8.1 Primary Structure of RNA

All RNA molecules have…

1. a pentose sugar

   The pentose sugar in RNA is replaced by ribose, not deoxyribose!

2. nitrogenous bases

   The four standard bases are adenine, cytosine, uracil, and guanine.

3. a 5’ cap

   

   Figure 8.2: A 7-methylguanosine Cap

   The molecule 7-methylguanosine is joind to the 5’ end of almost all eukaryotic mRNAs in an unusual 5’-5’ triphosphate linkage.

   The purpose of the cap is to protect mRNA from being degraded.

The 2’ OH group in RNA makes the RNA more structurally stable than DNA; the OH group also makes the RNA less chemically stable than DNA

8.1.1 Poly(A) tail of mRNA

Figure 8.3: Formation of a Poly(A) Tail in mRNA

The poly(A) tail of mRNA (see figure 8.3) is added to mRNA in a multistep process. The initial mRNA transcript has a 3’ extension beyond the site where the poly(A) tail is to be added; this transcript is also added by an endonuclease.

The enzyme polyadenylate polymerase catalyzes the polyadenylation reaction in figure 8.3.

The tail serves to protect the mRNA from being degraded during transportation (from the nucleus to the cytosol).