This is the first and longest phase of mitosis .At its beginning all 10 nm segments of chromatids (= chromonemata) of the duplicated chromatin fibres of interphase start shortening and thickening (= condensing) by folding and coiling becoming 30 nm thick like the heterochromatic segments of interphase. Hence the chromatin reticulum of interphase disappears and the duplicated fibres start appearing as chromosomes whose length can now be measured .As mentioned before the DNA is packaged 40 fold in 30nm fibers .Further condensation chromosomes continues throughout prophase, ultimately achieving a 5000 fold to 10000 fold packaging of DNA molecules in metaphase chromosomes ,opinions differ as to how higher levels of packaging are achieved. It might occur by intensive clustering of chromomeres and spiralization of the chromatids . opinion is however, now emerging that further condensation of 30 nm fibres might be achieved first by folding and coiling to form chromomeres as described before and ultimately by radial looping around a central core of nonhistone structural proteins . Secondary constrictions of the chromatids appear at late prophase.
Other changes which occur during prophase include gradual dispersion and ultimate disappearance of nucleoli and formation of a fibrous spindle apparatus , The latter is a system of microtubules, the two centrioles, located in the centrosome of interphase, get duplicated probably in the s period , but the new ones grow during g2 period and attain full size only as prophase begins ,Prophase probably begins with the appearance, around the centrosome , of short radiating fibres, each comprising a bundle of microtubules .These fibres are called astral rays, because these impart a starlike appearance to the centrosome , which is now called an aster ,the latter now splits into half asters each containing a daughter centrosome and a pair of centrioles .The half aster. Containing the new centriole pair , now starts moving away from the other half that contains the original pair of cenbtrioles and as it does so several other fibres connecting the two half asters appear. These fibres form a spindle shaped framework and are called continuous or polar spindle fibres. The migrating half aster keeps moving along the nuclear membrane throughout prophase and ultimately occupies a position at the opposite pole of the nucleus, With increasing distance between the two daughter centrosomes, the continuous spindle fibres get gradually j stretched, The complete framework (with spindle astral rays, daughter centrosomes ,and the centriole pairs) finally formed is termed mitotic apparatus or bipolar spindle apparatus or acromatic figure or amphiaster .Cells of higher plants do not have centrioles. Hence asters are not formed but a spindle is formed. Their division is , therefore, of anastral type.
Evidences suggest that the microtubules which form the astral rays and spindle fibres are derived from those of the cytoskeleton of the cell, Towards the close of prophase, called prometaphase. By some nuclear envelope breaks down into small fragments resembling those of endoplasmic reticulum. This results in a free mixing of nucleoplasm with cytosol.