Sept 24, 2009
-In E. Coli the rRNA makes up approximately 80% of all the RNA present and
the tRNA is about 15% and the mRNA/sRNA is about 5%.
-Due to the relative small size and amount of mRNA it was quite difficult to
discover the existence of mRNA; mRNA is quite unstable, one of the reasons for
this is that it is a useful tool for gene regulation (control the level of initiation and
transcription, the rate of turnover also causes the transcript to be quickly
degraded). This is not always the case with the higher eukaryotes that have can
have long lived mRNA
Subunits of RNA (fig.1.19 Singer & Burk)
-RNA subunits are linked together in the same manner as DNA (5’-3’ phosphate
bonds) the main difference is that there is a 2’ OH that allows it to have catalytic
activity; the OH-group also means that the molecule cannot be degraded by pH,
but makes it sensitive to hydrolysis of the phosphate bond by alkaline substances.
-Modifications that can occur are:
1) methyl, thiol, and hydrogen substitutions in the bases
2) methylation at the 2’ C in the sugar
3) altered linkage between the base and the sugar groups
-Many of the alterations listed above are vital to the function of tRNA.
-Dystrophin is the missing protein in individuals with DMS, the primary structure
for this molecule is approximately 2 x 10 nt in size; to date this is one of the
largest molecular transcript found. ( half the length of Ecoli genome)
-Most of the RNA molecule have secondary structures and it can be quite
extensive in complexity (hairpin, double hairpin, multi-branched structure)
-One of the unique features of RNA is that it can have non-conventional base
pairing because the sugar phosphate backbone is flexible (“non Watson & Crick
pairing” such as GU, GA, AC, AA, GG).
-When RNA forms a double strand structure whether by nature or due to
secondary interactions (RNA:RNA) it will resemble the A form of DNA.
-Refer to fig 1.22 Singer & Bu