Three main classes of mutations affecting plant height have been selected:

Gibberellin-insensitive dwarfs

Gibberellin-deficient mutants in which the gibberellin deficiency has been overcome by a second “suppressor” mutation, so the plants look closer to normal

Mutants with a constitutive gibberellin response (“slender” mutants)

TLS Online TPP Program

#Id: 6308

#Unit 3. Fundamental Processes

RNA polymerase performs two proofreading functions on that growing transcript. 

The first of these is called pyrophosphorolytic editing. In this, the enzyme uses its active site, in a simple back-reaction, to catalyze the removal of an incorrectly inserted ribonucleotide, by reincorporation of PPi. The enzyme can then incorporate another ribonucleotide in its place in the growing RNA chain. Note that the enzyme can remove either correct or incorrect bases in this manner, but spends longer hovering over mismatches than matches, and thus removes the former more frequently.

TLS Online TPP Program

#Id: 6309

#Unit 3. Fundamental Processes

In hydrolytic editing, the polymerase backtracks by one or more nucleotides and cleaves the RNA product, removing the error-containing sequence. 

TLS Online TPP Program

#Id: 6310

#Unit 3. Fundamental Processes

in E. coli, GreA and GreB enable the polymerase to cleave a few ribonucleotides from the 3’ terminus of the RNA product, thereby allowing the catalytic site of RNA polymerase to be realigned with the 3’–OH to release stalled polymerase

TLS Online TPP Program

#Id: 6311

#Unit 3. Fundamental Processes

he consequences of arrest can be catastrophic if the gene being transcribed is essential as no product will be made by the arrested polymerase, and that same enzyme will cause a roadblock to other polymerases attempting to transcribe the same gene.The cell has machinery that removes the arrested polymerase and at the same time recruits repair enzymes (in particular, the endonuclease Uvr(A)BC); the repair that follows is called transcription-coupled repair. Both polymerase removal and repair enzyme recruitment are performed by a single protein called TRCF.

TLS Online TPP Program

#Id: 6312

#Unit 3. Fundamental Processes

TRCF has an ATPase activity. 

Base-pairing of something to the transcript to destabilize RNA-DNA hybrid

Causes hairpin to form

Causing the transcription to pause

Causes a string of U’s to be incorporated just downstream of hairpin

TLS Online TPP Program

#Id: 6313

#Unit 3. Fundamental Processes

Sequence of a Rho-independent terminator