STRUCTURE AND FUNCTIONS OF NUCLEUS OF A CELL.pptx

The nucleus is the part of a cell that contains DNA organized into chromosomes and is located in the middle of the cell. It is surrounded by the nuclear envelope, which is a double membrane that separates the nucleus from the cytoplasm. The nuclear envelope contains nuclear pores, which are gateways that allow molecules to move into and out of the nucleus.

The Cell: The Histology Guide
Nucleus - The Cell: The Histology Guide - University of Leeds
The nucleus is found in the middle of the cells, and it contains DNA arranged in chromosomes. It is surrounded by the nuclear envelope, a double nuclear membrane (outer and inner), which separates the nucleus from the cytoplasm. The outer membrane is continuous with the rough endoplasmic reticulum.
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Nucleus – BIO109 Biology I Introduction to Biology
The boundary of the nucleus, called the nuclear envelope, is a double membrane that contains small openings called nuclear pores. These pores are gateways that allow molecules to move into and out of the nucleus, enabling it to communicate with the rest of the cell.
The nucleus has three main parts:
Nuclear membrane: A protective barrier of the nucleus
Nucleoplasm: The cytoplasm of the nucleus, which is a semifluid matrix that contains chromatin, the less condensed form of DNA that organizes into chromosomes during mitosis or cell division
Nucleolus: A spherical structure that produces and assembles the cell's ribosomes
The nucleus controls and regulates the activities of the cell, such as growth and metabolism.
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1. STRUCTURE AND FUNCTION OF NUCLEUS RASHMI M G P01AG23S028017 MAHARANI’S SCIENCE COLLEGE FOR WOMEN MYSORE

2. INTRODUCTION Cells are the building blocks of life. the biological science which deals with the study of structure, function, molecular organization, growth, reproduction and genetics of the cells, is called cytology or cell biology. Cell is the basic unit of organization or structure of all living matter. Within a selective and retention semi permeable membrane, it consists of a complete set of different kinds of units necessary to permit its own growth and reproduction from simple nutrients. So the cell is defined as a unit of biological activity delimited by a semi permeable membrane and capable of self reproduction in a medium free of other living systems. Cell theory- In 1838, a German botanist, Mathias Jacob Schleiden put forth the idea that cells were the units of structure in the plants. In 1839, his co worker a German zoologist, Theodor Schwann applied Schleiden’s thesis to the animals. Both of them, thus postulated that the cell is the basic unit of structure and function in all life. This simple, basic and formal biological generalization is known as cell theory or cell doctrine.

3. STRUCUTRE OF NUCLEUS

4. NUCLEUS The nucleus (nux= nut) is the heart of cell. It is here that almost all the cell’s DNA is confined, replicated and transcribed. The nucleus, thus controls different metabolic as well as hereditary activities of the cell. The synonymous term for this organelle is the Greek word “karyon”. Nucleus serves as the main distinguishing feature of eukaryotic cells that is this is the true nucleus as opposed to the nuclear region, prokaryon or nucleoid of the prokaryotic cells.

5. DISCOVERY OF NUCLEUS Nuclei was first discovered and named by Robert Brown in 1833 in the plant cells and were quickly recognized as a constant feature of all animal and plant cells. Nucleoli were described by M J Schleiden in 1838, although first noted by Fontana (1781). The term nucleolus was coined by Bowman in 1840. In 1879, W Flemming coined the term chromatin for chromosomal meshwork.

6. OCCURRENCE AND POSITION The nucleus is found in all the eukaryotic cells of the plants and animals. However, certain eukaryotic cells such as the mature sieve tubes of higher plants and mammalian erythrocytes contain no nucleus. In such cells nuclei are present during the early stages of development. Since mature mammalian red blood cells are without any nuclei they are called red blood corpuscles rather than cells (corpus =body, especially dead body or corpse). Usually the nucleus remains located in centre. But its position may change from time to time according to the metabolic state of the cell. For example. in embryonic cells.

7. MORPHOLOGY Number- usually cell contain single nucleus but the number of nucleus may vary from cell to cell . According to the number of nuclei following types of cells have been recognized. Mononucleate cells –normal plant and animal cells. Binucleate cells- ex. Paramecium, cartilage and liver cells. Polynucleate cells- Polynucleate cells of animals- syncytial cells. Polynucleate cells of plants- coenocytes.

8. MORPHOLOGY Shape- shape is normally remains related with the shape of cell, but certain nuclei are almost irregular in shape. The spheroid, cuboid or polyhedral cells (isodiametric) contain the spheroid nuclei. The nuclei of the cylindrical, prismatic or fusiform cells are ellipsoid in shape. The cells of the squamous epithelium contain the discoid nuclei. The leucocytes, certain infusoria, glandular cells of some insects and spermatozoa contain the irregular shaped nuclei. Nucleus of cells of silk glands of silk worm have finger like extensions that greatly increases their surface area.

9. MORPHOLOGY Size- generally nucleus occupies 10% of the total cell volume. Nuclei vary in size from about 3 µm-25µm in diameter, depending on cell type and contain diploid set of chromosomes. Moreover the size of the nucleus is related with the number of chromosomes or ploidy. The haploid cells contain small sized nuclei than the diploid cells. Thus, the size of the nucleus of a cell depends on the volume of the cells, amount of the DNA and proteins and metabolic phase of the cell.

10. Nuclear envelope The nucleus, whatever its shape, is segregated from the cytoplasm by a double membrane, the nuclear envelope, and the two membranes separated from each other by a perinuclear space of varying width. Each layer is about 100-300Å apart, leaving a discontinuous perinuclear space that communicates with the channels of cytoplasmic membrane. The envelope is absent only during the time of cell division for a very brief period.

11. Structure of nuclear envelope The two membranes of the nuclear envelope are roughly parallel, separated by perinuclear space which is discontinuous except some areas where the membranes join to form pore complex. The two membranes around these pores are in continuity and form a rounded lip around the circular area. The cytoplasmic side of the nuclear envelope is frequently coated with ribosomes, indicating its relation with the endoplasmic reticulum. The inner membrane possess a crystalline layer, often coated with filaments and fibrous structures, some of which extend deeper in the nucleus and others may attach to the chromatin material.

12. Pore complex The nuclear envelope is interrupted at intervals by nuclear pores which act as passageways for transport of various types of material. In plant cells these pores are irregularly and sparsely distributed over the surface of the nucleus. The pores are aligned with the nucleoplasmic channels. it seems that the pore number is related to pore density and the number of RNA molecules being transported from the nucleus. The pores are also arranged in rows or hexagonal clusters. The pores contain some electron dense materials and are enclosed by some annular material, which is granular. These circular structures are called annuli, which along with pores form a pore complex. The annulus is ring like and consists of eight granules. The matrix of the annulus may be amorphous or fibrillar. The pore complex is a rigid structure.

13. Nuclear matrix The body of the nucleus within the nuclear envelope is made up of a dense jelly- like mass, which is composed of 2 elements, each of which is dynamic in nature and has a complex chemical composition. These elements are the nuclear gel or karyolymph and the chromatin. The nuclear gel is highly granular, containing fibrous material. It is rich in proteins, which account for more than 90% of the material. A small percentage of DNA, RNA and phospholipids is also present. The proteins are largely acidic and they have an unusual property of expanding and contracting under the influence of Ca 2+ and Mg 2+ ions. In this region events like replication of DNA, transcription and transport of substances take place.

14. Nucleolus In addition to nuclear gel and chromatin, the nucleus contains one or more dense bodies called nucleoli or plasmosomes, which are present in all eukaryotic nuclei except sperms, some algae and cleavage cells of amphibians. Normally there is one nucleolus per nucleus, cell lacking nucleolus show little or no protein synthesis. It is characterized by a dense, heavy staining interior surrounded by a light staining external layer. A typical eukaryotic nucleus contains one large or two smaller nucleoli, which are situated in the nucleus along certain chromosomes. These chromosomes have specific sites or nucleolar organizing regions, which are not only responsible for their synthesis but also organize them into dense bodies. In some cases the nucleolar region is surrounded by a ring of chromatin material, which is Feulgen positive. This chromatin is fibrillar and represents the heterochromatin region, sometimes extending into the nucleolus. The fibrils are supposed to be the DNA, representing the nucleolar organizing region, and the granular region consists of ribosomal precursor particles.

15. Chromatin It is regarded as the most important constituent of the nucleus because it forms the basis of heredity. It contains DNA, RNA and protein in a compact form, in which most of the DNA sequences are functionally inactive and inaccessible. The inaccessible is due to binding of proteins. It is constructed of several subunits each of which is composed of 200 base pairs organized by basic proteins called histones, into a beadlike structure. The DNA cannot be directly packages into chromatin, therefore it has several layers of organization.

16. Nucleosome Experimental evidence indicates that the chromatin is like a string of beads, each bead is discrete unit called nucleosome. Individual nucleosomes are connected by a fine thread, the free DNA duplex. Nucleosomes can be isolated by treating chromatin with an endonuclease, micrococcal nuclease, which cuts the DNA in between the nucleosomes. The monomer of nucleosome consists of 200 bases with associated histone octamer, comprising 2 copies of each H2A, H2B, H3 and H4 called core histones. Associated with each nucleosome is a single molecule of H1 histone.

17. FUNCTIONS Nuclear envelope - it separates the cell interior into 2 compartments- cytoplasm and nucleoplasm. It acts as a means of transport of substances between the 2 compartments. Normally materials passes from the nucleoplasm enter the perinuclear space and from there to the cisternae of ER. It may acts as a diffusion barrier to small cations as well as anions. Nuclear matrix - DNA replication, transcription and post transcriptional processing. RNA undergoes post transcriptional changes in the matrix itself where it is processed to smaller RNA pieces, which ultimately pass through the nuclear pores into the cytosol. Nucleolus - transcription of genes that code for rRNA, processing of periribosomal molecule and assembly of ribosomal subunits. The genes associated with nucleolus are rich in specific bases and located on specific regions of the chromosome. It is the site for synthesis of ribosomal precursors whereas other regions of the cell contribute to the components of ribosomes. It is the location where the molecular components are assembles into ribosomal particles which are exported to cytosol to be engages in protein synthesis.

18. CONCLUSION Nucleus is considered to be the core of the cell which commands all metabolic events. It was described in 1831 by Robert Brown. As early as 1869, Miescher suggested that DNA is an important component of the nucleus. In 1888, Waldeyer observed rod like structures in the nucleus, and designated them as chromosomes. In prokaryotes, the information centre resides in a region of the cell called the nucleoid or genophore. 2 important functions of nucleus are- it is repository of genetic information It directs all biosynthetic activities of the cell. The role of nucleus in controlling all metabolic activities was elucidated by nucleus.

19. REFERENCE CELL BIOLOGY 2 nd edition, S C RASTOGI , new age international publishers. page no. 259-289. CELL BIOLOGY ( cytology, biomolecules and molecular biology), Dr. P S Verma, Dr. V K A garwal. S chand & company pvt. ltd. page no.631-647.