Double-stranded helical regions, complementary

Differences in the capacity of inhibition by polynucleotides not involved in complementary hydrogen bonds and by double-helical complexes of synthetic polyribonucleotides, or double-stranded viral RNA allow the conclusion that it is above all the regions of associated base pairs which are recognized in the RNA by anti-poly I poly C antibodies. Such complementary double-stranded helical regions have been described especially in tRNA but they have also been shown to exist in ribosomal RNA. These two kinds of RNA were therefore isolated and studied separately. Although both fractions precipitate anti-poly I poly C antibodies, their reactivity is nevertheless very different and rRNA precipitates eight times as much antibody as tRNA. Since tRNA possesses an important tertiary structure, this low reactivity could be explained by the non-accessibility of antigenic sites. 

Unlike the double-stranded nature of DNA, RNA molecules usually occur as single strands. This does not mean they are unable to base-pair as DNA can. Complementary regions within an RNA molecule often base-pair and form complex tertiary structures, even approaching the three-dimensional nature of proteins. Some RNA molecules, such as transfer RNA (tRNA) possess several helical areas and loops as the strand interacts with itself in complementary sections. Other hybrid molecules such as the enzyme RNase P contain protein and RNA portions. The RNA part is highly complex with many circles, loops, and helical regions creating a convoluted structure. 

RNA The secondary structure of RNA consists of a single polynucleotide. RNA can fold so that base pairing occurs between complementary regions. RNA molecules often contain both single- and double-stranded regions. The strands are antiparallel and assume a helical shape. The helices are of the A-form (see above). 

The principal structural difference between DNA and RNA is the 2 OH group of ribose in RNA molecules. In DNA, which lacks the 2 OH group in the deoxyribose sugar, hydrogen-bonded complementary strands can easily adopt the B-form double helix. In contrast, double-stranded regions of RNA molecules cannot adopt this conformation because of steric hindrance. Instead, they adopt the less compact A-helical form in which there are 11 bp per turn and the base pairs tilt 20° away from the horizontal. 

RNA chains are usually single-stranded and lack the continuous helical structure of double-stranded DNA. However, RNA still has considerable secondary and tertiary structure because base pairs can form in regions where the strand loops back on itself. As in DNA, pairing between the bases is complementary and antiparallel. But in RNA, adenine pairs with uracil rather than thymine (Fig. 12.18). Basepairing in RNA can be extensive, and the irregular looped structures generated are... 

The reactive -NHj, -OH and -NH groups of purine and pyrimidine bases are responsible for certain properties of N.a., e.g. formation of specific hydrogen bonds between purines and pyrimidines, leading to secondary structures. Thus complementary linear chains can form a double helix (see DNA), or a linear strand can fold on itself, forming alternate linear and helical regions (RNA). Other forces involved in the... 

Palindromes, or inverted repetitions are characteristic components of the genome. They are formed by two complementary sequences which run in opposite directions. These two sequences are on the same strand of DNA but can be either very close or at great distances from each other. Palindromes range from 50 to several thousand nucleotides in length. They can result in the formation of a loop or a double helical region (Hardman and Jack, 1977). 

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