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Cell Biology
Plasma Membrane, ER, & Golgi
| Question | Answer |
|---|---|
| How does the phospholipid bilayer serve the function of the plasma membrane? | the phospholipid serves as a barrier between two aqueous compartments and naturally viscous fluid |
| What as served as a model for the study of the plamsa membrane? Why? | red blood cells (erththrocytes), because they do not contain nucei or internal membranes |
| Who discovered that once the plasma membrane was broken down into a monolayer the surface area was always twice that which occupied by the regular plasma membrane, leading to the conclusion of a lipid bilayer? | Gorter and Grendel |
| What are the four major phospolipids of the plasma membrane? | phosphatidylcholine, phosphataidylethanolamine, phoshatidylserine and shpingomyelin |
| how are these phospholipids distributed? | The outer leaflet of the membrane contains mainly phosphatidylcholine and sphingomyelin while the inner leaflet contains primarily phosphatidythanolamine and phospatidylserine. |
| Which side of the plasma membrane contains a negative charge? | the inner leaflet |
| What fifth phospholiipid located in the inner half of the plasma membrane plays a role in endocytosis, cell junctions and cell signaling? | phosphatidylinositol |
| Where are glycolipids found? | in the outer leaflet with their carbohydrate portion exposed on the cell surface |
| What are the functions of the plasma membrane? | defines the boundary of the cell and environment, serves as a selective barrier thus determining the composition of cell and mediates interactions between the cell and environment |
| Who thought that proteins were only found of the surface of the cell creating a coat on both sides? | Davson-Danilli's plasma membrane model |
| What were some flaws to the Davson-Danielli Model? | thought proteins were in beta sheets when actually alpha helices, thought protiens where linked by ionic bond increasing the ions did not release them, and surface protein should protect from phospholipase but didn't |
| Who discovered the Fluid Mosaic Model? | Singer and Nicolson |
| How does cholesterol affect the plasma membrane fluidity at different temperatures? | At high temperatures it interferes with the fatty acid causing the outer part of the membrane less fluidand less permeable while at low temperatures it prevents membrane freezing and maintains fluidity |
| What are lipid rafts? | clusters of cholesterol and sphingolipids with GPI-anchored protein |
| What is the fluid mosaic model? | model that states the membrane is 2-dimensional fluids in which proteins are inserted into the lipid bilayer |
| What are peripheral membrane proteins? | Proteins that are not inserted into the hydrophodic interior of the lipid bilayer |
| What are integral membrane proteins? | proteins that can span the bilayer |
| What are commonly used reagent that can break up the plasma membrane and desociate the integral proteins? | detergents |
| What are proteins that are a form of integral proteins that can span the lipid bilayer with portions exposed on both sides of the membrane? | transmembrane proteins |
| What technique allows us to reveal proteins in the membrane? | freeze-fracture analysis |
| How does freeze fracture analysis work? | A cell is frozen then fractured in half and then fractured with a knife that separates the two lipid bilayers |
| How did freeze-fracture provide support for the fluid mosaic model? | By demonstrating that the majority of the proteins associated with the cell membrane are integral proteins, it showed holes or intact proteins protruding from the membrane in the wrong direction |
| How did the experiments of Frye and Edidin demonstrate protein mobility in the membrane? | They took human cells and mouse cells fused them together and the two types of proteins mixed together instead on staying with the distinct species type |
| What are some peripheral membrane proteins found in red blood cells that serves in the structure of shape and link between the membrane and the cytoskeleton? | actin, ankyrin, and spectrin |
| What provides an additional link with the cytoskeleton that binds to the junctions of spectrin and actin as well as glycophorin? | band 4.1 |
| What are the two major integral proteins of red blood cells? | glycophorin and band 3 |
| What is the function of band 3? | transport of bicarbonate and chloride ions across the red blood cell membrane |
| What is a class of proteins that from a channel in the outer membranes of some bacteria? | porins |
| What limits diffusion along a membrane? | tight junctions |
| What are lipid/glycolipid attachments added to proteins to anchor them to the plasma membrane? | glycosylphosphatidylinositol (GPI) anchor |
| How did Frye and Edidin determine which proteins were human and which where mouse? | By using antibodies ladeled with different fluorescent dye that specifically recognized human and mouse proteins |
| What are the two distinct regions of an epithelial cell that each contain different functions and proteins? | apical and basolateral domain |
| What is a carbohydrate coat formed by oligosaccharides of glycolipids and transmembrane glycoproteins? | Glycocalyx |
| Where is glycocalyx found? | on the outer face of the plasma membrane |
| What is the main function of glycocalyx? | protects the cell from ionic and mechanical stress and is a barrier to invading microorganisms |
| What are cell surface proteins that bind to oligosaccharides on other cells? | Selectins |
| What is the simplest mechanism by which molecules can cross the plasma membrane? | Passive diffusion |
| What are the characteristics of molecules that preform passive diffusion? | small, relatively hydrophodic molecules Example: O2, benzene, H2O and ethanol |
| What involves the movement of molecules in the direction ddetermined by their relative concentration gradients inside and outside the cell, but do not dissolve in the lipid bilayer but the transport is mediated by proteins? | facilitated diffusion |
| What are the two classes of proteins that mediate facilitated diffusion? | carrier proteins and channel proteins |
| Which protein specifically binds to it and transports it to the other side? | carrier protein |
| Which protein forms an open pore through the membrane, allowing free diffusion of any molecule of the appropriate size and charge? | channel protein |
| What can influence passive diffusion? | phospholipid order |
| What are water channel proteins through which water molecules are able to cross more rapidly than diffusion? | aquaporins |
| What is the best characterized protein channel that mediates the passage of ions across plasma membranes and can demonstrate selectivity and can be gated? | ion channels |
| What are three properties of ion channels? | transport= rapid, highly selective b/c of narrow pores, and they are not permanently open |
| What are ion channels that open in response to the binding of neurotransmitters of other signaling molecules? | ligand-gated channels |
| What are ion channels that open in response to changes in electric potential across the plasma membrane | voltage-gated channels |
| What are the difference between Na+ channels and K+ channels? | Na+= selctive based on size Na=smaller then K, K+ are selective based upon interactions with carbonyls that line the channel and take the water and letting it pass through |
| What is active transport? | where molecules are transported against their concentration gradient through the use of energy |
| What are the coupled reactions that provide energy? | ATP hydrolysis and cotransport of another substance with its concentration gradient |
| What is an example of a direct ATP dependent active transport? | Na+ & K+ pump/ATPase |
| How does Na+ & K+ pump/ATPase work? | uses energy from ATP hydrolysis to transport 3 Na+ out of the cell and 2 K+ into the cell against their gradients |
| What percentage of all the ATP produced does the Na K pump use? | 25% |
| Why are the Na and K gradients important to the cell? | They are necessary for propagation of electric signal, to drive active transport of other molecules and to maintain osmotic balance and cell volume |
| What do ABC transports do in prokaryotic cells and eukaryotic cells? | -transport nutrient molecules into the cell -they transport toxic substances out of the cell |
| How do ABC transporters affect cancer? | Two transporter are multidrug resistant genes and in cancer cells thay are often over exposed leading to efflux of a variety of chemotherapy drugs |
| What is indirect active transport? | active transport driven by an ion gradient, the flow of one ion with its gradient provides the opportunity for a second substance to move against its gradient |
| What is symport, antiport? | -in indirect active transport where both substances move in the sam direction -opposite directions |
| What is an example of an indirect active transport symport system? | Co-transport of glucose and Na+, the transmembrane protien binds to two Na+ and one glucose to allow glucose into the cell |
| What are the three domains of the ER? | rough ER, smooth ER and transitional ER |
| In which part of the ER are proteins synthesized? lipids synthesized? and vesicle formation? | Rough ER, Smooth ER, Transitional ER |
| Who first demostrated the role of the endoplasmic reticulum in protein processing and sorting? | George Palade and colleages |
| What is the secretory pathway? | Cytosol, Rough ER, vesicles, Golgi Apparatus, vesicles, plasma membrane |
| How did Palade discover the secretory pathway? | by studying pancreatic acinar cells, and labeling the newly formed protiens with radioactive amino acids, then the location of the radioactivity was etermined ing autoradiography using a pulse chase method |
| In Palade's experiment what is a pulse? Chase? | -expose to a radioactive material -removal of radioactive sub and replace with non- radioactive |
| What is a reductionist approach? Genetic approach? | -isolating different organelles -finding a cell that doesn't have the enzyme through screening or selecting for it |
| What is a sequence located at the amino terminus of the growing polypeptide chain that signals for the ribosome to engage with the rough ER? | signal sequence |
| What is the process where proteins can be translocated into the ER during their synthesis on the membrane bond ribosome? | Cotranslational translocation |
| What is the process where the proteins are completely translated before entering into the ER?? | post translational translocatiion |
| What is the function of signal recognition particle (SRP)? | binds to the signal sequence, stops translation and moves the ribosome to the rough ER |
| What does the SRP bind to at the ER? | the SRP receptor |
| What is the composition of the signal sequence and what happens to it once the protein enters the rough ER? | -short stretches of nonpolar amino acids followed by a basic residue signal for cleavage -it is cleaved |
| What is the Sec 61 translocon? | Is the passage that the synthesizing protein enters into to pass into the rough ER, which also contains three transmembrane proteins called Sec 61 |
| What is the signal sequence cleaved by? | signal peptidase |
| What does the ER break up into when the cell is disrupted? | microsomes |
| What are some of the differences between post-translational targeting and co-translational targeting? | Co-translational: utilizes SRP, SRP, receptors and signal peptidase, Post- translational: Chaperone proteins, signal recognized by Sec 62/63 complex, BiP, |
| What is the function of BiP? | acts as a molecule ratchet to drive protein translocation into the ER |
| how are integral proteins in bedded in the membrane? | by hydrophodic sequences that span the lipid bilayer, stop transfer sequence |
| What is the process of cleaving the signal sequence? | proteolytic cleavage |
| What facilitates protein folding and assembly with the ER? | chaperone BiP |
| What facilitates the formation of disulfide bonds? | protein disulfide isomerase |
| Why do disulfide bonds form in the ER and not the cytosol? | The Cyotosol= a reducing environment while ER=oxidizing environment |
| How is a protein N-linked Glycosylated in the ER? | 1)oligosaccharide is synthesized on a dolichol carrier anchor in the ER membrane 2) then it is transfered as a unit to acceptor Asn-x-Ser/Thr by a membrane bound oligosaccharyl transferase |
| What are the three glucose residue used for on the N-linked Glycosylation? | Quality control to make sure the protein is folded right these will be removed before the protein can leave for the golgi |
| How does a protein receive an GPI anchor addition? | 1)after protein synthesis GPI added to C-terminus of proteins that are retained in the membrane by a C-terminus hydrophobic sequence 2) This is then cleaved from the protein and exchanged for the GPI anchor |
| What quality control protein recognizes the partially-processed olisaccharides on the newly translated glycoprotein and assists the glycoprotein in folding correctly? | calreticulin |
| What signals the release of calretculin from the glycoprotein and allows it to be recognized by a protein folding sensor? | removal of the terminal glucose |
| What is the function of the protein folding sensor? | if protein is folded correctly to pass the glycoprotein on to the transitional ER, If not folded correctly add another glucose and send back to calretculin |
| how are phospholipids formed? | 1)Coenzyme A carries fatty acids to glyserol-3-phosphate and bound by membrane bound proteins 2)Enzymes convert phosphatidic acid to diacylglycerol and catalyze the addition of differnt polar head groups |
| What enzyme transfers polar head groups through the membrane to ensure even growth of both halves of the bilayer? | flippase |
| Where do proteins exit the ER? | in vesicles that bud from the transitional ER |
| How are proteins marked for exit or retention in the ER? | the are marked by sequences that signal their export from the ER, some possess di-acidic or di-hydrophodic amino acids seq. in cytosolic domain that are ER |
| What are three ER export signals? | Met-Met for transmembrane proteins, Glu-Glu for lumenal proteins, and Asp-Asp for GPI anchored proteins |
| What signal directs ER proteins back to the ER? | Lys-Asp-Glu-Leu |
| What organelle functions as a factory in which proteins recieved from the ER are further processed and sorted for transport to their eventual destinations? | Golgi Apparatus |
| What are the four distinct regions of the Golgi | cis Golgi network, medial and trans cisternae and trans Golgi network |
| What occurs in the cis Golgi network? medial and trans cisternae? transGolgi network | -receives molecules from the ERGIC -most modifications are done here -the sorting and distribution center |
| What is the ER Golgi intermediate compartment (ERGIC)? | is the nucleus facing cis face that receives proteins directly from the ER and transports them to the cis Golgi network |
| How are secretory proteins thought to be transported through the Golgi apparatus? | Cisternal maturation and vesicular transport |
| What is cisternal maturation? | Where proteins are carried in golgi cisternae which gradually mature and progressively move through the Golgi |
| What are the functions of the Golgi apparatus? | take proteins from the ER process and sort them, most glycolipids and sphingomyelin are synthesized as well as cell wall polysaccharides |
| What is the processing of N-linked oligosaccharides in the golgi? | 1)four mannose residues are removed 2) N-acetylglucosamine is added 3)2 additional removal of mannose 4)Addition of Fucose and 2 N-acetylglucosamine 5) 3 galatose residues and 3 sialic acid are added |
| What enzyme carries out the addition of sugar residue for N-linked oligosaccharide in the Golgi? | Glycosyltransferase |
| What carries out the removal of sugars for N-linke oligosaccharide in the Golgi? | glycosidases |
| What is the process of N-linked oligosaccharides of lysominal proteins in the golgi? | 1)N-acetylglucosamine phosphates are added to specific mannose residue 2) removal of N-acetylglucosamine leaving mannose-6-phosphate residues on the N-linked oligosaccharides 3)The mannose are then recognized by a receptor in the trans Golgi |
| How is sphingomyelin produced in the Golgi? | By taking ceramide produce in the ER,and transferring a phosphorylcholine group to it |
| how are different glyoclipids produced in the Golgi? | By adding carbohydrates to ceramide |
| What are the three route from the Golgi to the cell surface? | direct transport, recycling endosomes and regulated secretory pathways |
| What happens to proteins that need to stay in the Golgi? | They associate with the membrane and contain signals that prevent packaging and trasnport |
| What happens to lysosomal proteins after they have been tagged and are in the trans Golgi network? | a specific receptor recognizes it, then the receptor and protein are packaged into transport vesicle destined for late endosomes |
| how do transport vesicles recognize their correct targets? | by recognizing and fusing only with the appropriate target membrane |
| What are the five distinct experimental approaches to vesicular transport? | Isolation of yeast mutants defective in protein transport/sorting, reconstitution of it in cell-free system, biochemical analysis of synaptic vesicles, tracing path of specific GFP fusion proteins through secretory network, Proteomic analysis of cmpartmnt |
| What stores neurotransmitters such as acetylcholine within neurons? | synaptic vesicles |
| What is the function of a vesicle coat? | allows the vesicle to travelalong mircotubules to their targetes by interacting with specific tubulin-based molecular motors |
| What regulates the formation of coated vesicles? | GTP-binding proteins related to Ras and Ran |
| how does the coat form? | the coat assembles as the secretory protein containing vesicle buds off the donor membranes |
| What are the three families of vesicle proteins? | Clathrin, COPI and COPII |
| What form of vesicl carries proteins from the ER to the ER-Golgi intermediate compartment and on to the Golgi? | COPII-coated vesicles |
| What type of vesicle buds from the ER-golgi intermediate compartment of Golge and carries their cargo backwards to the ER? | COPI-coated vesicles |
| What type of vesicle is responsible for the transport in both directions between the trans Golge network endosomes, lysosomes and the plasma membrane? | Clathrin-coated vesicles |
| how is the clathrin coat assembled? | 1GDP/ARF binds to pr in Goli mmbrn 2 ARF-guanine nt exchange factor in mmbrn stims exchange of GDP for GTP 3 GTP initiates budding process by recruiting adaptor pr which serve as binding sties for trnmmbrn & clathrin |
| What does clathrin do to the budding process once it is bond? | by assembling into a basketlike structure that distorts the membrane and initiates the bud |
| how is the clathrin coat deassmbled from the vesicle>? | GTP bound to ARF1 is hydrolyzed and ARF/GDP is released, loss of ARF1 weakens the clathrin coat allowing chaperone to dissociate the coat |
| What are the two events involved with vesicle fusion? | 1) transport vesicle recognizes the correct target membrane 2) the vesicle and target membrane must fuse |
| What two proteins combine to provide enough energy to bring two bilayers sufficiently close to destabilizing them and fuse? | v-SNAREs and t-SNAREs |
| What GTP-binding proteins plays a key role in the docking of transport vesicles, interactions with SNAREs to regulation and facilitating the formationn of SNARE-SNARE complexes? | Rab family |
| If the incorrect Rab/GDP complex is present what can remove it and carry it to another membrane? | GDI |
| What removes Rab from GDI? | GDI displacement factors |
| How is Rab/GTP formed? | At a membrane Rab is removed by GDI displacement factor, specific G nt exchange factors localize and act on the RAb family to form Rab/ GTP |
| How is the transport vesicle fusion initiated? | 1 Rab/GTP on t-vesicle interacts w/ effector proteins and v-SNAREs to assemble a pre-fusion complex, same is done on the corresponding membrane, when they encounter the effector proteins link membranes, this stims Rab/GTP hydrolysis & allows SNARE-SNARE |
| What happens is vesicle fusion once the SNARE-SNARE forms? | the lipid bilayers fuse |
| What deassembles SNARE? | NSF/SNAP complex |
| What are membrane-enclosed organells that contain an array of enzymes capable of breaking down all types of biological polymers? | Lysosomes |
| What is the function of a lysosome? | it functions as the digestive system of the cell serving both degrade material take up from outside the cell and to digest obsolete components of the cell itself |
| What is the structure of a lysosome? | dense spherical vacuoles, but can display a variety of forms and sizes as a result of difference in the materials |
| What enzymes are present in the lysosome? | over 50 different types of degradtive enzymes: acid hydrolases |
| Mutations in genes that encode lysosomal enzymes resul in... | lysosomal storage diseases |
| What activates acid hydrolases and why don't they work in the cytoplasm? | active at pH 5 cytosol = pH7.2 which means it is inactive |
| how does a lysosome maintain its pH? | by using a proton pump in its membrane that actively transports protons into it |
| how were lysosomes discovered? | by analyzing subcelluar fractions prepared by differential centrifugation |
| What is the digestion of material taken up from outside the cell? | endocytosis |
| how are lysosomes formed? | when transport vesicles from the trans-Golgi network fuses with a late endosome which contains molecules taken up by endocytosis at the plasma membrane |
| What is the lysosomal digestion where specialized cells such as macrophages take up and degrade large particles that need to be eliminated from the body? | phagocytosis |
| What is another lysosomal digestion of the cell's own components? | autophagy |
| What represents an intersection between the secretory pathway and the endocytic pathway? | endosomes |
| What results from phagosomes fusing with lysosomes? | phagolysosomes |
| Where are molecules destined for degradation pransported too? | late endosomes |
| What is the process of cell drinking? | pinocytosis |
| What is a mediated endocytosis that is a mechanism for selctive uptake of specific macromolecules? | Receptor-mediated endocytosis |
| In receptor-mediated endocytosis, the macromolecule first binds to receptors concentrated in specialized regions of the plasma membrane called? | clathrin-coated dynamin |
| What helps the pits bud from the membrane? | dynamin |
| What are receptors and their bound macromolecules? | ligands |
| What are small invaginations of the plasma membrane organized by a protein called caveolin | Caveolae |
| Cholesterol is transported through the bloodstream in the form? | low-density lipoprotein |
| What are a small family of proteins that interact with cholesterol in lipid rafts, insert into the cell membrane and interact with one another to form caveolae | caveolin |
| Caveolae carry out receptor-mediated endocytosis via.. | transmembrane receptors |
| What is the advantage of having a nucleus? | allows gene expression to be regulated by unique mechanisms, and posttranscriptional mechanisms |
| What has a complex structure consisting of two nuclear membranes and underlying lamina and nuclear pore complexes? | nuclear envelope |
| What is the nucleus surrounded by that consists of a system of 2 concentric membranes | nuclear membrane |
| What is the difference between the two nuclear membranes? | The outer is continuous with with the ER and functionally similar to the membrane of the ER, the inner carries proteins specific to the nucleus |
| What underlies the inner nuclear membrane and is a fibrous meshwork that provides structural support to the nucleus? | nuclear lamina |
| What is the nuclear lamina composed of? | 60 to 80 kilodalton fibrous proteins called lamins along with associated proteins |
| What is a coiled-coil? | where two lamins associate to form a dimer in which the alpha helical regions of 2 polypeptide chains are wound around each other |
| What is the only channel through which small polar molecules, ions and macromolecules can travel between the nucleus and the cytoplasm? | nuclear pore complex |
| What is the nuclear pore complex composed of? | 30 different pore proteins called nucleooporins, |
| What types of molecules can pass freely through nuclear pores of the aqueous channels? | small proteins and small molecules |
| How do larger proteins and RNAs get out or into through the nuclear pore complex? | pass through a central pore by an active process in which the appropriate protein and RNAs are recognized and selectively transported in a specific directiion |
| What targets a protein to the nucleus by a specific amino acid sequence? | nuclear localization signal |
| What recognizes the nuclear localization signal and directs the protein through the nuclear pore complex? | nuclear transport receptor |
| What was the first NLS to be discovered in the T anitgen? | Pro-Lys-Lsy-Lsy-Arg-Lys-Val |
| What is an NLS that is composed of of two separate elements? | bipartite |
| What type of nuclear transport proteins recognizes NLS because they carry proteins into the nucleus? | importins |
| What protein regulates the movement of macromolecules through the nuclear pore? | Ran |
| For Ran which side hydrolyzes GTP to GDP? GDP to GTP | cyoplasmic side, nuclear side |
| How does Ran regulate movement of macromolecules? | by controlling the activity of the nuclear transport receptors |
| how does the import of proteins through the nuclear pore complex begin? | when a specific nuclear importin binds to the nuclear localization signal in the cytoplasm |
| How is the protein released from importin? | by the complex binding to Ran/GTP causing a change in the conformation of importin |
| What happens to the importin -Ran/GTP complex? | it is exported back though the nuclear pore complex and GTP is hydrolyzed releasing imporin |
| What maintains an unequal distribution of Ran/GTP across the nuclear envelope in the cytoplasm? in the nucleus | Ran GAP, Ran GEP |
| What hydrolyzes Ran/GTP to Ran/GDP? Ran/GDP to Ran/GTP? | Ran GAP, Ran GEP |
| What targets proteins for export from the nucleus by specific amino acids? | nuclear export signals |
| What recognizes NES that are receptors in the nucleus? | exportins |
| What are many exportins a member of that are nclear transport receptors? | karyopherins |
| How are RNAs transported out of the nucleus? | tRNAs, rRNAs, miRNAs and smRNAs transport similar to proteins, mRNAs:is exported by mRNA exporter complex |
| how is chromatin organized in the nucleus? | into large loops of DNA and special regions of these loops are bound to the lamin matrix by lamin-binding proteins in chromatin |
| What is highly condensed chromatin? decondensed chromatin? | heterochromatin, euchromatin |
| What is a chromosome territory? | The nonrandom distribution of chromatin wiht in the interphase nucleus with centromeres and telomeres attached to opposite sides of the nuclear envelope |
| Where are actively transcribed genes located? | In mammalian cell nuclei, actively transcribed genes are localized to the periphery of the territories, adjacent to channels separating the chromosomes |
| What transports RNAs across the Nuclear membrane? | RNPs |
| What associates with ribosomal proteins and specific RNA processing proteins in the nucleolus? | rRNAss |
| What are associated with at least 20 proteins during processing and eventual transport to the cytoplasam? | mRNA |
| snRNA are transported to the cytoplams by, where they associate with proteins to form snRNPs and return to the nucleus? | exportin (CRM1) |
| Where is RNA processing and transport thought to occur? | interchromosomal domains (channels) that separate the chromosomes |
| Where is most of the heterochromatin located? | locatized to peripheral of the nucleus because its proteins bind to the matrix of the nuclear lamina |
| Why does the heterochromatin vary in location | Different cell types express different genes, so their facultative heterochromatin is different, and varying regions of the chromosomes interact with the nuclear lamina in different cells and tissues |
| What is a looped domain? | where actively transcribed DNA can be visulaized as large loops of extended Chromatin |
| What is the function of the nucleolus? | it is the site of rRNA transcription and processing as well as aspects of ribosome assembly |
| What is the structure of the nucleolus? | not surrounded by a membrane,and is associated with the chromosomal regions that contain genes for 5.8S,18S, and 28S rRNAs |
| how are these genes for 5.8S,18S, and 28S rRNAs transcribed? and yield? | single unit bye RNA polymerase 1and yielding a 45S ribosomal precursor RNA |
| What is a nucleolar organizer region? | a region in the nucleoli that associates with the chromosomal regions that contain the 5.8S, 18S and 28S genes? |
| how are the 3 types of eukaryotic rRNA produced? | 45Spre-rRNA is transcribed by RNA polymerase 1, , cleavages within the external transcribed spacer near the 5' end and the removal of the ETS at the 3' end plus the additional cleavage of the internal transcribed spacers lead to the resulting rRNAs |
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sfitzpatrick
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