STRUCTURE AND FUNCTION OF THE NUCLEUS
The nucleus was the first organelle to be discovered in 1632 by Antonie Van Leeuwenhoek who was a microscopist. He observed the nucleus in red blood cells of salmon. Furthermore nucleus is the most prominent organelle in the cell as it houses the cell`s chromosomes and is the place where all deoxyribose nucleic acid (DNA) replication and Ribose nucleic acid (RNA) synthesis occur. According to Solomon et al (2008) the nucleus averages 5 micrometers (Aµm) in diameter. The nucleus can be classified as the control centre of the cell as it helps in maintaining the integrity of the genes and by controlling the activities of the cell by gene expression.
Moreover the nucleus is spheroid in shape and separated from the cytoplasm by a membrane called nuclear envelope. The nucleus consists of 4 inner parts as shown in the picture below. It consists of the nuclear envelope, nucleolus, nucleoplasm and chromatin.
Pictures adapted from: http://www.agen.ufl.edu/~chyn/age2062/OnLineBiology/OLBB/www.emc.maricopa.edu/faculty/farabee/BIOBK/nucleus_1.gif last accessed on Wednesday 20.1.10
The nuclear envelope consists of two concentric membranes that separate the nuclear contents from the surrounding cytoplasm. Becker et al. (2005) illustrated that these membranes are separated by about 20 to 40 nanometres (nm).At intervals these membranes come together to form nuclear pores which consist of protein complexes. Nuclear pores control the course of materials between the nucleoplasm and cytoplasm. According to Lamb et al. (2006) the nuclear pores is approximately 80 nm in diameter. The nuclear pore complex (NPC) is formed by union of the inner and outer nuclear membranes. Therefore it facilitates the movement of certain molecules in both ways between the nucleus and the cytoplasm by the use of a 9 nm channel opening. Solomon et al. (2008) stated that the nuclear pore complex is composed of nearly 100 proteins also it consists of the cytoplasmic ring nucleoplasmic ring and the middle ring. Palgrave et al. (2007) further noticed that the nucleoplasmic side of the pore consists of a nuclear basket whereas the cytoplasmic side exhibits fibres extending into the cytoplasm. As the transporter protein is situated in the inner core it is responsible for transferring proteins into and out of the nucleus via receptor-mediated transport.
The nucleolus is a secondary organelle that can change its size basing in ribosomal requirements. For instance if a cell produces huge amounts of protein t it definitely requires big amounts of ribosomes thus the nucleolus will increase its size to provide accommodation for the production of the proteins. Palgrave et al. (2007) stated that most of the cell`s ribosomal RNA is synthesised in the nucleolus. Moreover the nucleolus is the site mainly responsible for the assembly of ribosomes. The nucleolus consists of four components such as fibrilla centres, Pars fibrosa, Pars granulosa and nuclear matrix. The picture shown below illustrates the purpose of the nucleolus in ribosome and other ribonucleoprotein synthesis.
Picture adapted from: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A972&rendertype=figure&id=A1047 last accessed on Wednesday 20.1.10
The fibrilla centres are composed of inactive DNA where minimal synthesis of DNA and RNA takes place. The Pars fibrosa is consisted of 5 nm fibrils surrounding the fibrillar centres which contain transcriptional active DNA and the rRNA precursors are being transcribed. According to Palgrave et al. (2007) the Pars granulosa are composed of 15 nm maturing ribosomal precursor particles and the nucleolar matrix is a fibre network which participates in the organisation of the nucleolus. In addition to that the nucleoplasm is a highly viscous liquid that surrounds the chromosomes. Many substances such as the nucleotides which are necessary for replication of DNA and enzyme which direct activities that occurs in the nucleus are dissolved in the nucleoplasm. The nucleoplasm is the protoplasm within the nuclear envelope and it contains a nuclear matrix. The matrix acts as a scaffold that helps in organising the nucleoplasm. Lamb et al. (2006) noted that it contains other components such as the structural components and functional components. The structural components include fibrillar elements, nuclear pore, nuclear lamina complex, residual nucleoli, and a residual ribonucleoprotein (RNP) network whereas the functional components are involved in the transcription and processing of mRNA and rRNA, steroid receptor binding sites, carcinogen binding sites, heat-shock proteins and viral proteins (1st antigen).
DNA replication occurs when the genetic code is copied exactly before the cell division. In RNA transcription and splicing, genes are copied and adapted to form complementary strands of messenger mRNA (mRNA) which can then be translated into proteins. Lamb et al. (2006) noted that chromosomes are long strands of DNA that carry the genetic code. In eukaryotes DNA is complexed with histone and non histone proteins to form chromatin.Histones are DNA binding proteins that are important for DNA packaging and other DNA associated proteins function as enzymes for replication and transcription. Lamb et al. (2006) further noted that the nucleoli are dense staining areas within the nucleus where ribosomal RNA (rRNA) is made. Clegg et al. (1994) described chromatin as the composite combination of DNA and protein that formulate chromosomes. Chromosomes are separated between heterochromatin (condensed) and euchromatin (extended) forms. The euchromatin is the transcriptionally active form of chromatin that appears in the electron light microscope as a lightly stained region of the nucleus. The major components of chromatin are DNA and histone proteins. The main functions of the chromatin are to package DNA into a smaller size ratio to fit into the cell to strengthen DNA thus allowing mitosis and meiosis to occur. It also acts as a system to control expression and DNA replication. Smith et al. (1992) noted that alterations in chromatin structure are influenced by chemical modifications of histone proteins such as methylation (DNA and proteins) and acetylation (proteins) and by non-histone DNA- binding proteins.
Finally it can be concluded that the nucleus is one of the most important organelles in the cell because inside of it holds the nucleic acids (DNA and RNA) which makes possible to reproduce. In addition to that it also holds chromatin which forms chromosomes which are the ones in charge of the hereditary information. DNA replication processes take place inside the nucleus and it stores all the information that is to be transferred to the next generation. The information generated by nucleus governs the formation of enzymes and other proteins that carry out all the processes of body. Lamb et al. (2006) elaborated that the nucleus is indeed the most prominent organelle in the cell as it sequesters and replicates DNA, transcribes and splices RNA which allows facilitated selective exchange of molecules such as transfer RNA (tRNA) within the cytoplasm.
Becker, Wayne M. The world of the cell / Wayne M. Becker, Lewis J. Kleinsmith, Jeff Hardin. – 6th edition. – New York : Pearson/Benjamin Cummings, 2005 pg 82,83
Cell biology / [edited by] Smith and Wood. – London : Chapman and Hall, 1992
Clegg, C. J. (Christopher James), 1938- Advanced biology: principles & applications / C.J. Clegg with D.G. Mackean. – London : John Murray 1994 pg 155,156
http://www.agen.ufl.edu/~chyn/age2062/OnLineBiology/OLBB/www.emc.maricopa.edu/faculty/farabee/BIOBK/nucleus_1.gif Nucleus picture last accessed on Wednesday 20.1.10
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4&part=A972&rendertype=figure&id=A1047 nucleolus last accessed on Wednesday 20.1.10
Lamb, Neil Cell biology and genetics / Neil Lamb. – Edinburgh : Elsevier Mosby, 2006 pg 6,7
Molecular cell biology. – 6th ed. -Basingstoke : Palgrave Macmillan, 2007 pg 378
Solomon, Eldra Pearl Biology / Eldra Pearl Solomon, Linda R. Berg, Diana W. Martin. – 8th ed. (International). – New York : Brooks Cole, 2008 pg 84,85