If the cell does not duplicate its DNA before it divides, the daughter cells will be haploid (one copy of each chromosome) instead of diploid (two copies of each chromosome). This is assuming the organism is diploid; some animals such as certain species of frogs are tetraploid, meaning there are 4 copies of each chromosome in every cell!

DNA must be duplicated to ensure that each cell gets the correct number of chromosomes. Humans are diploid, meaning that we inherit one set of chromosomes from mom and the other from dad. Because of this, we always have two alleles for every gene. This is why you could have blue eyes even though your parents both have brown eyes - if each parent gives you one copy of a recessive "blue eye" allele, you will end up with this recessive trait. If the cell only gets one set of chromosomes, it will only express the genes from that set. Most likely this cell will not survive unless it is a gamete, which must be haploid because two gametes will eventually come together to form a diploid embryo.

Sister chromatids are joined so that they can be aligned at the metaphase plate, allowing them to be separated appropriately into the two daughter cells. When the cell is not undergoing mitosis, all the chromatin will unravel and none of the chromosomes will be joined - this is only something that happens to enable cell division.