A complex of nucleic acids (e.g. DNA or RNA) and proteins (e.g. histones) is called chromatin. During cell division, the chromatin condenses to become a chromosome. The basic structural unit of chromatin is a nucleosome. Each nucleosome is made up of a DNA segment wound around the histone protein cores. The main function of chromatin is to package DNA into a smaller volume to fit into the cell. There are two major forms of chromatin: the euchromatin and the heterochromatin. The euchromatin is structurally loose to allow transcription and replication whereas the heterochromatin is more condensed and therefore less active.
Nuclear DNA
The nuclear DNA comprises a large fraction of the cell’s genome (the small fraction comes from the extranuclear DNA in the mitochondria and/or chloroplasts). The DNA outside the nucleus is called extranuclear DNA. This extranuclear DNA, such as cpDNA in chloroplasts and mtDNA in mitochondria, occurs in multiple copies since there are several chloroplasts and mitochondria while there is usually just one nucleus inside a cell. A cell would, therefore, contain several copies of mtDNA and cpDNA, often in thousands. Nuclear DNAs are compacted into chromatin structures through histones whereas mtDNA and cpDNA are not.
Nuclear bodies
A nuclear body is defined as a non-membraned mostly proteinaceous structure in the nucleus. As already mentioned above, the nucleolus is regarded as one of the nuclear bodies and is most prominent. It is characterized by its round granular appearance. Its function is largely for the synthesis of ribosomes, which in turn are one of the key players in protein synthesis.
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