Cell Nucleus

Abstract

The cell nucleus is a double membrane‐bound organelle that contains the genetic information of the cell packaged in the form of chromatin. The nucleus is a characteristic feature of most eukaryotic cells.

The nucleus is considered to be one of the most important structures of eukaryotic cells as it serves the function of information storage, retrieval and duplication of genetic information. It is a double membrane‐bound organelle that harbours the genetic material in the form of chromatin. It consists of a dynamic mix of nonmembranous subcompartments of varying functional capacity. The specific features of a cell nucleus, especially in terms of the nature and distribution of the subnuclear compartments and the positioning of the chromosomes, depend upon their differentiated state in the organism. The key functions of the cell nucleus include deoxyribonucleic acid replication and further to control gene expression during the cell cycle. Transcription and further post‐transcriptional processing of pre‐messenger ribonucleic acids (mRNAs) occur inside the nucleus and the mature mRNAs are transported into the cytoplasm where the translational events occur. Thus, nucleus provides functional compartmentalisation inside the cell allowing higher levels of gene regulation.

Key Concepts:

  • The cell nucleus contains nonmembranous distinct substructures that are characterised by having a specific subset of proteins and RNA.

  • The nuclear envelope is a highly regulated membrane barrier that allows the compartmentalisation of the nucleus from the cytoplasm.

  • Nuclear pore complexes (NPCs) are large proteinaceous channels that perforate the nuclear membrane and allow transport of molecules into and out of the nucleus. In general, proteins destined for import contain nuclear localisation signal (NLS) and proteins destined for export contain nuclear export signal (NES).

  • Genes are located in chromosomes inside the nucleus, which consists of a single long DNA that is highly coiled and folded by proteins. A chromosome is confined to a distinct ‘territory’ within the interphase nucleus.

  • Histone modifications of the nucleosomes differentiate between the euchromatic and heterochromatic chromatin states and allow higher order of gene regulation.

  • The genetic material, DNA is transcribed into pre‐messenger RNA (pre‐mRNAs) inside the nucleus. Further intron splicing of the pre‐mRNA takes place according to a precise and complex arrangement of proteins and ribonuclear particles. The mature mRNA is exported to the cytoplasm for translation.

  • Gene expression is regulated at several steps including pre‐mRNA processing, nucleoplasmic export and degradation of RNA.

  • The clearest substructure in the nucleus in most eukaryotes is the nucleolus, which is the site of rDNA transcription and ribosome biosynthesis. Apart from that it is also involved in biogenesis of other RNP complexes, mRNA surveillance, stress sensing and control of cell proliferation.

  • Nuclear speckles are highly dynamic, irregularly shaped nuclear domains enriched with pre‐mRNA splicing factors.

  • SR family splicing factors are phosphoproteins that regulate both constitutive and alternative splicing and also the export of mRNAs from the nucleus to cytoplasm.

  • Nuclear‐retained regulatory noncoding RNAs (ncRNAs) constitute a fraction of ncRNAs and play pivotal role in the control of gene expression, affecting chromatin structure, post‐transcriptional modifications and stability of RNA.

Keywords: nuclear domains; transcription; chromatin; noncoding RNA; nuclear‐retained RNA; pre‐mRNA splicing; nuclear lamina

Figure 1.

Nuclear domains immunolabelled with specific antibodies (green). (a) Immunostaining by lamin B1 representing nuclear lamina. (b) Heterochromatin labelled with antibody recognising HP1α. (c) Immunostaining by nucleolin representing Nucleolus. (d) Nuclear speckles labelled with antibody‐recognising SON protein. (e) Immunolocalisation with P54/NRB representing paraspeckles. (f) Immunolacalisation with p80 coilin representing Cajal bodies. The DNA is counterstained with DAPI (Blue). Bar, 10 μm.

Figure 2.

Nuclear‐retained regulatory RNAs. (a) RNA‐FISH using probe against the Xist RNA. (b) RNA‐FISH analysis reveal Nuclear speckle localisation of MALAT1RNA and (c) Paraspeckle localisation of NEAT1 RNA and (d) CTN‐RNA. (e) Hsrw RNA‐FISH representing its localisation to omega‐speckles. (f) RNA‐FISH reveals the nuclear distribution of Gomafu ncRNA and (g) GRC‐RNA. DNA is counterstained with DAPI (Blue). Bar, 10 μm. (a) (Xist‐RNA), (f) (Gomafu‐RNA), courtesy of Shinichi Nakagawa Riken, Japan. (e) (Hsrw), courtesy of Lakhotia SC, Banaras Hindu University, Varanasi, India.

Figure 3.

Patterns of DNA replication identified in HeLa cells by BrdU labelling followed by immunofluorescence. (a) Early S‐phase pattern, the sites of replicating DNA are distributed throughout the nucleoplasm. However, there is no apparent labelling of the nucleolus and nucleolar periphery. (b–d) Mid S‐phase pattern. There is a decrease in internal and an increase in peripheral sites of replication. The sites of replication are associated with the periphery of the nucleus and nucleolus. (e and f) Late S‐phase, in this pattern the sites of replication are characterised by large areas of replicating DNA that span portions of the interior as well as smaller areas of replication at the periphery of the nucleus. Bar, 5 μm. Images courtesy of Dr Prasanth SG, UIUC, Urbana‐Champaign, IL, USA.

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Tripathi, Vidisha, and Prasanth, Kannanganattu V(Jul 2011) Cell Nucleus. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001337.pub2]