They all have 6 in its outer shell because they are all in the same group in the periodic table. Your VERY VERY welcome (:
A sister chromatid refers to the identical copies formed by the replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad. A full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated. The two sister chromatids are separated from each other into two different cells during mitosis or during the second division of meiosis. Compare sister chromatids to homologous chromosomes, which are the two different copies of a chromosome that diploid organisms (like humans) inherit, one from each parent.
First, homologous (like) chromosomes pair up at the cell equator where they actually exchange genetic information. Then, one chromosome from each pair is pulled toward each pole. At the end of this reduction division, each daughter cell receives only one homologous chromosome from each pair, ending up with one set. Meiosis halves the set of chromosome and randomly assorts homologous chromosomes into sex cells. The full chromosome number is restored when sperm and egg unite. This exactly mirrored the behavior of genes as deduced by Mendel three decades earlier.