Two areas of investigation are currently of great interest to biologists. The first being what determines the identity of a particular cell, and the second being the ability of various cells to maintain their identities during cell division. Since theoretically every cell in a given organism contains the same genetic information (i.e. DNA or genome sequence), the answers do not lie within the DNA sequence itself, but rather reflect the organization of chromatin into active and inactive components, i.e. epigenetics. So, the questions become: How is the genetic information organized so that it is interpreted correctly to give a cell a particular identity and how is that organization inherited? These questions are important to understand the relationship between genotype and phenotype and are also very important for medical sciences. For example, epigenetic abnormality has been linked to cancer and other human diseases. But the underlying mechanisms remain a mystery. So, in the long-term, an understanding of these topics will have positive medical applications with respect to both diagnosis and treatment.
Epigenetics is an enormous and rapidly growing field. It started from DNA methylation about 20 years ago. Now this concept includes histone modifications, histone variants, imprinting, replication timing, and more. However when you look at these closely, you will notice that all these properties could very likely be controlled by one thing: DNA replication, when chromatin is assembled. At the time of DNA synthesis, Mother Nature could efficiently assemble all these important packaging components that give a cell the identity.