Cell Nucleus PDF

Cell Nucleus  Learning Objectives Define the term, ‘interphase nucleus’ List the components of the interphase eukaryotic cell nucleus Differentiate between euchromatin and heterochromatin on the functional basis of gene transcription and on a visual basis by electron mic...

Cell Nucleus  Learning Objectives Define the term, ‘interphase nucleus’ List the components of the interphase eukaryotic cell nucleus Differentiate between euchromatin and heterochromatin on the functional basis of gene transcription and on a visual basis by electron microscopy Diagram how DNA is packaged into chromosomes and define the contents of the nucleosome, the fiber, and the chromosome List the function and the parts of the nucleolus Describe how proteins and ribosomes are transported across the nuclear pore complex Define the terms karyolysis, pyknosis and karyorrhexis Be able to identify the nucleus, nucleolus, heterochromatin, and euchromatin in a hematoxylin and eosin stained tissue section  The interphase nucleus consists of the following: The Nucleus Chromatin Nucleolus Nuclear Envelope ▪ Is a membrane limited Nucleoplasm compartment that contains the genetic information in eukaryotic cells ▪ DNA replication and RNA transcription happens here ▪ The nucleus of a non- dividing cell is called the interphase nucleus Components of the Nucleus Summary of the functions of the Nucleus Chromatin ▪ Chromatin is a highly organized complex of DNA and proteins ▪ A principal component of the cell nucleus ▪ Histone proteins help organize DNA into structural units: ▪ Nucleosomes: ▪ Which are then assembled into a compact structure: Chromatin: ▪ and eventually into very large, high-order structures: ▪ Chromosomes Chromatin cont. ▪ Under the microscope in its extended form, chromatin looks like beads on a string ▪ The beads are called nucleosomes ▪ Each nucleosome is composed of DNA wrapped around eight proteins called histones ▪ Nucleosomes are supported by an additional histone called H 1 histone: ▪ Chromatosome ▪ The nucleosomes then fold into a 30 nm spiral called a solenoid: ▪ During cell division, the structure of Folding the chromatin and chromosomes are visible under a light microscope ▪ and they change in shape as the DNA is duplicated and separated into two cells © 2013 Nature Education Adapted from Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed  Summary Folding Name this structure  Euchromatin vs. Heterochromatin Heterochromatin: Definition: Condensed and transcriptionally less active Densely packed and looks dark and granular This is DNA that is not being actively transcribed, and resting cells will have more of it Euchromatin: Definition: Decondensed and display more active gene expression Looks “clear” or lighter in microscopy It is stretched out so that gene transcription can take place It is prominent in active cells, like liver Heterochromatin and Euchromatin in cells plasma cell  Which is where? A Heterochromatin is BB spatially concentrated in two regions in the Nucleolus nucleus – Loci enriched at the perinucleolar Nucleolus associated region: Nucleolus- Karyosome associated domains (NADs) – Loci enriched at the Marginal nuclear periphery: Lamina-associated domains (LADs) Heterochromatin cont. Heterochromatin is divided into two subcategories: – Constitutive and – Facultative Constitutive heterochromatin: – Unchanged in its genomic location across the cell cycle or during differentiation – Adopts characteristic sub-cellular localizations – Mainly found at the following locations of a chromosome: Telomeres Centromeres, and Adjacent silent regions Facultative heterochromatin: – Transcriptionally inactive loci that are less condensed – Can become transcriptionally active at specific developmental stages – Ex: The inactive X chromosome in a 46, XX genotype  Barr bodies are facultative heterochromatin present in all genetically female (46, XX) somatic cells and easily visualized in some cell types ▪ Genetic females have two X chromosomes in each cell ▪ Only one of the chromosomes is expressed ▪ The other is maintained as facultative heterochromatin: ▪ Barr body or Drumstick ▪ The presence of Barr bodies in cells Neutrophils of blood smears and can be used to identify the genetic sex epithelial cells of cheek mucosa of an individual are commonly used cells to find Barr bodies Nucleolus ▪ The nucleolus is a generally spherical, highly basophilic subdomain of nucleus ▪ Non-membranous region of the nucleus ▪ Contains transcriptionally active ribosomal RNA (rRNA) genes ▪ Primary site of ribosomal subunit production ▪ Can vary in size and number across different cell types, but is usually very visible in cells that are active in protein synthesis (like the hepatocytes shown in this picture)  Nucleolus cont. NO Shows 3 morphologically distinct regions: PF PF ▪ A Fibrillar Center (Nucleolar Organizer - NO): ▪ Contains: ▪ Chromatin containing rRNA genes ▪ RNA polymerase I PG ▪ A Dense Fibrillar Component (Pars Fibrosa - PF): ▪ rRNA processing happens here ▪ Contains new rRNA ▪ A Granular Component (Pars Granulosa - PG): ▪ Is the site of the initial assembly of ribosomal subunits (small subunit and large subunit)  Ribosomal Assembly ▪ First, the transcription of genes for ribosomal proteins occurs outside the nucleolar region ▪ Then, the mRNAs cross the nuclear envelope through the pore complexes and are translated by free ribosomes in the cytoplasm ▪ These new ribosomal proteins enter into the nucleus and reach the nucleolus, where they are associated with the rRNAs to form a complete ribosomal subunit, either the small (40S) or the large (60S) subunit ▪ Once assembled, ribosomal subunits cross the nuclear envelope toward the cytoplasm, where they start working in translation  The Nuclear Envelope ▪ Comprises two membranes separated by a perinuclear space: ▪ * Impairment of the structure of the nuclear ▪ An outer nuclear membrane (ONM): Lamina or its function can cause tissue- ▪ Continuous with the rough endoplasmic reticulum specific diseases (e.g. nerve and skeletal ▪ An inner nuclear membrane (INM) muscle development, premature aging) ▪ Mutations of the Lamin gene or Lamin ▪ Attached to the inner fibrous nuclear lamina* receptors are associated with Laminopathies ▪ The nuclear pore complex (NPC) mediates the transport of ▪ Ex: Emery-Dreifuss muscular dystrophy cellular material between the nucleus and the cytoplasm  The Nuclear Pore Complex (NPC) Fu, et al. (2018). International Journal of Molecular Sciences. 19. 1445. 10.3390/ijms19051445. ▪ About 50 different proteins make up the complex – these are collectively called nucleoporins (or NUP proteins) ▪ A central framework, surrounding the central pore is inserted between the cytoplasmic ring and the nuclear ring: ▪ The nuclear ring complex anchors a “nuclear basket”. ▪ The complex acts as a close-fitting Looks like a fish basket? diaphragm or gated channel (similar to what is observed in the cellular membrane) and allows for the transport of small molecules Note: Lamin is a fibrous protein that provides structure and regulates transcription in a cell nucleus, while laminin is a glycoprotein found in the basement membranes of most animal tissue. Outer membrane Inner membrane  Transport of molecules across the nuclear pore complex (NPC) ▪ Any protein that needs to get into the nucleus has a nuclear localization amino acid sequence (NLS) sequence: ▪ This binds to the nuclear import receptor (importin, or Imp), and the protein is directed to a NPC ▪ A GTP-dependent mechanism then transports them through the pore and recycles the Imp protein ▪ Ribosomal subunits that need to leave the nucleus have a nuclear export sequence (NES): ▪ This binds to a protein called exportin (Exp), and the process occurs in reverse ▪ Ions and water-soluble molecules (less than 9 Da in size) can cross by simple diffusion Fahrenkrog, B.(2006). Canadian journal of physiology and pharmacology. 84. 279-86. 10.1139/y05- 100.