Cell nucleus

Nucleus and it's components .

1. NUCLEUS NAME-BASANT KUMAR MSC APPLIED GENETICS

2. INTRODUCTION The  nucleus is a membrane-enclosed organelle found in eukaryotic cells. The  nucleus  (pl.  nuclei) is derived from Latin word  nucleus  or  nuculeus which means kernel  or seed. The nucleus is a highly specialized organelle that serves as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).

3. The spherical nucleus typically occupies about 10 percent of a eukaryotic cell's volume, making it one of the cell's most prominent features. Only the cells of advanced organisms, known as  eukaryotes , have a nucleus. Generally there is only one nucleus per cell, but there are exceptions, such as the cells of slime molds and the  Siphonales  group of algae. Simpler one-celled organisms ( prokaryotes ), like the bacteria and cyanobacteria, don't have a nucleus. In these organisms, all of the cell's information and administrative functions are dispersed throughout the cytoplasm.

4. STRUCTURE OF NUCLEUS The nucleus is the largest cellular organelle in animal cells. In mammalian cells, the average diameter of the nucleus is approximately 6 µm, which occupies about 10% of the total cell volume.  The viscous liquid within it is called nucleoplasm (or karyolymph ), and is similar in composition to the cytosol found outside the nucleus. It appears as a dense, roughly spherical or irregular organelle.

5. …. CONTD The composition by dry weight of the nucleus is approximately: DNA -9%,RNA-1%,Histone protein-11%,residual protein-14%,acidic protein-65% In some types of white blood cells specifically most granulocytes the nucleus is  lobated and can be present as a bi-lobed, tri-lobed or multi-lobed organelle.

6. COMPONENTS OF NUCLEUS 1. NUCLEAR ENVELOPE:- The nuclear envelope also known as nuclear membrane, consists of two cellular membranes, an inner and an outer membrane. The nuclear envelope is a highly regulated membrane barrier that separates the nucleus from the cytoplasm in eukaryotic cells. It contains a large number of different proteins that have been implicated in chromatin organization and gene regulation.

7. (a) OUTER NUCLEAR MEMBRANE:- The nuclear membrane is a lipid bilayer.It consists of two layers of lipid molecules. The outer layer of lipids has ribosomes, structures that make proteins on its surface. It is connected to the endoplasmic reticulum, a cell structure that packages and transports proteins.

8. (b) INNER NUCLEAR MEMBRANE:- The inner membrane contains proteins that helps to organize the nucleus and tether genetic material in place. (c) NUCLEAR LAMINA:- The network of fibers and proteins attached to the inner membrane is called the nuclear lamina. It structurally supports the nucleus, plays a role in repairing DNA, and regulates events in the cell cycle such as cell division and the replication of DNA. The nuclear lamina is only found in animal cells, although plant cells may have some similar proteins on the inner membrane.

9. (d) PERINUCLEAR SPACE:- The perinuclear space is the lumenal space between  the inner and outer nuclear envelope bilayers that separate the nucleus from the cytoplasm. Because the outer nuclear membrane is continuous with the endoplasmic reticulum, the perinuclear space is continuous with the lumen of the endoplasmic reticulum.

10. 2. NUCLEOPLASM:- The nucleoplasm is a highly viscous liquid contained within the nucleus that surrounds the chromosomes and other subnuclear organelles. A network of fibers known as the nuclear matrix can also be found in the nucleoplasm.The nucleoplasm acts as a suspension medium for components of the nucleus including the  nucleolus,and  chromatin.   Nucleotides required for DNA replication and enzymes involved in other nuclear processes are also found dissolved within the nucleoplasm.

11. 3. NUCLEOLUS:- The nucleolus is the nuclear subdomain that assembles ribosomal subunits in eukaryotic cell. The nucleolus, whose primary function is to assemble ribosomes, is the largest structure in the cell nucleus  The nucleolar organiser regions of chromosomes, which contain the genes for pre‐ribosomal ribonucleic acid ( rRNA ), serve as the foundation for nucleolar structure.

12. The nucleolus disassembles at the beginning of mitosis, its components disperse in various parts of the cell and reassembly occurs during telophase and early G1 phase. Ribosome assembly begins with transcription of pre‐ rRNA . During transcription, ribosomal and non‐ribosomal proteins attach to the rRNA . Subsequently, there is modification and cleavage of pre‐ rRNA and incorporation of more ribosomal proteins and 5S rRNA into maturing pre‐ribosomal complexes. The nucleolus also contains proteins and RNAs that are not related to ribosome assembly and a number of new functions for the nucleolus have been identified.  These include assembly of signal recognition particles, sensing cellular stress and transport of human immunodeficiency virus 1 (HIV‐1) messenger RNA .

13. 4. CHROMATIN:- Chromatin  is a mass of genetic material composed of DNA and proteins that condense to form chromosomes during eukaryotic cell division. Chromatin is located in the nucleus of our cells. The primary function of chromatin is to compress the DNA into a compact unit that will be less voluminous and can fit within the nucleus. Chromatin consists of complexes of small proteins known as histones and DNA .

14. Histones help to organize DNA into structures called nucleosomes by providing a base on which the DNA can be wrapped around. A nucleosome consists of a DNA sequence of about 150 base pairs that is wrapped around a set of eight histones called an octamer. The nucleosome is further folded to produce a chromatin fiber. Chromatin fibers are coiled and condensed to form chromosomes. ……CONTD

15. Chromatin makes it possible for a number of cell processes to occur including DNA replication, transcription, DNA repair, genetic recombination, and cell division. The primary protein components of chromatin are histones, which bind to DNA and function as "anchors" around which the strands are wound. ……CONTD

16. CHROMATIN ORGANIZATION In general, there are three levels of chromatin organization: DNA wraps around histone proteins, forming nucleosomes and the so-called "beads on a string" structure ( euchromatin ). Multiple histones wrap into a 30-nanometer  fibre consisting of nucleosome arrays in their most compact form (heterochromatin).

17. Higher-level DNA supercoiling of the 30-nm fiber produces the metaphase chromosome (during mitosis and meiosis). The overall structure of the chromatin network further depends on the stage of the cell cycle. During interphase, the chromatin is structurally loose to allow access to RNA and DNA polymerases that transcribe and replicate the DNA. ……CONTD

18. HETEROCHROMATIN Heterochromatin is very tightly packed and condensed. The formation of heterochromatin involves modifications to histones and the recruitment and spreading of silencing complexes, which cause changes to the structure of chromatin. These changes affect various DNA processes, including gene transcription.

19. Euchromatin consists of loosely wrapped chromatin, so the DNA is more accessible. Modifications to the histone tails allow the chromatin to be more open. Euchromatin is the part of the chromatin involved in the active transcription of DNA into mRNA. As euchromatin is more open in order to allow the recruitment of RNA polymerase complexes and gene regulatory proteins, so transcription can be initiated. EUCHROMATIN

20. It controls the heredity characteristics of an organism. It is responsible for protein synthesis, cell division, growth and differentiation.  Stores heredity material in the form of deoxy -ribonucleic acid (DNA) strands.Also stores proteins and ribonucleic acid (RNA) in the nucleolus.  It is a site for transcription process in which messenger RNA (m RNA) are produced for protein synthesis. FUNCTION OF NUCLEUS

21. REFERENCES:- CELL BIOLOGY BY CB.POWAR THE CELL BY GEOFFREY M.COOPER CELL & MOLECULAR BIOLOGY BY ROBERTIS https//www.ncbi.nlm.nih.gov