A chromosome is the thread - like strand that is housed in the nucleus of a cell and carries genetic information. It is composed partially of DNA (deoxyribose nucleic acid). This DNA looks like a double helix. It is made of a sugar-phosphate backbone (sides of the ladder) and nucleotide base pairs as its rungs (adenine and thymine, or cytosine and guanine). The DNA within a chromosome is tightly wound around histone proteins. Each species has a unique number of chromosomes. For example, humans have 46 chromosomes.
A chromatid, on the other hand, is a replicated copy of a chromosome. This occurs at the beginning of cell division. Thus, a chromosome is made of two sister chromatids. The two copies are joined in the center by what is called a centromere.