Question

Partial double stranded regions of single stranded RNA are because of 1) Nucleotide double helix 2) Presence of scaffold proteins 3) Its genetic nature 4) Folding back of single strand

Answer 1

Answer:

Partial double stranded regions of single stranded RNA are because of Folding back of single strand

Answer 2

Explanation:

Local structural transitions from the common B-DNA conformation into other DNA forms can be functionally important. This chapter describes the structures of DNA forms called alternative DNA conformations that are different from the canonical B-DNA helix. Also discussed are the requirements for the formation of alternative DNA structures, as well as their possible biological roles. The formation of non-B-DNA within certain sequence elements of DNA can be induced by changes in environmental conditions, protein binding and superhelical tension. Several lines of evidence indicate that alternative DNA structures exist in prokaryotic and eukaryotic cells. The data on their involvement in replication, gene expression, recombination and mutagenesis continues to accumulate.

Introduction

Genetic information is generally stored in long double-stranded DNA molecules. Hydrogen bonding between nucleobases keeps the complementary DNA strands organized into a right-handed helical structure called B-DNA. Structural transitions into other DNA forms can occur within certain sequence elements of DNA and these can be functionally important. Several non-B-DNA structures (oftentimes called unusual or alternative DNA structures) can be important for interactions with proteins involved in replication, gene expression and recombination. They may also play different roles in the formation of nucleosomes and other supramolecular structures involving DNA. DNA sequences characterized as “random” or “mixed sequence” typically only form A-DNA or B-DNA. Special sequence characteristics or defined symmetry elements are required to form alternative structures such as left-handed Z-DNA, cruciforms, intramolecular triplexes, quadruplex DNA, slipped-strand DNA, parallel-stranded DNA, and unpaired DNA structures.1 Together with variations in DNA supercoiling, local alternative structures provide enormous potential for autoregulation of DNA functions. This chapter will briefly review major alternative DNA structures and their potential involvement in biology.

B-DNA and A-DNA

Structure

Table 1 lists structural parameters for three structural families of DNA helices. B-DNA is the term given for the canonical right-handed DNA helix that is the most common form of DNA. Canonical B-DNA is a double helix made of two antiparallel strands that are held together via hydrogen bonding in the A•T and G•C base pairs (fig. 1). One helical turn of B-DNA contains about 10.5 base pairs that are buried inside the helix and are almost perpendicular to the helical axis. DNA exists as a cylinder of 20 Å in diameter with two grooves, a major and a minor groove, spiraling around the cylinder. In B-DNA the distance between the bases (rise) is 3.4 Å. Studies of oligonucleotide duplexes in crystals showed significant sequence-dependent variability of the structural parameters listed in Table 1 that define the structure of the B-DNA helix. In bent DNA, for example, certain B-DNA parameters add up over a length of several base pairs to produce a permanently curved DNA helix. A- and Z-DNA are also double-helical but the spatial arrangement of base pairs differs significantly from that for B-DNA. Other DNA structures may have regions of unpaired strands or be composed of three and even four strands.