(a) If nuclear membrane fails to disintegrate, the spindle fibres would not be able to reach chromosomes and they would not move towards opposite poles of the cell. In certain protozoans, such as Amoeba, the spindle is formed within the nucleus and this is called intranuclear mitosis or pre-mitosis.

(b) If DNA duplication does not occur than cell might not be able to surposs S-phase of cell-cycle as no chromosome formation will take place, and will not be able to enter M-(mitotic phase) or if in case it enters mitosis the cycle will cease,

(c) If the centromeres do not divide,one of the daughter cell will receive a complete pair of chromosomes and other cell would not get any of them. This may result in trisomy.

(d) If cytokinesis does not occur, then multinucleate condition called coenocyte, syncytium is produced, as in Rhizopus and Vaucheria, etc.

In unicellular organisms, the type of cell division is known as amitosis in which somatic cell is divided into two parts directly. In multicellular organisms occurs which is an indirect process.

Both cell division in unicellular and multicellular organisms undergo mitosis have differences as below

Cell Division in Unicellular Organism (Amitosis)	Cell Division in Multicellular Organism (Mitosis)
It is direct division of cellular and nuclear content. Without the formation of chromosomes.	Nucleus and cellular content do not divide directly and involves formation of chromosomes.
Different phases of cell divisions are not seen.	It involves different phases of cell division.

Cancer is the pathological condition in which the cells loose control over cell division and this results into malformation of the organ in which such cell division occurs.

DNA replication takes place in nucleus and duplication of centriole occurs in the cytoplasm. Centriole forms spindle fibres during cell division in animal cells which direct the movement of chromosomes within the cell. The position of centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of the cell.

Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms. Even though the process results in reduction of chromosome number by half, it is gradually conserved by union of male gamete $(n)$ and female gamete $(n)$ in next generation. Meiosis also increases the genetic variability in the population of organisms from one generation to the next.