Hydrogen bonding between complementary base pairs

Ribosomal RNA (rRNA) is involved in the protein synthesis. It is found in the ribosomes which occur in the cytoplasm. Ribosomes contain about 35% protein and 65% rRNA. Experimental evidence suggests that rRNA molecules have structures that consist of a single strand of nucleotides whose sequence varies considerably from species to species. The strand is folded and twisted to form a series of single stranded loops separated by sections of double helix, which is believed to be formed by hydrogen bonding between complementary base pairs. The general pattern of loops and helixes is very similar between species even though the sequences of nucleotides are different. However, little is known about the three dimensional structures of rRNA molecules and their interactions with the proteins found in the ribosome. 

FIGURE 9.22 The cloverleaf depiction of transfer RNA. Double-stranded regions (shown in red) are formed by folding the molecule and stabilized by hydrogen bonds between complementary base pairs. Peripheral loops are shown in yellow. There are three m or loops (numbered) and one minor loop of variable size (not numbered). 

Fl9Ure 11.8 Base pairing. Hydrogen bonding between complementary base pairs holds the two strands of a DNA molecule together. 

Step 1. Unwinding of the double helix Replication begins when the enzyme helicase catalyzes the separation and unwinding of the nucleic acid strands at a specific point along the DNA helix. In this process, hydrogen bonds between complementary base pairs are broken, and the bases that were formerly in the center of the helix are exposed. The point where this unwinding takes place is called a replication fork (see I Figure 11.11). 

The red dotted lines in Figure 20.7 represent hydrogen bonds between complementary base pairs. 

Figure 25.35 Hydrogen bonding between complementary base pairs. The hydrogen bonds shown here are responsible for formation of the double-stranded helical structure of DNA, as shown in Figure 25.34(b). 

It si known that temperature largely affects the conformation of nucleic acids in solution. Heating can break the hydrogen bonds between complementary base pairs (thermal denaturation) leading the molecules to a less ordered conformation. At the same time, an increase in the u.v. absorption spectrum occurs, this due mainly to the decrease in the interaction between neighbouring bases. The analysis of the difference spectrum upon denaturation (denaturation spectrum) may give information on the amount of the ordered and stacking-stabilized structure initially present in the RNA moiety, ... 

FIGURE 18.21 Hydrogen bonds between complementary base pairs hold the polynucleotide strands together in the double helix of DNA. 

The Watson-Crick model portrays the double-stranded DNA molecule as a right-handed helix. This means that when the molecule is viewed from one end the top surface of the helix spirals towards the right. Each turn of the helix contains ten base-pairs, measures 3.4 nm in length and corresponds to the major periodicity (Section 4.5) attained by X-ray crystallography. The distance of 0.34 nm occupied by one nucleotide is consistent with the experimental minor periodicity. The two helical strands are held together by the hydrogen bonds between complementary base-pairs. In addition, within a polar environment vertical hydrophobic... 

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