate and begin to move to opposite sides of the cell, outside the vanishing nucleus.
mid-prophase - the chromatin threads are now condensed enough to be distinguished as individual chromosomes. Each chromosome consists of two identical halves called chromatids, that are connected and held together by a constriction called a centromere.
The location of the centromere along the length of the chromosome and chromatids is a distinctive characteristic of many chromosomes, and can sometimes be used (with other factors) to identify them.
The pairs of centrioles continue to move around the almost vanished nucleus to opposite sides of the cell.
In animal cells a system of thin protein fibers begins to radiate out from the centrioles forming a pattern in the cytoplasm of the cell that looks like a star or aster.
late prophase - the nuclear membrane and the nucleolus finally vanishes completely. The chromosomes are very distinct, easy to recognize and have clear "arms" composed of the two parts of the sister chromatids.
The centrioles, and asters, are at opposite ends of the cell and the thin protein spindle fibers are reaching out and attaching to the centromeres of each chromosome from opposite directions.
Prophase is generally considered to be over when the chromosomes are fully condensed, clear, and the nuclear membrane is gone or almost gone.