Q.At which stage of mitosis does the nuclear envelope begin to disintegrate and the mitotic spindle begin to assemble, while chromatin progressively condenses into visible chromosomes, each already consisting of two chromatids?

Mitosis is the equational nuclear division in which the daughter cells end up with exactly the same chromosome number as the parent cell, making it responsible for growth, tissue repair and asexual reproduction. In plants it occurs mainly in meristematic tissue (shoot and root tips); it can be closed (spindle forms inside an intact nuclear envelope, as in yeasts and slime moulds) or open (the nuclear envelope breaks down and later re-forms, as in most plants and animals). Mitosis proceeds through four phases. In prophase, chromosomes condense into visible mitotic chromosomes, the nuclear envelope breaks down, and in animal cells the centrioles organise an aster of microtubules at each pole (plant cells, lacking centrioles, do not form asters). In metaphase, chromosomes attach to spindle fibres via their kinetochores and align at the equatorial metaphase plate, checked by the spindle assembly checkpoint. In anaphase, the enzyme complex APC/C degrades cohesin, each centromere splits, and sister chromatids are pulled to opposite poles. In telophase, the spindle disappears, nuclear …