The Nucleus

by Vibha Tyagi

Department of Molecular & Cellular Biology, University of Guelph, N1G 2W1. Canada.


The nucleus, suspended by cytoplasmic strands observed moving around a cotyledon cell in Arabidopsis.
Autofluorescent chloroplasts have been false coloured blue.




The nucleus is a highly specialized, multifunctional organelle serving as the information hub and administrative center of the cell. It is a cohesive centre for nuclear genes aligned on chromosomes and acts as an organizer of cytoplasmic and nuclear activities during the cell cycle. The nucleus helps in shuttling of regulatory factors & gene products via nuclear pores, aids production of mRNAs and ribosomes and organizes the uncoiling of DNA to replicate key genes. It thus coordinates innumerable metabolic pathways to achieve growth, division and differentiation of the cell. The nucleus of the plant cell shows a complex organization [1]. The study of particular proteins in the nucleus of chemically fixed cells aided by fluorescence microscopy uncovered many vital details of nuclear architecture [2]. These studies clearly established the existence of several morphologically distinguishable intra-nuclear structures.

Chromatin and Chromosomes – Most of the eukaryotic genome is packaged into chromatin which is formed of DNA and proteins. Most of the proteins of chromatin are the histones—positively charged proteins rich in basic amino acids (arginine and lysine) that bring about binding to the negatively charged DNA molecule. DNA strands wound around a basic core of histone and other small protein molecules form bead-like structures known as nucleosomes. The further winding of the string of nucleosomes into an even denser structure known as a solenoid results in the formation of a structure that serves several functions. The overall negative charge of the DNA is neutralized by the positive charge of histone molecules; the packaged DNA takes up much less space, and inactive DNA can be folded into inaccessible locations until it is needed [3]. A histone2B::YFP fusion protein has been one of the most useful nuclear localizing probe for plant research [4].

The Nucleolus - The nucleolus is the densest region of chromatin that is responsible for the synthesis of ribosomes. A nucleus may contain multiple nucleoli, but within each species the number of nucleoli appears to be fixed. The nucleolus is morphologically separated into three distinct components; fibrillar centres (FC) - containing hundreds of rRNA genes in tandem arrays found at several chromosomal loci (termed nucleolar organising regions (NORs); the dense fibrillar component (DFC), which contains actively transcribing rRNA genes and nascent rRNA transcripts; and the granular component (GC), which is the site of late processing events in the biogenesis of rRNAs [4]. The nucleolus is also responsible for the trafficking of various small RNA species and has been implicated in the control of cellular survival and proliferation and senescence [5].

The Nuclear Envelope – The contents of the nucleus are enclosed by double-layered membrane called the nuclear envelope [3, 14-18]. The peri-nuclear space (space between the two layers) of nuclear envelope connects with the rough endoplasmic reticulum [3]. It creates a separate biochemical environment from the cytoplasm, and allows for a selective and regulated exchange of macromolecules between the two compartments. Numerous minute opening called nuclear pores form passages through the nuclear envelope connecting the nucleoplasm with the cytoplasm and facilitating the exchange of materials like proteins RNA etc. The nuclear lamina is a protein lining on the inner surface of nuclear envelope which binds to chromatin and other nuclear components. [4, 5].

Nuclear Pores – These are circumscribed points where the outer and inner nuclear membranes of the nuclear envelope are fused together. Nucleo-cytoplasmic transportation of various macromolecules takes place through the nuclear pore complexes which are multi-protein structures embedded in the nuclear pores [7].

The nucleus may also contain a variety of other smaller components, such as Cajal bodies (or coiled bodies [8]), Gemini bodies (Gems [9]), and inter-chromatin granule clusters [10].

References

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10.    Saitoh, N. et al., (2004) Proteomic Analysis of Interchromatin Granule Clusters Spector. Mol Biol Cell. 15(8): 3876–3890.

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15.    Gindullisa, F. et al. (1999) MAF1, a Novel Plant Protein Interacting with Matrix Attachment Region Binding Protein MFP1, Is located at the Nuclear Envelope Plant Cell, Vol. 11, 1755-1768.

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18.    Irons, S.L. et al. (2003) The first 238 amino acids of the human lamin B receptor are targeted to the nuclear envelope in plants. J. Expt. Bot. 54, 943-950.

19.    Gonzalez-Melendi, P. et al. (2000). The nucleus: a highly organized but dynamic structure. Journal of Microscopy, Vol. 198 3, 199-207.

20.    Zhang C et al. (2005) Cell type-specific characterization of nuclear DNA contents within complex tissues and organs. Plant Methods 1:7