First stage
BiologyLec 2
28/12/2015
د. بثينة
The nucleusThe Nucleus is highly specialized organelle that serve as the information processing and administrative center of the cell. This organelle has two major functions: it stores the cells hereditary material, or DNA, and it coordinated the cells activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division). Its position may change from time to time according to the metabolic status of the cell. Usually the cells contain single nucleus but the number of nucleus may vary from cell to cell. On the basis of number of nuclei present, cells are classified as mononucleate(example;smooth muscle cell). Binucleate(example; liver cells, cartilage cells) and polynucleated cells(syncytial cells)(example; osteoclast).The shape of the nucleus normally related to the shape of the cell, but certain nuclei are almost irregular in shape. In a tumor, the presence of nuclei with irregular features (e.g., variable size, atypical chromatin patterns) and the capacity to invade neighboring tissues are the main morphologic characteristic used by pathologists to estimate the malignancy.
Ultra structure of the nucleus
The nucleus is composed of four main parts, the nuclear envelope, nuclear sap or nucleoplasm, the nucleolus and the chromatin fibers.
The nuclear envelope:
The nuclear envelope has a complex structure, consisting of:
(1)- Two nuclear membranes
(2)- Underlying nuclear lamina
(3)- Nuclear pore complexes.
The nuclear envelope is a double-layered membrane that encloses the content of the nucleus during most of the cell's life cycle. The space between the layers is called peri-nuclear space and appears to connect with the rough endoplasmic reticulum, where protein synthesis occurs. The inner surface has a protein lining called nuclear lamina, which binds to chromatin and other nuclear components. During mitosis, or cell
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Structure of the nucleus
division, the nuclear envelope disintegrates, but reforms as the two cells complete their formation.Lamin gene expression depends on the cell type and stage of development. All nuclei of higher eukaryotes, including early embryos, have a lamina that contains lamin B-family subunits, loss of which is lethal. Lamins A&C typically appear only later in development as cells begin to differentiate. This variation in lamina composition may affect chromosome organization, possibly contributing to different patterns of gene expression.
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Nuclear pores:
The envelope is perforated with tiny holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and the cytoplasmDetailed structural studies indicate that the nuclear pore complex consists of assembly of eight spokes arranged around a central channel.
The nuclear pores( nuclear pore complex) appear circular in surface view and have a diameter between 10-100nm.the nuclear pore complex is generally permeable for molecules of 5-500 Daltons size.
The nucleotides, ions and many other molecules have easy access into the nucleus. But proteins of more than 50 KD cannot diffuse passively; they are transported through ATP dependent (active process) pathway.
Nuclear pores
Nucleoplasm:
Nuclear sap is colorless, transparent, granular, semifluid matrix found inside the nucleus is called nucleoplasm. Within the nucleoplasm, most of the nuclear material consists of chromatin, the less condensed form of the
cell's DNA that organizes to form chromosomes during mitosis or cell division. It consists of mainly the nucleoproteins and other organic and inorganic substances like nucleic acids, proteins, enzymes and minerals.
The nucleolus:
Through the microscope, the nucleolus looks like a large dark spot within the nucleus, that manufactures ribosomes. The number of nucleoli in the nucleus depends on the species and the number of chromosomes.
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After a cell division, a nucleolus is formed when chromosomes are brought into nuclear organizing regions.
Nucleoli are typically composed of three morphologically distinct regions which can be visualized by electron microscopy(EM):
Fibrillar center(FC):it is highly stained inner most region of nucleolus composed of fibrils that occupies 1-2% of the total volume. The RNA genes of nucleolar organizer of chromosomes are located in this region.
Dense fibriller centers(DFC): it surrounds the Fc's, composed of densely packed fibrils and occupies a large fraction of the nucleolus (about 17%). The biogenesis of RNA takes place in this region.
Granular region (GR):it is the largest and outer most fraction of the total nucleolus volume (about 75%). At this region the processing and maturation of pre-ribosomal particles occurs.
Chromatin and chromosomes:
They are thread-like structures located inside the nucleus of animal and plant cells. The complexes between eukaryotic DNA& Proteins are called chromatin, which typically contains about twice as much as much protein as DNA. The major proteins are the histones, small proteins containing a high proportion of basic amino acids ( arginine and lysine) that facilitate binding to the negatively charge DNA molecules, there are
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five major types of histone called H1, H2A, H2B, H3, H4 , which are very similar among different species of eukaryotes. Histone proteins are involved in a range of activities, including DNA replication and gene expression.
Partial digestion of chromatin with micrococcal nuclease, the enzyme can attack DNA on sites separated by approximately 200 base pairs.. consistent with this notion, electron microscopy revealed that chromatin fibers have a beaded appearance, with the beads spaced at intervals of approximately 200 base pairs. Thus, both nuclease digestion and the E.M studies suggested that chromatin is composed of repeating 200-base pairs units, which are called (nucleosome).
More extensive digestion of chromatin with micrococcal nuclease was found to yield particles (nucleosome core particles). Each one contain 146 base pairs of DNA wrapped 1.75 times around a histone core consisting of two molecules each of the H2A,H2B,H3,H4 (the core histone).
One molecules of H1 is bound the DNA as it enters and exits each nucleosome core particles, this form as chromatin subunit known as
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Structure of nucleosome
(7)( Chromatosome) consists of 166 base pairs of DNA wrapped around the histone core held by H1(a linker histone).
Nonhistone proteins are major component of genetic regulatory system, acting as, activator, inhibitors and enzymes.
The chromatin can then further condensed by coiling into 30nm fibers (chromosomes). The number and characteristic of chromosomes encountes in an individual are known as the karyotypes (alterations associated with ( tumors, leukemia's, and several types of genetic diseases)
human cell contains 46 chromosomes arranged in 23 pairs, each pair are identical with each other.
The extent of chromatin condensation varies during the life cycle of the cell.
In interphase (non living cell) most of the chromatin called (euchromatin),is genetically active, is relatively decondensed and distributed through the nucleus. During this period of the cell cycle, gene
are transcribed and the DNA is replicated in preparation for cell division.
A bout 10% of interphase chromatin (called heterochromatin) is a very highly condensed state that resembles the chromatin of cell undergoing mitosis.
Genetic defects in nuclear envelope
Genetic defects in nuclear envelope proteins are known to cause at least 14 disorders, including muscular dystrophies, lipodystrophies, and neuropathies ( diseases of striated muscle, fatty tissue & the nervous system). The most dramatic of these is Hutchinson-Gilford progeria. Affected individuals are essentially normal at birth, but they appear to age rapidly and die in their early teens of symptoms (including atherosclerosis & heart failure) that are typically associated with extreme age.
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عاند الدنيا وابتسم ان بعد الليل فجر يرتســـم
لاتقل حظي قليل انما قل هذا قدري وماقسمMohammed J Al-Rawi
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