The Nucleus Nucleolus: (arrow) site of rRNA transcription and processing and some aspects of ribosome assembly. The nucleolus is not membrane-bound, but is associated with regions of the chromosomes bearing genes for ribosomal RNAs.(arrows). These genes are present in multiple copies, and larger nucleoli are found in cells with high rates of protein synthesis. Euchromatin: regions where DNA is decondensed and genes are being actively transcribed. This DNA does not stain darkly in an electron micrograph. Heterochromatin: regions of highly condensed DNA not being transcribed. (arrowheads).
Nature of the double membrane surrounding the nucleus. *Nuclear pores link interior with cytoplasm
The Inner Nuclear Membrane The inner nuclear membrane has integral proteins that anchor the nuclear lamina, a network of protein fibers to which chromatin is attached.
Nuclear Lamina: EM visualization of the network just inside the inner nuclear membrane
Isolated lamin B structure P, phosphorylation sites that regulate disassembly in mitosis Dimerization: self- assembly of two proteins into functional unit Membrane and nuclear targeting regions (NLS= nuclear localization sequence – this allows the protein to return to the nucleus after it has been synthesized in the cytoplasm). Lamin B will attach to Lamin A
Nuclear Lamina Diseases X-linked muscular dystrophy traced to mutations in a transmembrane protein – still little understood, although the defective protein is identified. Different mutations in the same gene are causes for a lipodystrophy and a premature aging disease. The diverse effects and involvement of different tissues are as yet unexplained.
Nuclear Pore Complexes Each nuclear pore consists of eight subunits surrounding a central aperture containing some additional structures
Transport through nuclear pores is passive (small molecules) or energy-dependent (large molecules)
Regulation of traffic through pores More than 1 million molecules/min. pass through 3000-5000 nuclear pores of the typical cell. Outbound: mRNA, tRNA, ribosomes…proteins exported must bear a Nuclear Export Sequence) Inbound: Nuclear and ribosomal proteins (which must bear a Nuclear Localization Sequence that makes it “cargo”. This is recognized by an adapter “Importin” that binds it to an import receptor)
Nuclear business: To store the DNA. Chromatin is DNA that is attached to the nuclear lamina in a condensed form associated with proteins (histones). To serve as the site for DNA transcription to RNA and processing of the RNA. In the nucleolus, ribosomes are assembled from >40 proteins and 3 RNA molecules. Inbound traffic (through nuclear pores) includes proteins produced in the cytoplasm, including transcription factors and ribosomal proteins. Outbound traffic (through nuclear pores) includes Messenger RNAs and ribosomal subunits. All is cool until conditions inside and outside the cell trigger the chain of events leading to DNA replication and cell division…..
The outer nuclear membrane is continuous with the Endoplasmic Reticulum (ER)
ER Compartments Rough ER is studded with ribosomes and is specialized to support protein synthesis and protein sorting. Smooth ER is specialized for steroid synthesis, drug metabolism (the liver) or calcium storage (muscle). Transitional ER is where vesicles are budding off to carry cargo to the Golgi Apparatus.
Differences in appearance of rough and smooth ER
Golgi Apparatus Stacked membrane compartments that are polarized: in cis) and out (trans). Illustration includes coated pre- Golgi intermediates and transitional elements.
Golgi Functions Protein processing Lipid synthesis Distribution of proteins and lipids: vesicles
Lysosomes: a specialized offspring of the Golgi Apparatus that form when vesicles containing lysosomal proteins fuse with endosomes that result from endocytosis.
Lysosomal function: hydrolysis of material taken up by endocytosis or phagocytosis and also to recycle worn-out cell debris taken into endosomes by a process called autophagy. The lysosomes are acidified by a vacuolar-type H + ATPase.
Lysosomal Storage Diseases If a genetic defect leads to malfunction in one of the enzymes used by lysosomes to break down a particular class of substances, that substance will accumulate, i.e., be stored, in the lysosome. This accumulation leads to malfunction. In response, the cell produces more lysosomes and these also become clogged, and eventually the cell itself becomes dysfunctional. The tissues that produce the most of the unhydrolyzed material will be most affected.
Gaucher’s Disease, the most common lysosomal storage disease The primary lesion of the disease is seen in macrophages that ingest damaged leucocytes and erythrocytes and then cannot digest their membrane lipid. Ultimately this results in symptoms that involve almost every organ system. It is most common in people of Ashkenazi Jewish lineage. Gaucher’s disease can be treated by IV infusion of the missing enzyme. Unfortunately, such treatments are quite expensive.
Peroxisomes Peroxisomes are membrane-bound organelles formed on the ER which then mature through accumulation of additional proteins (tailored to the needs of the cell) that are targeted to them from the cytoplasm. Fatty acid oxidation and other oxidative reactions, including breaking down uric acid, amino acids, purines and methanol occur in the controlled environment of the peroxisomes. Oxidative reactions that lead to the production of H 2 O 2 occur in peroxisomes, but the potential toxicity of peroxide is managed by the presence of catalase, an antioxidant that decomposes H 2 O 2 and to water and O 2 or uses the extra oxygen to oxidize another compound.
More obvious components of mitochondrial structure
Where did organelles of eukaryotic cells come from? Revisiting the origin of eukaryotic cells The Margulis Hypothesis: intracellular symbionts became mitochondria, and perhaps other organelles Evidence: –Mitochondria have bacterial-type chromosomes and the genes are most closely related to bacterial ones –Some mitochondrial genes are part of the nuclear DNA – this could reflect the activity of mobile elements in the pre-eukaryotic cells which had acquired symbionts –Mitochondria have a double membrane – which could represent a membrane-bound structure (bacterial cell) that got engulfed by a host cell. –In bacteria, a proton gradient across the plasma membrane is used to phosphorylate ADP; in mitochondria, the same principle operates across the inner mitochondrial membrane
Who were the host cells? Family tree analysis of genomes places the Archaea in closer relationship to plants and animals than bacteria. So, the cells that took in the endosymbionts initially were probably Archaea rather than bacteria. Archaea typically live in anaerobic environments, so it’s probable that, initially, the endosymbionts offered protection against the damaging effects of oxygen free radicals, with ATP being a bonus that became available only when the host cells began to express the carriers that exchange ADP, ATP and phosphate across the mitochondrial membranes.
Why do eukaryotic cells have a nucleus? “mobile elements” or transposons are components of the genome that can cause genes to move about within a genome (“jumping genes”). With two different genomes within one cell, transposons could create havoc by moving genes randomly between the genomes. It seems likely that the nuclear membrane evolved as a defense against this.