Nucleas and chromosome.pptx

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1. welcome

2. Nucleus and Chromosome their Structure, Morphology, Origin, Composition and Function

3. Introduction: Discovery of Nucleus by Ernest Rutherford ( 1871-1937) Nucleus theory given in 1910, An atom’s mass is mostly in the nucleus and it has a positive charge. Prominent and Characteristics Features Eukaryon means True nucleus Basis of eukaryote- membrane bounded nucleus Imp function: -Physically separates DNA from the cytoplasm’s complex metabolic machinery - Nuclear membrane serve as boundry

4. Definition: The nucleus is the genetic control centre of a eukaryotic cell. In most cells, there is only one nucleus. It is spherical, and the most prominent part of the cell, making up 10% of the cell’s volume. It has a unique structure and function that is essential for the ce ll .

5. Components of Nucleus: 1. Nuclear Envelope – pore riddled 2. Nucleoplasm – Fluid interior portion 3. Nucleolus – Dense cluster of RNA & Proteins 4. Chromatin – all DNA+ Proteins Average diameter of nucleus is 6 µm, which occupies around 10% of cell volume.

6. Ultrastructure Of Nucleus:

7. Nuclear Membrane: Also known as nuclear envelope or nucleolemma . Separates the nuclear material from cytoplasm. Consists of two lipid bilayers: Outer membrane, Inner membrane The nuclear envelope is a double-layered membrane perforated with pores, which control the flow of material going in and out of the nucleus. The outer layer is connected to the endoplasmic reticulum, communicating with the cytoplasm of the cell. The exchange of the large molecules (protein and RNA) between the nucleus and cytoplasm happens here.

8. Function Of Nuclear Membrane:  Shape and Stability: Helps the Nucleus From Collapsing Compartmentalizing: Separates The Nuclear Material From Cellular Material Regulation Of Substances: Allow The Exchange Of Materials Communication: Develops A Chemical Connection Between Nucleus And Cell

9. The Nuclear Pore: Most distinctive feature of NE Small cylindrical channels- direct contact cytosol & Nucleoplasm Readily visible – freeze fracture microscopy (a specimen is frozen rapidly and cracked on a plane through the tissue) Mammalian nucleus – 3000 to 4000 pores Inner & outer membranes fused Structural complexity – control transport of key molecules

10. Function Of Nuclear Pore:  Exchange of materials between nucleus and cytoplasm Passive diffusion of low molecular weight solutes Efficient passage through the complex it requires several protein factors.

11. Nucleoplasm: A jelly-like (made mostly of water) matrix within the nucleus Just like the cytoplasm found inside a cell, the nucleus contains nucleoplasm, also known as karyoplasm All the other materials “float” inside Helps the nucleus keep its shape and serves as the median for the transportation of important molecules within the nucleus The nucleoplasm is a type of protoplasm that is made up mostly of water, a mixture of various molecules, and dissolved ions It is completely enclosed within the nuclear membrane or nuclear envelope

12. Nucleolus: Ribosome factory Large, prominent structures Doesn’t have membrane Most cells have 2 or more Directs synthesis of RNA The nucleolus takes up around 25% of the volume of the nucleus. This structure is made up of proteins and ribonucleic acids ( rna ). Its main function is to synthesis ribosomal RNA( rrna ) and combine it with proteins.

13.   Function of nucleolus: Site for transcription ( cell makes RNA copy from fragment of DNA) Association of ribosomes Synthesis of ribosomes Synthesis of RNA

14. Chromosome: Chromosome means: (chroma - colour, some - body) A chromosome is a thread-like self-replicating genetic structure containing organized DNA molecule package found in the nucleus of the cell. Chromosomes are seen during metaphase stage of mitosis when the cells are stained with suitable basic dye and viewed under light microscope. E. Strasburger in 1875 discovered thread-like structures which appeared during cell division. In all types of higher organisms (eukaryote), the well organized nucleus contains definite number of chromosomes of definite size and shape.

15. H. G. Waldeyer coined the term chromosome first time in 1888. The somatic chromosome number is the number of chromosomes found in somatic cell and is represented by 2n (Diploid). The gametic chromosome number is half of the somatic chromosome numbers and represented by n (Haploid). The two copies of chromosome are ordinarily identical in morphology, gene content and gene order, they are known as homologous chromosomes.

16. Chromosomes are of two types: Autosomes: that control characters other than sex characters or carry genes for somatic characters. Sex chromosomes ( Gonosomes ): Chromosomes involved in sex determination. Humans and most other mammals have two sex chromosomes X & Y, also called heterosome . Females have two X chromosomes in diploid cells; males have an X and a Y chromosome. In birds the female (ZW) is hetero-gametic and male (ZZ) is homo-gametic.

17. The size of chromosome is normally measured at mitotic metaphase and may be as short as 0.25 µm in fungi and birds, or as long as 30 µm in some plants like Trillium. Each chromosome has two arms - p (the shorter of the two) and q (the longer). Chromosome shape is usually observed at anaphase, when the position of primary constriction (centromere) determines chromosome shape. This constriction or centromere can be terminal, sub-terminal or median in position.

18. Organism No. chromosomes  Human 46 Chimpanzee 48 Dog 78 Horse 64 Chicken 78 Goldfish 94 Mosquito 6 Round worm 2 Organism No. chromosomes  wheat 42 Rice 24 Carrot 20 Tomato 24 Tobacco 48 Maize 20 Bajara 14

19. Chromosome Morphology: Mitotic metaphase is the most suitable stage for studies on chromosome morphology. The chromosome morphology changes during cell division. Chromosomes are thin, coiled, elastic, thread-like structures during the interphase. As cells enter mitosis, their chromosomes become highly condensed so that they can be distributed to daughter cells. In mitotic metaphase chromosomes, the following structural features can be seen under

20. Chromatid: Each metaphase chromosome appears to be longitudinally divided into two identical parts each of which is called chromatid. Both the chromatids of a chromosome appear to be joined together at a point known as centromere. The two chromatids of chromosome separate from each other during mitotic anaphase (and during anaphase II of meiosis) and move towards opposite poles. Since the two chromatids making up a chromosome are produced through replication of a single chromatid during synthesis (S) phase of interphase, they are referred to as sister chromatids. In contrast, the ch r omatids of homologous chromosomes are known as non-sister chromatids.

21. Two types of chro matids Euchromatin which undergoes the normal process of condensation and decondensation in the cell cycle. Heteo chromatin which remain in a highly condensed state throughout the cell, even during interphase. Chemical composition of chromatid : DNA= 20-40 %- most important chemical constituent of chromatin. RNA=05-10 %-associated with chromatin as; Ribosomal RNA-( rRNA) Messenger RNA- (mRNA) Transfer RNA- (tRNA). PROTEINS=55-60%-associated with chromatin as, Histones : - Very basic proteins + ve charged at neutral P H, constitute about 60% of total protein, almost 1:1 ratio with DNA. Non-Histones : - They are 20% of total chromatin protein

22. Centromere (Primary constriction ) Centromere is the landmark for identification of chromosome. Each chromosome has a constriction point called the centromere (Synonym: Kinetochore), which divides the chromosome into two sections or arms. The short arm of the chromosome is labeled the "p" arm. The long arm of the chromosome is labeled the "q" arm. Telomere The two ends of a chromosome are known as telomeres, they play critical roles in chromosome replication and maintenance of chromosomal length. The telomeres are highly stable and telomeres of different chromosomes do not fuse. The telomeric region of chromosome is made up of repetative sequence of T and G bases.

23. Secondary constriction In some chromosome addition to centromere / primary constriction, one or more constrictions in the chromosome are present termed secondary constrictions. Satellite The chromosomal region between the secondary constriction and nearest telomere is called as satellite and chromosomes that possess this region called as satellite chromosome or sat chromosome. A small chromosomal segment separated from the main body of the chromosome by a secondary constriction is called Satellite.

24. Size of the chromosome: The size of the chromosome varies from stage to stage of cell division. The chromosomes are the longest and thinnest during interphase (resting stage) and hence not visible under light microscope. Chromosomes are the smallest and thickest during mitotic metaphase. Chromosome size is not proportional to the number of genes present on the chromosome. The location of the centromere on each chromosome gives the chromosome its characteristic shape.

25. Types of chromosome based on centromere: Metacentric chromosome:   The centromere is located in the centre of chromosomes, i.e. the centromere is median. The centromere is localized approximately midway between each end and thereby two arms are roughly equal in length. Metacentric chromosome take V shape during anaphase .

26. Submetacentric Chromosome Centromere is located on one side of the central point of a chromosome. Centromere is sub median giving one longer and one shorter arms. Submetacentric chromosome may be J or L shaped during anaphase. Acrocentric Chromosome The centromere located close to one end of chromosomes. The centromere is more terminally placed and forms very unequal arm length (The "acro-" in acrocentric refers to the Greek word for "peak"). The p (short) arm is so short that is hard to observe, but still present. Acrocentric chromosome may be rod shape during anaphase.

27. Telocentric Chromosome Centromere located at one end of chromosome (at terminal part of chromosome) lies at one end. Telocentric chromosome may be rod shape during anaphase . According to the number of the centromere the eukaryotic chromosomes may be: Acentric: without any centromere Mono centric: with one centromere Dicentric : with two centromeres Polycentric: with more than two centromeres

28. Special type of chromosome: 1. Giant chromosomes : These were first discovered by E. G. Balbiani in 1882. They are made up of several dark staining regions called “bands”. It can be separated by relatively light or non-staining “ interband ” regions. The bands in Drosophila giant chromosome are visible even without staining, but after staining they become very sharp and clear. T hese chromosomes are also known as “Polytene chromosome”, and the condition is referred to as “Polytene” 2. Lampbrush Chromosome : These were first observed by W. Flemming in 1882. It was given this name because it is similar in appearance to the brushes used to clean lamp chimneys in centuries past. These are found in oocytic nuclei of vertebrates (sharks, amphibians, reptiles and birds)as well as in invertebrates (Sagitta, sepia, Ehinaster and several species of insects).

29. 3. Accessory chromosomes :- In many species some chromosomes are found in addition to normal somatic chromosomes. These extra chromosomes are called accessory chromosomes or B- chromosomes or supernumerary chromosomes. These chromosomes are broadly similar to normal somatic chromosomes in their morphology For instance, presence of several such chromosomes often leads to reduction in vigour and fertility in males. Origin of these chromosomes in most species is unknown . 4. Isochromosomes :- An isochromosome is the one in which two arms are identical with each other in gene content and morphology. Every isochromosome is metacentric. The attached ‘x’ chromosome of Drosophila is a classical example of an isochromosome. However its origin is uncertain.

30. 5. Allosomes / sex chromosomes :- Chromosomes differing in morphology and number in male and female are called allosomes . They are responsible for determination of sex. e g : X and Y chromosomes in human beings and Drosophila. Chromosomes which have no relation with determination of sex and contain genes which determine somatic characters of individuals are called autosomes and are represented by letter ‘A’. 

31. Variation in chromosome number: Organism with one complete set of chromosomes is said to be euploid (applies to haploid and diploid organisms). Aneuploidy - variation in the number of individual chromosomes (but not the total number of sets of chromosomes). The discovery of aneuploidy dates back to 1916 when Bridges discovered XO male and XXY female Drosophila, which had 7 and 9 chromosomes respectively, instead of normal 8.

32. Nullisomy - loss of one homologous chromosome pair. (e.g., Oat ) Monosomy - loss of a single chromosome (Maize). Trisomy - one extra chromosome. (Datura) Tetrasomy - one extra chromosome pair. 

33. Chromosomal Aberrations: The somatic (2n) and gametic (n) chromosome numbers of a species ordinarily remain constant. This is due to the extremely precise mitotic and meiotic cell division. Somatic cells of a diploid species contain two copies of each chromosome, which are called homologous chromosome. Each chromosome of a genome contains a definite numbers and kinds of genes, which are arranged in a definite sequence.

34. Sometime due to mutation or spontaneous (without any known causal factors), variation in chromosomal number or structure do arise in nature. - Chromosomal aberrations. Chromosomal aberration may be grouped into two broad classes: 1. Structural 2. Numerical

35. Structural Chromosomal Aberrations: Chromosome structure variations result from chromosome breakage. Broken chromosomes tend to re-join; if there is more than one break, re-joining occurs at random and not necessarily with the correct ends. The result is structural changes in the chromosomes. Chromosome breakage is caused by X-rays, various chemicals, and can also occur spontaneously. There are four common type of structural aberrations: 1. Deletion or Deficiency 2. Duplication or Repeat 3. Inversion 4. Translocation.

36. Consider a normal chromosome with genes in alphabetical order: a b c d e f g h i 1. Deletion: part of the chromosome has been removed: a b c g h i 2. Dupliction : part of the chromosome is duplicated: a b c d e f d e f g h i 3. Inversion: part of the chromosome has been re-inserted in reverse order: a b c f e d g h i 4. Translocation: parts of two non-homologous chromosomes are joined: If one normal chromosome is a b c d e f g h i and the other chromosome is u v w x y z, then a translocation between them would be a b c d e f x y z and u v w g h i .

37. A New Chromosome Model Chromosomes and their function have been well known for more than 100 years. The three-dimensional architecture of chromosomes however, still a matter of intense discussion. In general, metaphase chromosomes consist of the following elements: A pair of two chromosome arms of either equal or unequal length which splits into two chromatides during metaphase. There is one obligatory constriction which defines the “centromere” and a facultative constriction which represents the “satellite region.” The chromatides are terminated by telomeres. G. Wanner and H. Formanek August 22, 2000 Munich, Germany Case study: 1

38. The structural compound of chromosomes, the chromatin, consists of equal amounts of DNA, histones and nonhistone proteins (Earnshaw, 1991). Chromatin consists of “euchromatin,” which condenses (and stains intensely) during mitosis, and “heterochromatin,” which remains condensed during interphase also ( Passarge , 1979; Traut , 1991a).

39. Methodology: Human and plant chromosomes were prepared for high-resolution scanning electron microscopy as described (Martin et al., 1994, 1996). Controlled decondensation of (unfixed) metaphase chromosomes was achieved by treatment with citrate buffer (60 mM, pH 7.2) for 60 min at room temperature. For proteinase treatment chromosomes were first fixed with glutaraldehyde (2.5% in 75 mM cacodylate buffer) and then treated with proteinase K (0.1–1 mg/ml) for 30 min at 37°C. For separate visualization of DNA, chromosomes were stained for 30 min at 20°C with 1% zirconiumchloridoxide in 1% hydrochloric acid.

40. Proteins were separately visualized after staining for 12 h at 60°C with 20% aqueous silver nitrate solution. The colloidal silver solution was prepared in the following way: 0.5 g silver nitrate dissolved in 1.5 ml water was slowly added to 25 ml of an aqueous solution of 0.25% tannic acid and 2% sodium carbonate by continuous stirring at room temperature. A slight precipitate forms within 24 h which is removed by centrifugation (Lea, 1891; Gmelin , 1971). After the specimens were washed with aqua dest . (three times for 5 min at 2°C) they were dehydrated through a graded acetone series (20–100%) and critical point dried with liquid CO 2 . Chromosome spreads were preselected with a light microscope.

41. Result From their investigations it is obvious that there are two dominant structural elements in metaphase chromosomes: coiled chromomeres and parallel matrix fibers . Since 1929 these heterochromatic structures have been called “chromomeres” During metaphase they can be observed only (if at all) in the centromeric and satellite regions. They become visible best after controlled decondensation of metaphase chromosomes by proteinase K treatment.

42. According to our ultrastructural investigations, they propose a new model for the three-dimensional structure of chromosomes. The model can simply explain the enormous variety of chromosome morphology in plant and animal systems by varying only a few cytological parameters.

43. The Structure of the Nucleus Studied by Electron Microscopy in Ultrathin Sections with Special Reference to the Chromonema-An Advocation of " Subchromonema " and " Protochromonema " The nucleus was the first intracellular structure discovered and was originally described by Franz Bauer in 1802 and later popularized by Robert Brown. The purpose of study to obtain a precise knowledge concerning the structure of the nucleus, especially (a) the finest structure of the chromosome, (b) the chromosomal constrictions and (c) various nucleolar structures found in resting nuclei. Kyoto, Japan Shigeyasu Amano et. al March 30, 1956 Case study: 2

44. Methodology: Plasma cells, monocytes and lymphocytes in mice were utilized in this study. Referring to the procedure to get ultrathin sections and electron microphotographs. ( Dohi , 1955) Observations: Chromonema and subchromonema in the nucleus of plasma cell Chromosome, chromonema and subchromonema in the mitotic nucleus of lymphocyte Chromonema and subchromonema in the resting nuclei of lymphocytes.

45. Result: In their previous phase microscope studies, various interkinetic nuclei were described, in which the chromonema structure is either (a) uncoiled completely. The cells belonging to the latter group (b) such as lymphocytes, monocytes and plasma cells in mice, referring especially to their chromonema structure as seen in ultrathin section by an electron microscope. The proto chromonemata (coiled chromatin thread within a single chromosome) belong to the morphological limit capable of being observed by an electron microscope in ultrathin sections. observation was confirmed also with a structure of the more or less loosely coiled chromosomes found in mitotic lymphocytes in prophase. The size and structure of the chromonema and subchromonema in mitotic and interkinetic stages were studied.

46. Reference: Genome 4 by T. A. Brown Life Science (Fundamental and practice) by Pranav Kumar and Usha meena B D Singh