Single-strand helices, melting

Double-helical DNA can be denatured by conditions that disrupt hydrogen bonding and base stacking, resulting in the melting of the double helix into two single strands that separate from each other. No covalent bonds are broken in this process. Heat, alkaline pH, and chemicals such as formamide and urea are commonly used to denature DNA. 

Fig. 3 Thermal denaturation transition of a DNA helix, (a) UV absorbance increases with temperature, following the unstacking of bases, following a sigmoidal shape. AD and Au are lower and upper baselines, also slightly dependent on temperature, (b) Fraction of single strands 6 extracted from data in (a), which defines the melting temperature corresponding to 9 = 0.5. Adapted with permission from [7]...

Figure 5.17. Hypochromism. (A) Single-stranded DNA absorbs light more effectively than does double-helical DNA. (B) The absorbance of a DNA solution at a wavelength of 260 nm increases when the double helix is melted into single strands.

Replication begins when DnaA monomers bind to oriC forming a nucleosome-like structure. This requires ATP and HU. Localized melting permits the binding of the DnaB/DnaC complex. Dissociation of DnaC allows the DnaB helicase to unwind the helix in preparation for DNA replication. The DnaA complex dissociates, SSB keeps the single strands apart and the replication fork is now ready for the assembly of the replisome. 

Singl-e-strand helix can melt noncooperatively because each bond is independent... 

Single-strand binding proteins (SSBP), also called helix-destabilizing proteins, are proteins which bind much more tightly to single-stranded polynucleotides than to double-stranded ones, thus lowering the melting... 

In this section we discuss the change that occur in the melting of DNA and RNA. The term melting is used to refer a thermally induced transition of a double-helical nucleic acid from an ordered double helix to a disordered state. Native RNA is usually single stranded, but it folds back on itself to form double helical sequences. Raman spectroscopy does not appear to be sensitive to the tertiary structure of RNA, but it can tell what fraction of the RNA is in double-helical form. ... 

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