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Continuation of Chapter 12 and 13 (Summer2011)

6 Pages

Biological Sciences
Course Code
Rosada Silva

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BIOB11Summer2011 Chapter 12 and Chapter 13 DNA Replication and Repair mRNA Stabilityo Lifetimes of eukaryotic mRNA vary o polyA tail length may play a role in the lifetimepolyA tail is reduced as the mRNA resides in cytoplasmcertain destablilizing proteins in the 3 UTR may affect the rate of shortening of the polyA tailwhen the tail reaches about 30A residues the mRNA is degraded rapidly o Once the 3 tail is removed message is decapped and degraded from the 5 end o Deadenylation decapping and 5 to 3 degradation occur within a small transient cytoplasmic granule called Pbodies microRNAs o miRNAs act by binding to site in the 3UTR of their target mRNAs o studies suggest that miRNAs carry out translational level control by inducing the degradation of target mRNA o post initiation inhibition binding of RISC complex containing Argonaute protein AGO leads either to degradation of the nascent protein or to the release of the ribosomes from the mRNA o inhibition of initiation RISC complex either inhibits the interaction between eIF4E and the 5 end of the mRNA or prevents the assembly of the complete ribosome from its subunits o mRNA degradation RISC complex has recruited proteins that either promote the decapping of mRNAproteins DCP12 abdor degrade the polyA tailGW182 is attached and is a component of the Pbody RNA interference Petunias o tried to make flowers more purple with extra gene copyinstead they ended up white o both RNAs were being made but being degradedo Mello and Fire injected worms with sense antisense and double stranded RNA complementary to transcript they wanted to knock downthe double stranded RNA worked o The dsRNA was taken up by the cells selectively the degraded mRNAs had the same sequence o May have evolved as a genetic immune system against viruses or transposonso Both strands of dsRNA were cleaved by Dicercreating overhanging ends o then it becomes associated with preRISC a protein complex containing an arogonaute proteino passanger miRNA strand is then removedo guide strand associated with RISC binds the target RNAtranslation is blocked or mRNA destabilizedmiRNAs vs siRNAs o miRNAs are encoded by genomeothe transcript from double stranded RNA through complementarity othe guide strand is often not 100 identical to target DNA oit can block translation and not just destabilize RNAsiRNA just destabilizes the RNA it does not block translation DNA Replication o All cells need to reproduce o DNA replication is the process of DNA duplicationsame machinery used for replication is often also used for DNA repair o DNA replication involves separation of the strands and synthesis of 2 daughter strands whichare complementary to the 2 parental templates o There are 3 ways replication could occur semiconservative conservative and dispersive
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