(50)
20. Beskrive RNA turnover (endo- og exonucleases) og forklare hvorfor RNA molekylers levetid er kortere end DNA molekyler
Devlin, s.229-30
 

The different roles of DNA and RNA in genetic expression are reflected in their metabolic rates.

DNA is a very stable molecule and has a longer life-time because of the following reasons:

• DNA is the genetic information in our cells that need to be given from generation to generation.
   

• It also serves as a template for replication and transcription during the entire life. The DNA molecule must therefore be preserved and various DNA-repair systems are found in the cell.
   

• When the DNA is not replicating or being transcribed, it is a metabolically inert molecule.
   

• It doesn’t have a 2’OH group as RNA, so it isn’t that prone to hydrolysis
   

• It is wrapped around the basic histones that protect the molecule from interaction with other basic molecules.

RNA isn’t that stable and has therefore a shorter life. There are several reasons for this:

• RNA, unlike DNA, has a 2’-OH group attached to its ribose, which makes it more susceptible to hydrolysis.
   

• RNA is a dispensable molecule and can be synthesized over and over from the DNA template. Therefore there is no RNA repair systems found in the cell which makes RNA more prone to mutation. This decreases the stability and life time of a RNA molecule. If there is a mutation in the RNA molecules, fx. mRNA, it will only affect the protein product of the respective mRNA and nothing else. The cell would waste too much energy and time to repair the mutation in mRNA which doesn’t have that bad affect overall.

The defective RNAs are degraded into nucleotides, which are reused for synthesizing new molecules.

So, all in all, RNA molecules are unstable. However, even the stable ones are degraded (turn over) pretty fast. Fx. tRNA in the liver has a half-life of 5 days, while mRNA has a half life of 30 hours.

Removal of RNAs from the cytoplasm is accomplished by cellular ribonucleases. There are two types:

• exonucleases – that degrade RNA from the 5’-3’ end

• endonucleases – that cleave phosphodiester bond within the RNA-molecule
   

Turnover number – number of substrate molecules converted into product by an enzyme molecule in a unit of time when the enzyme is fully saturated with substrate. This definition can be applied for RNA molecules, if we think of RNA as substrate molecules, while the exo- and endonucleases are the enzymes that degrade the mRNA.

So, the RNAs turnover number must be number of mRNA molecules that are converted by the endo- and exonucelases in a unit of rime, when the nucleases are maximaly saturated with RNA.

tilbage til molekylær biologi