4-5.1. Introduction

Library screening is the process of identification of the clones carrying the gene of interest. Screening relies on a unique property of a clone in a library. The DNA libraries consist of a collection of probably many thousand clones in the form of either plaques or colonies on a plate. Screening of libraries can be done by following approaches based on-

•  Detecting a particular DNA sequence and

•  Gene expression.

4-5.2. Methods for screening based on detecting a DNA sequence

4-5.2.1. Screening by hybridization

•  Nucleic acid hybridization is the most commonly used method of library screening first developed by Grunstein and Hogness in1975 to detect DNA sequences in transformed colonies using radioactive RNA probes .

•  It relies on the fact that a single-stranded DNA molecule, used as a probe can hybridize to its complementary sequence and identify the specific sequences.

•  This method is quick, can handle a very large number of clones and used in the identification of cDNA clones which are not full-length (and therefore cannot be expressed).

The commonly used methods of hybridization are,

a)  Colony hybridization

b)  Plaque hybridization.

4-5.2.1(a). Colony hybridization

Colony hybridization, also known as replica plating, allows the screening of colonies plated at high density using radioactive DNA probes. This method can be used to screen plasmid or cosmid based libraries (Explained in detail in Module 3-Lecture 4 as point 3-4.4).