Answer:
Prophase
Step-by-step explanation:
Prophase is usually the longest phase in mitosis,the chromatin threads condense to form distinct chromosomes,long and thin at first,later become shorter and fatter.
Each chromosome consist of a pair of chromatids held closely together at the centromere. Nucleolus disappears, nuclear membrane disintegrates, series of spindle (protein) fibres are formed spanning the cell from end to end. In cells where centrioles are present,they lie at each of the two spindle poles.