Chapter 11 and 12 - Nucleus and Control of Gene Expresion

Polypeptide is then released at this point gtp of

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Unformatted text preview: point GTP of class II RF (RF3 and eRF3) is hydrolyzed The final step is the dissociation of the mRNA from the ribosome and the disassembly of the ribosome Polyribosomes o o o o o o Ribosomes initially assemble at the initiation codon mmoving towards 3 end of mRNA As each ribosome moves forward another can assemble at the initiation codon When mRNA being translated is examined by EM , any ribosomes are attached complex of ribosomes and mRNA is referred to polyribosome or polysome Rate at which translation initiation occurs varies with the mRNA Polysomes increase the rate of protein synthesis In prokaryotes, it all happens in the same place, which is very efficient as mRNA comes out, ribosomes attach to make protein (in eukaryotes the mRNA has to exit the nucleus first) Quality Control o o o o o o o o o o o o o Single base mutations can change an amino acid codon into a termination codon nonsense mutations Nonsense mutations are responsible for % of inherited disorders Premature termination codons can also be introduced during splicing Cells posses an mRNA surveillance mechanism that can detect premature termination codons mRNA with premature termination codons generally are only translated once before mRNA is selectively destroyed nonsense mediated decay (NMD) NMD protects the cells from non-functional proteins When an intron is removed by the splicesosome, a protein complex is deposited on the transcript 20 -24 nucleotides upstream of exon-exon junction exon junction complex (EJC) EJC stays with mRNA until translation In normal mRNA, termination codon is just downstream of the EJC As mRNA undergoes translocation EJC is displaced by the ribosome If mRNA has a premature termination codon, ribosome would dissociate and EJC would still be attached to mRNA this initiates NMD NMD is generally a good thing but sometimes its better to have a short protein than no protein at all Clinical trials ongoing for cystic fibrosis and muscular dystrophy drugs interfere with NMD so they have abnormally short protein which may have enough enzyme activity to rescue patients from fatal conditions The Nucleus o o o o Multicellular organisms have diverse, specialized cell types Despite the diversity ,each cell has a full set of genetic instructions Differentiated cells only turn on a subset of genes/proteins Therefore cells must have a means to control what genetic information is expressed 5 o o o Most of the regulatory machinery is in the nucleus The nucleus has simple morphology Nonmitotic (nondividing) cell contains: chromosomes (nucleoprotein fibers called chromatin), one or more nucleoli, nucleoplasm (fluid interior), nuclear matrix (fibrillar protein-containing network) The Nuclear Envelope o o o o o o o o o o o Two membranes parallel to one another they are to 50 nm apart Acts as barrier to ions, solutes and macromolecules 2 membranes are fused at sites of circular nuclear pores contain a complex assembly of proteins and there are several thousand in an average mammalian cell The outer membrane is continuous with the RER therefore it is studded with ribosomes Space between outer and inner membrane is continuous with the RER lumen The inner surface of the nuclear envelope is lined by nuclear lamina The nuclear lamina supports the nuclear envelope, it is composed of lamins (intermediate filaments), and it strong and flexible and made of rope like fibers The integrity of nuclear lamina is regulated by phosphorylation and dephosphorylation The nuclea...
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This document was uploaded on 04/10/2014.

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