BIOL 300 - Chapter 20

what is stability?

the extent to which RNA molecules retain its structural integrity and resists degredation

how is RNA stability controlled?

through increases and decreases in stability

what are RNases?

nucleases that can degraade RNA, and are secreted to kill things outside of the cell

what is the purpose of controlling mRNA stability/duration?

controls the amount of proteins produced, or how many mRNA are produced from a single gene

how is mRNA able to undergo fast regulation?

it’s half time is much shorter than a regular cell cycle

when does mRNA become less stable?

if the mRNA is missing its cap or polyA tail

when is polymerase 2 transcription shut down?

immediately, when mRNA starts degrading

how does mRNA become more stable?

with higher translation speed, as it causes more dense ribosomes

how can degradation rate be accurately measured?

when synthesis is shut down

what is the degradosome made up of?

RNase, PNPase, enolase, helicase

why is very stable RNA bad?

it can cause human disease

what can increase stability in mRNA?

sequences that form secondary and tertiary RNA structures, 5’ cap, polyA tail, stabilizing elements, and RNA binding proteins

what is the degradosome?

major machinery that degrades mRNA in prokaryotes

what mRNA factors affect degradosome activity?

translation efficieny, secondary structures, secondary structures, or short polyA/oligoA tails

what is a RNP (ribonucleoproteins) particle?

the form mRNA takes in the cell - complex form with RNAs and proteins

what are RBPs?

RNA binding proteins

what is the purpose of RBPs?

catalogue tags - signal cellular processes and locatinos

how many tags does mRNA have?

several - different mRNAs have distinct but overlapping RBPs

what is a regulon?

set of mRNAs that share a specific type of RBP tag

what are the two steps in a mRNA degradation event?

recognizing and tagging mRNAs that need to be degraded, and recruitment of the degradation machinery by the tags

what is involved in the second step in a mRNA degradation event?

two machineries are involved; a 5’ to 3’ exonuclease Xrn1 that acts after decapping, and a 3’ to 5’ decay occurs by the exosome after deadenylation

what is the exosome?

a highly conserved complex involved in degradation in eukaryotes

which degradation is the major route, from Xrn1 or the exosome?

if each route is blocked, it’s determined that the 5’ to 3’ end is the major route from Xrn1

what is involved in the first step in a mRNA degradation event?

degredation systems will degrade mRNA that’s deemed as no longer needed or supposedd to be downregulated, through deadenylation independent or dependent pathways

what is deadenylation-dependent degradation?

occurs when a shortened polyA tail is tagged - responsible for the degradation of most mRNAS, and is triggered when the polyA tail reaches 10 nucleotides

what is deadenylation-independent degradation?

occurs when tagged by other mechanisms, which eventually recruit Xrn1 or exosome for degradation. 4 additional pathways are involved, including independent decapping, oligoU-dependent degradation of histone mRNAs, endonucleolytic cleavage, and miRNA mediated silencing

what is miRNA?

short RNA strand that was first discovered in C elegans, and regulates 50% of mRNA

what is RISC?

RNA-induced silencing complex - a protein complex made up of miRNA, which targest mRNAs that have complementary sequences with miRNA tags

what is the pathway of microRNA mediated silencing?

drosha and dicer cleae off different parts of pre-miRNA, and the strand separaes before becoming mature. proteins are then associated, and the miRNA runs around the cell lookoing for complementary sequences to degrade.

what is RNAi?

RNA interference - it can insert and sequence into pre-miRNA, which leads to it becoming selective to the mRNA inserted

is mRNA localized?

yes - it can localize to specific regions of the cell

what are the three purposes of subcellular localization?

localized translation in specific cell compartments, different inheritance of specific mRNAs, and the setting up of embryonic body plan