✍ Scribed by Hsueh Jei Li
No coin nor oath required. For personal study only.
Thermal denaturation of native or partially dehistonized nucleohistones shows two melting bands a t 66 and 81" in 2.5 X These correspond to the melting of DNA segments bound by the less basic and the more basic half-molecules of histones, respectively. These two melting bands combine into a broad melting band from around 70 to 85" when these nucleohistones are pretreated with formaldehyde. A formaldehyde reaction which fixes histones on DNA by covalent bonds account for the effect. Formaldehyde fixation also increases the melting temperature of some free DNA segments from around 42 to around 55". This is interpreted as a result of closed or rigid boundaries between free DNA and formaldehyde-reacted histone-bound DNA segments. MgClz dissociates histones from DNA more effectively arid leaves longer free DNA segments than does NaCI. Thermal denaturation of a formaldehyde-reacted nucleoprotein thus provides an effective tool for comparing the relative size of free DNA regions on nucleoproteins. The effect of reversible binding of ligands on helix-coil transition of DNA is discussed and found not adequate for theimal denaturation of nucleohistones.
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