To solve this problem, we need to identify which statement about gel electrophoresis is not true. Let's analyze each option:$\newline \bullet$Option 1: The process of extraction of separated DNA strands from gel is called elution.$\newline$ - This statement is true. Elution refers to the process of extracting DNA fragments from the gel after electrophoresis.$\newline \bullet$Option 2: The separated DNA fragments are stained by using ethidium bromide.$\newline$ - This statement is true. Ethidium bromide is a common stain used to visualize DNA fragments in gel electrophoresis.$\newline \bullet$Option 3: The presence of chromogenic substrate gives blue coloured DNA bands on the gel.$\newline$ - This statement is not true. Chromogenic substrates are not typically used to stain DNA in gel electrophoresis. Instead, ethidium bromide or other fluorescent dyes are used, which do not produce blue bands.$\newline \bullet$Option 4: Bright orange coloured bands of DNA can be observed in the gel when exposed to UV light.$\newline$ - This statement is true. When DNA stained with ethidium bromide is exposed to UV light, it fluoresces bright orange.$\newline$Therefore, the statement that is not true is Option 3.$\newline$The correct option is Option 3.