Chromosome literally means “colored body”. These densely packed structures are composed of highly organized DNA and have distinct regions we can make reference to. An unreplicated chromosome looks a bit like a cucumber that has been constricted in the middle (or two breakfast links, still connected). The central constricted region is called the centromere. When the chromosome is replicated, it will become two identical sister chromatids that are attached at the centromere region. You can count the number of chromosomes present by counting the number of centromeres. Two attached sister chromatids are one chromosome, but the instant they separate, two centromeres are visible and the sister chromatids each become individual chromosomes. This will become important later.
The main body of the chromosome above and below the centromere are called arms and the end of each arm is the telomere. Telomeres are of interest to many scientists studying aging, as it is possible that the degeneration of telomeres during replication eventually leads to the suite of symptoms we connect with the aging process.