a diploid in biology means a cell or a body that has chromosomes in paired conditions.in humans it is 24 pairs of chromosomes.in creatures like roundworm the numbers are indifferent(range from 2 to 100).
secondly,at the time of mitotic division each chromosome makes its duplicate,that is,each dna molecule of each chromosome makes a copy of it self and gets joint to protien.until and unless the cell divides fully,these copies get attached to each other and remain attached on their centromere(at the middle in a pair).until and unless they get detached from each other they are called sister chromatid and individually chromatid.chromatids are also called deoxyribonucleicprotiens(dna + protiens)
