This results in a very deep and narrow major groove and a shallow and wide minor groove. A second consequence of the presence of the hydroxyl group is that in conformationally flexible regions of an RNA molecule that is not involved in formation of a double helix it can chemically attack the adjacent phosphodiester bond to cleave the backbone. RNA and DNA differ in three main ways. First unlike DNA which is doublestranded RNA is a singlestranded molecule in most of its biological roles and has a much shorter chain of nucleotides. Secondly while DNA contains deoxyribose RNA contains ribose there is no hydroxyl group attached to the pentose ring in the position in DNA whereas RNA has two hydroxyl groups. These hydroxyl groups make RNA less stable than DNA because it is more prone to hydrolysis. In light of this several types of RNA tRNA rRNA contain a great deal of secondary structure which help promote stability.
Thirdly the complementary nucleotide to adenine is not thymine as it is in DNA but rather uracil which is an unmethylated form of thymine. Like DNA most biologically active RNAs including tRNA rRNA snRNAs and other noncoding RNAs such as the SRP RNAs are extensively base paired to form double stranded helices. Structural analysis of these RNAs have revealed that they are not singlestranded but rather highly structured. Unlike DNA this structure is not just limited to long doublestranded helices but rather collections of short helices packed together into structures akin to proteins. In this fashion RNAs can achieve chemical catalysis like enzymes. For instance determination of the structure of the ribosome an enzyme that catalyzes peptide bond formation revealed that its active site is composed entirely of RNA.
Synthesis of RNA is usually catalyzed by an enzyme RNA polymerase using DNA as a template. Initiation of synthesis begins with the binding of the enzyme to a promoter sequence in the DNA usually found upstream of a gene. The DNA double helix is unwound by the helicase activity of the enzyme. The enzyme then progresses along the template strand in the direction synthesizing a complementary RNA molecule with elongation occurring in the direction. The DNA sequence also dictates where termination of RNA synthesis will occur.There are also a number of RNAdependent RNA polymerases as well that use RNA as their template for synthesis of a new strand of RNA.
