Eukaryotic Nuclear Organization

Nuclear organization refers to the spatial distribution of chromatin within a cell nucleus. There are many different levels and scales of nuclear organisation.

At the smallest scale, DNA is packaged into units called nucleosomes. The quantity and organisation of these nucleosomes can affect the accessibility of local chromatin. This has a knock-on effect on the expression of nearby genes, additionally determining whether or not they can be regulated by transcription factors.

At slightly larger scales, DNA looping can physically bring together DNA elements that would otherwise be separated by large distances. These interactions allow regulatory signals to cross over large genomic distances - for example, from enhancers to promoters.

In contrast, on a large-scale, the arrangement of chromosomes can determine their properties. Chromosomes are organised into two compartments labelled A ("active") and B ("inactive"), each with distinct properties. Moreover, entire chromosomes segregate into distinct regions called chromosome territories.

Each human cell contains around two metres of DNA, which must be tightly folded to fit inside the cell nucleus. However, in order for the cell to function, proteins must be able to access the sequence information contained within the DNA, in spite of its tightly-packed nature. Hence, the cell has a number of mechanisms in place to control how DNA is organized.

Moreover, nuclear organization can play a role in establishing cell identity. Cells within an organism have near identical nucleic acid sequences, but often exhibit different phenotypes. One way in which this individuality occurs is through changes in genome architecture, which can alter the expression of different sets of genes. These alterations can have a downstream effect on cellular functions such as cell cycle facilitation, DNA replication, nuclear transport, and alteration of nuclear structure. Controlled changes in nuclear organization are essential for proper cellular function.

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