MA2 Nucleus Flashcards
Nucleus - definition
a major membrane-bound organelle of eukaryotic cells containing a cell’s linear DNA (chromatin) that is separated from the cytoplasm by the nuclear envelope
Components of the nucleus
nuclear envelope, nucleolus, nuclear matrix, chromatin
Structure of the nucleus
a complex of DNA, RNA, and associated proteins enclosed in a double membrane
Functions of the nucleus
Dynamic center of the cell: synthesis, processing and storage of DNA and RNA; and synthesis of proteins (structural and regulatory)
Nuclear Envelope
It consists of two parallel membranes (outer and inner nuclear membranes) separated from each other by a narrow perinuclear cisterna (space). It is perforated at intervals by openings called nuclear pores. It forms the barrier that separates the nuclear material from the cytoplasm and anchors the nucleus within the cell
Outer nuclear membrane
6nm thick; a membrane that is continuous in some locations with the membrane of the ER and may have associated ribosomes:
Ribosomes synthesize proteins for outer and inner nuclear membranes
Surrounded by a loosely arranged mesh of intermediate filaments (vimentin)
Inner nuclear membrane
6nm thick; membrane that is continuous with the outer membrane at perforations (nuclear pore complexes):
A unit membrane facing the nuclear contents - it’s separated from the nuclear material and supported on its inner surface by the nuclear (fibrous) lamina
Nuclear Lamina
a 100nm thick fibrous protein network composed primarily of lamins, intermediate filament-like proteins that form the meshwork.
Lamina means a flat sheet or layer
The nuclear lamina helps organize the nuclear envelope and perinuclear chromatin
It serves as attachment sites for nuclear RNAs and chromosomes
Emery-Dreifuss Muscular Dystrophy (EDMD)
A single heterozygous splicing mutation leads to a major loss of lamin A expression; EDMD is a laminopathy, an alteration in the nuclear lamina structure leading to distended perinuclear space and nuclear fragility causing wasting and heart failure by early adulthood
Perinuclear Cisterna
20-40 nm; the space between outer and inner nuclear membranes, continuous with the lumen of the ER; proteins are found in this space and it is perforated by nuclear pores
Nuclear pore complex (NPC)
~100 nm in diameter; sites of nucleo-cytoplasmic communication and attachment points for nucleoskeletal proteins; large proteinaceous channels through nuclear envelope that act as selective barriers
Roughly 2000 per nucleus (can change depending on the cell’s needs)
Transport of macromolecules between nucleus and cytoplasm
NPC structure
Formed by fusion of the inner and outer membranes
Made up of 30 distinct proteins (nucleoporins), and are organized with eight-fold radial symmetry
Proteins form a small (9nm) pore through which proteins pass
NPC function (nucleocytoplasmic communication/ transport)
Numerous small molecules are transported bidirectionally by passive diffusion
Other molecules are transported by active transport (energy-required, receptor-mediated, and selective barrier)
Example: selective and receptor mediated nuclear import - a nuclear localization signal (NLS) of proteins serves as a signal for transport; it binds to importin-alpha in the cytoplasm and finds importin-beta (docked with the NPC) and nuclear translocation occurs. Importin alpha is recycled
Example: in nuclear export, the nuclear export signal (NES) is coupled to exportins
Nucleolus
Site of ribosomal RNA synthesis and assembly of ribosomal subunits
Consists of rRNAs (synthesized in the nucleolus) and ribosomal proteins (synthesized in the cytoplasm and transported into the nucleus) which are complexed together to form the ribosomal subunits
The subunits are transported out of the nucleus and are assembled in the cytoplasm
There may be more than 1 nucleolus in a cell
It contains 3 regions: a pale-staining region - nucleolar-organizing region (NOR; portions of 5 chromosomes), a fibrillar region (rRNA precursors are transcribed), and a granular region (assembly of ribosomal subunits)
Nucleoplasm
Nuclear matrix and nuclear granules are its components; structural and regulatory proteins that surrounds the chromosomes and nucleolus
Nuclear matrix
Part of the nucleoplasm, a scaffold/skeleton; it aids in organizing the nucleoplasm
Nuclear lamina, a residual ribonucleoprotein network and fibrillar elements
It’s associated with DNA replication sites, rRNA and mRNA transcription and processing, steroid receptor-binding sites, carcinogen binding sites, heat-shock proteins, DNA virus and viral proteins
May contain components that facilitate gene expression
Nuclear granules
Part of the nucleoplasm, may be sites of mRNA processing (contains enzymes and small molecules for DNA/RNA biosynthesis)
DNA storage
DNA and associated proteins in the nucleus are present as heterochromatin and euchromatin. The degree of packing/condensation of DNA is governed by the DNA-associated proteins (histones and non-histone proteins)
Histones have high positive charges and have a high affinity for the negatively charged phosphates on the DNA molecule.
Nucleosome
The basic structural unit of the chromatin, consisting of proteins and DNA. The functional unit of is a core made up of 8 histone molecules plus 166 bp of DNA wrapped twice around the core
‘beads on a string’ appearance of many nucleosomes linked by the DNA linker segment
Coiled nucleosomes form 30nm thick chromatin fibers and highly condensed form of chromatin fibers are chromosomes
Non Histone Proteins
Non-histone proteins include nuclear matrix (lamins) and DNA binding proteins (transcription factors)
Heterochromatin
Darkly staining, more coiled and less active chromatin in the interphase nucleus, usually concentrated at the periphery of the nucleus and around the nucleolus, as well as scattered through the nucleoplasm appearing as basophilic clumps (LM) or dense granular clumps (TEM). Example: Barr body
Euchromatin
the lightly staining. less coiled, more active chromatin in the interphase nucleus, appearing as lightly stained (LM) or electron-lucent regions(TEM). Are transcriptionally more active than heterochromatin
Chromosomes
thread-like, highly coiled, appear during cell division and carry genetic info with chromatin folded into the loops. humans have 46 chromosomes
Karyotype
refers to the number and morphology of chromosomes
