a) During S phase there is a full set of sisters chromatids, since they are created during the synthesis (S) phase of interphase. The dogs have 78 chromosomes in a diploid cell line up at the Equator during cell dividion, being that already replicated. If there is 78 chromosomes during meiosis I, therefore the number of sister chromatides is 78 duplicate for the dog chromosomes after the start of cell division. That occurs because the initial stages of cell division prepare the cell to be divided into two cells with the same number of chromosomes than the parent cell (78), and in the S phase the number of chromatidies is duplicated to make that possible.