Experimental studies have shown that both long and short complementary B-form DNA oligomers exhibit liquid crystals phases. We study the isotropic to cholesteric (I-Ch) phase transition of DNA solutions using Landau theory. We define relevant order parameters for this transition. It is shown that the I-Ch transition temperature is depressed and a two phase region is formed due to the presence of the DNA concentration. The concentration and oligomers length dependent transition temperature is calculated and compared with experimental results. The Kerr effect and Cotton-Mouton coefficient are calculated in the isotropic phase above the I-Ch transition. (C) 2013 Elsevier B.V. All rights reserved.