The assignment of mitochondria to daughter cells at mitosis also appears to be random. Indeed, it was the observation of somatic variation in plants that first suggested the existence

of genes that could be lost from one of the daughter cells because they were not inherited according to Mendel's laws.

TLS Online TPP Program

#Id: 6758

#Unit 3. Fundamental Processes

Degradation of bacterial mRNAs is initiated by removal of a pyrophosphate from the 5’ terminus. The monophosphorylated form stimulates the catalytic activity of an endonuclease RNase E

TLS Online TPP Program

#Id: 6759

#Unit 3. Fundamental Processes

PNPase a 3’ to 5’ exonucleases in E. coli, are unable to progress through double-stranded regions. Thus, the stem-loop structure at the 3’ end of many bacterial mRNAs protects the mRNA from direct 3’ attack.

TLS Online TPP Program

#Id: 6760

#Unit 3. Fundamental Processes

RNase E and PNPase, along with a helicase and another accessory enzyme, form a multiprotein complex called the degradosome.

TLS Online TPP Program

#Id: 6761

#Unit 3. Fundamental Processes

Cytoplasmic mRNAs are degraded by one of the three pathways. For most mRNAs, the deadenylation-dependent pathway is followed These dense regions of cytoplasm contain the decapping enzyme (DCP1/DCP2), activators of decapping (DHH, PAT1, LSM1-7), and the major 5′→3′ exoribonuclease XRN1, as well as densely associated mRNAs

TLS Online TPP Program

#Id: 6762

#Unit 3. Fundamental Processes

The exposed cap is then removed by a decapping enzyme (DCP1/DCP2), unprotected mRNA susceptible to degradation by XRN1, a 5′→3′ exoribonuclease. Removal of the poly(A) tail also makes mRNAs susceptible to degradation by cytoplasmic exosomes containing 3′→5′ exonucleases.

TLS Online TPP Program

#Id: 6763

#Unit 3. Fundamental Processes

The 5′→3′ exonuclease pathway predominates in yeast, and the 3′→5′ exosome pathway predominates in mammalian cells.  The decapping enzymes and 5′→3′ exonuclease are concentrated in P bodies