Cytology Notes for Nucleus

NUCLEUS 1,GENERAL INTRODUCTION 1. Nucleus was first seen by Anton Van Leeuwenhoek in RBC of fish. 2. The nucleus is the most important and essential part of the cell. It directs and controls all the cellular activities and carries the activity information of the cell. 3. It is called "Director of the cell" 4. It was discovered by Robert brown (1831) 2. OCCURENCE 1. The nucleus is found in all eukaryotic cells. 2. In some cells’ nucleus are present during early stage of development 3. In prokaryotic cell the nucleus is without distinct nuclear membrane and nucleolus. 3. POSITION Generally Nucleus is located in the center but change according to metabolic activities of the cell. Example (1) In embryonic cells nucleus is generally found in the center of the cell. (2) In some glandular cell nucleus located in the basal portion of the cell. 4. NUMBER Normally cell contains only one nucleus but it may vary in different cells. 1. Two nuclei are found in paramecium (protozoa) liver cell, cartilage cell. 2. Many nuclei are found in Osteoblast, Rhizopus. Vaucheria etc. 5. SHAPE AND SIZE 1. It differs from cell to cell. It could be spherical,cuboidal,ellipsoidal, discoidal or irregular. 0 2. The size depends on the volume of the cell. Polypoids generally have bigger nuclei. They are about 10 micron in length Formula - Np = Np/Cv - Nv Np = Nucleoplasmic Index Nv = Volume of nucleus Cv = Volume of cytoplasm The size of the nucleus depend on the volume of cellan DNA, protein and metabolic activity of cell. 6. STRUCTURE The interphase nucleus is composed of the following structure (6.1) 1 Nuclear membrane (6.2) Nucleoplasm (6.3) Chromatin Network (6.4) Nucleolus 6.1. Nuclear membrane 1. 2. 3. 4. It is nuclear envelope composed of two membrane outer membrane and inner membrane. Both membranes have a unit membrane structure Each membrane is about 50 Ao thick. Both membranes are separated by a space called perinuclear space. Perinuclear space is 100-300 A° wide. 5. Outer membrane is rough due to presence of ribosome and is continuous with the unit membrane of endoplamic reticulum 6.1.1. Nuclear pore 1. Nuclear membrane is perforated by pores called nuclear pore. Each pore is 600 Ao to 1000 A° in diameter The pore may be om or polygonal. 2. Pores allow exchange of substance between nucleus and cytoplasm. 3. They allow exit of mRNA and ribosomes unit and entry of nucleotides, and many other macromolecules 4. Nuclear pore possess ring like cylindrical structure called as annulus. 5. The inner nuclear membrane is smooth as it lacks ribosome. . 6.1.2 Origin of Nuclear Membrane 1. Nuclear membrane is derived from the membrane of endoplasmic reticulum 2. During telophase cisternae of endoplasmic reticulum get collected around the chromosome and fuse with the another and result in formation of nuclear membrane. Note: Nuclear membrane disappear in prophase stage of mitosis. 6.1.3. Function of Nuclear Membrane (i) It provides definite shape to the nucleus. (ii) Exchange of genetic material takes place through nuclear membrane in between nucleoplasm and cytoplasm. (iii) Nuclear pore also exchange the macromolecule. 6.3. Chromatin network 1. Nucleoplasm contain many thread like coiled and elongated structure, known as chromatin fibre. 2. During cell division mitosis & meiosis chromatin fibre become thick rod like structure known as chromosome. 3. The fibre of chromatin are twisted in nature and two type of chromatin. 6.3.1. Heterochromatin 6.3.2. Euchromatin 6.3.1. Heterochromatin Chromatin which remain condensed during interphase is called Heterochromatin 6.3.2. Euchromatin Chromatin which is not condensed during interphase is called euch The non condensed one are called Euchromosomes. 6.3.2.1 Difference between Euchromatin & Hetero chromatin S.No. Heterochromatin Euchromatin (1) Stain deeply Stainless deep (2) Contain small amount Contain large amount of DNA of DNA large amount of RNA (3) Found in condensed Diffused and lightly coiled region chromosome region of chromosome chromosome (4) Replication late at the Replication at early phase end of phase, not acetylated acetylated (5) Less active part More active part (6) Affected by temp. Not affected by temp. (7) Inert genetically Genetically active (8) Contain few genes Has more genes (9) It is found near to It is found in the centre of nucles nuclear envelope Note: Euchromatin and heterochromatin discovered by Amil Heitz, 6.3.2.2. Chemical composition of chromatin DNA-31% RNA-2 to 5% Histone protein - 36% Non Histones -28% 6.3.2.3. Some interesting facts 1. Histone proteins are basic protein composed of arginine, lysine, and histidine. 2. Histone protein was discovered by Kossel. Note: Kossel also discovered purine and pyrimidines. 3. On the basis of quantity of arginine and lysine histone proteins are of 5 types H2A, H2B, H3, H4, and H1. 4. Nucleosome model of chromatin was discovered by R Konberg 6.4. Nucleolus 1. General introduction (i) Nucleus contains large spherical acidophilic granule known as Nucleolus. (ii) It was discovered by Fontona (1781) (iii) Term nucleolus was given by Bowman. (iv) It is large in cells that are actively busy in protein synthesis (v) It is found in small amount or not found in cell that are not active in protein synthesis. e.g. sperm, muscular cells, blastomere. It is a site within the nucleus where ribosome are made. (vi) It is most conspicuous in a non dividing cell. (vii) Each cell may possess one or more Nucleoli. 6.4.1. Structure of nucleolus Structure of nucleolus is described by Day (1968) Bernhord and Granboulan (1968) According to them, there are four chief components of nucleolus. 6.4.1.1 Matrix 1. It is composed of protein 2. Granular and fibrils remain scattered in matrix. 6.4.1.2. Granular portion It is composed of protein and RNA in the ratio of 2:1 6.4.1.3. Fibrillar portion (i) The fibrils are 50-80 A0 long. (ii) It is made up of ribonucleo protein. (iii) lt gradually gets transformed into granules. 6.4.1.4. Chromatin It is composed of DNA (Feulgen positive) Two types (a) Perinucleolar chromatin - It surrounds the nucleolus like a shell (b) Intra nucleolar chromatin - Projected into the nucleolus 6.4.2 Chemical composition of nucleolus (i) Nucleolus composed of RNA, Protein and DNA (Very small percentage) RNA is 10%, protein is 85% and DNA is 5% (ii) The proteins are exclusively phosphoprotein. (iii) RNA is similar to ribosomal RNA in base sequence and composition. 6.4.3. Function of nucleolus Following are the function of nucleolus 1. RNA synthesis -Synthesis of Ribosomal RNA (rRNA) takes place inside the nucleolus, biogenesis of ribosome 2. It plays important role in spindle formation. 7. FUNCTION OF NUCLEUS (i) Nucleus controls all metabolic activites of the cell by controlling the synthesis of enzymes. (ii) Nucleus carries chromatid material (chromosome) which is carrier of hereditary unit genes. CHROMOSOME 1. INTRODUCTION (i) Chromosomes (Gr-chrom-colour) (soma-body) are darkly stained individualized protoplasmic bodies seen only during cell division (ii) They are most distinct in metaphase stage. (iii) It was at first observed by hofmeister and Karl Nageli (iv) The term chromosome was introduced by Waldeyer in 1888 (v) Fleming named it chromatin. 2. TYPE OF CHROMOSOMES (1) Viral Chromosome (i) In virus a single molecule of DNA or RNA represent the chromosome (ii) It may be linear or circular. (2) Bacterial chromosome In bacteria and blue green algae the material (DNA) is organized into a single circular chromosome. (3) Eukaryotic chromosomes (i) In eukaryotic cell the chromosome are formed of DNA and protein, (ii) In inter phase stage they are in form of an interwoven network of chromatin thread. (iii) During cell division the chromatin thread condenses in to distinct dark stained chromosome 3. STRUCTURE OF CHROMOSOMES Chromosome consists of following parts: (1) Pellicle (i) It is an outermost covering of chromosome. (ii) Its presence is controversial. (2) Matrix (i) It is composed of protein and lipid. (ii) In it chromonemata remain embedded. (iii) Its exact function is not known. (3) Chromonemata (i) Chromonemata is 2 in number, highly twisted coiled. (ii) Veidovsky gave the name chromonemata to the coiled structure. (iii) Along the entire length of chromonemata, dense granules are found called chromomere. (iv) The coil may be either plectonemic where the coiling can be easily separated, or paranemic where the coiling cannot be easily separated. (v) Chromomere contains gene. Note: The length of chromosome is directly proportional to number of genes. (4) Centromere (i) It is also called primary constriction or kinetochore. (ii) Centromere determines shape of the chromosome by dividing chromosome, into two arms. On the basis of position of centromere chromosome are of numerous type, which are as follows. (1) Metacentric - Centromere in middle, two arms are equal so chromosome is v shaped. (2) Submetacentric - Centromere slightly away from middle so chromosome is L-shaped. (3) Acrocentric - Centromere occupies sub terminal position so one arm is very long whereass other arm is extremely short. Acrocentric chromosome is J-shaped. (4) Telocentric - Centromere occupies terminal most position so chromosome is single armed. Telocentric is i-shaped. (5) Monocentric - One centromere is found (6) Dicentric - 2 centromeres are found. (7) Polycentric - More than 2 centromeres is found. (8) Acentric - It is without centromere. (9) Diffused - Centromere diffused in chromosome Note: Diffused centromere is found in insect, some algae and some group of plant. (5) Function of centromere (i) Centromere determines shape of chromosome. (ii) It serves to provide attachment to spindle fibre (iii) It helps chromosomal movement during cell division (6) Secondary constriction - I (i) It is also called NOR (Nucleolar organizer region) (ii) It is not found in all chromosomes. It is found on chromosome number 13, 14,15,21,22, and Y chromosome. (7) Secondary constriction - II (i) Position of secondary constriction-ll is fixed in a chromosome (ii) In human being secondary constriction Il is found in chromosome number 1, 10, 13, 16 and Y chromosome. (8) Satellite (i) Found in chromosome in which secondary constriction is found (ii) It is small spherical part of chromosome distal to secondary constriction (iii) Chromosome with satellite is called SAT chromosomes. (9) Telomere (i) Tip of chromosome is known as telomere. (ii) Perform two important functions. (a) It prevents ends of two chromosomes from sticking with each other. (b) Help chromosome in attachment to nuclear envelope. (10) Karyotype A complete set of chromosome of a species is called karyotype. e.g. 23 pair of chromosome in human being is karyotype. Idiogram: If homologous chromosome pairs arranged in decreasing order, then it is called idiogram. 4. MOLECULAR STRUCTURE OF CHROMOSOME (i) Chromosome is composed of nucleoprotein. (ii) Nucleic acid constitutes 40% (35% DNA, 5% RNA) histone and other proteins constitute 60%. (iii) In a chromosome two basic proteins are found of low molecular weight. These two proteins are histone and protoamines. (iv) Histone composed of Arginine and Lysine mainly. Protoamines composed of Arginine only. (v) Protein of high molecular weight are acidic and called non histone protein. Main component tryptophan + Tyrosin. (vi) On the basis of compositive quantity of Arginine and lysine. Histone can be divided into 3 groups. (1) Very lysine rich -H1 (2) Lysine rich -H2a and H2b (3) Arginine rich -H3, H4 Note: (i) RD Kornberg and J.O. Thomas proposed nucleosome model (ii) In Radiolarians 1600 chromosome are found which is Highest (iii) In Ascaris Megalicephale 2 chromosomes are found which is Lowest (iv) In plants chromosomes are larger and few in number, whereas in animals chromosomes are smaller and more in number 5. FUNCTION OF CHROMOSOME (i) The chromosomes control the physiological activities of the organism (ii) Chromosomes are carrier of hereditary units, genes on their surfaces and transmit the characters from parent to the next generation so chromosome are known as hereditary vehicles. SPECIAL TYPE OF CHROMOSOME (GIANT CHROMOSOME) 1. LAMPBRUSH CHROMOSOME (i) Lampbrush chromosome was first observed by Flemming (1882) in amphibian oocyte (ii) A detailed study was made by J.Ruckert (1892) in the oocyte of sharks. (iii) Found in the oocytes of insect, sharks, amphibian, reptiles and birds which produce yolky eggs. (iv) These may be as long as 1000 micron and about 20 micron in width (v) In Salamandar its length is 5,900 m. (vi) Lampbrush chromosome is composed of main axis which possess series of chromomere. (vii) Main axis formed of DNA and Protein. (viii)Main axis give rise to lateral loops. Lateral loops axis remain surrounded by matrix. Matrix is composed of RNA and Protein. (ix) Lateralloop is composed of DNA (x) It is concerned with protein synthesis, yolk formation and RNA synthesis. 2.POLYTENE CHROMOSOME (i) It was discovered by balbian in 1881 (ii) Kostoff described its concern with genetics. (iii) It is found in salivary gland of chironamous larva, salivary gland, malphigian tubule, epithelial lining of drosophila, fat bodies of certain Diptera. (iv) In drosophila melanogaster it is 1000 times larger than somatic chromosome. (v) The larger size of chromosome is due to the presence of numerous chromonemata So called polytene (Many standard) chromosome (vi) Numerous chromonemata presence is because of repeated division of chromosome without cytoplasmic division which is called endomitosis. (vii) Possess two types of transverse bands dark band, which is feulgen negative which possess more RNA (a) Balbiani ring (i) The bands get enlarged at certain place and because of uncoiling of chromonemata form swelling called puffs or Balbiani ring. (ii) Balbiani ring are concerned with synthesis of mRNA Note: 1. Ecdyson hormone of insect stimulates formation of balbiani ring. 2. Polytene chromosome is related with metamorphosis of an insect. (3) Supernumerary Chromosome (i) In certain plants and animal one or more additional chromosome were observed in addition to the normal number called supernumerary chromosome. (ii) It is genetically inert. (iii) It is present in the nucleus. (iv) Its presence does not effect an organism phenotypically if its number is too many they reduce the fertility and vigour. (v) Wilson in 1905 discovered it in metapodium insect. 3. MEGACHROMOSOME It was seen in hybrid tobacco plant. Its size is 15 times more. Incomparison to normal chromosome.