Which of the following statements about genomic DNA libraries is false?

Question

(a) The larger the size of the fragments used to make the library, the fewer colonies you will have to examine to find a clone that hybridizes to your probe.
(b) The larger the size of the fragments used to make the library, the more difficult it will be to find your gene of interest once you have identified a clone that hybridizes to your probe.
(c) The larger the genome of the organism from which a library is derived, the larger the fragments inserted into the vector will tend to be.
(d) The smaller the gene you are seeking, the more likely it is that the gene will be found on a single clone.

Answer 1

(c) The sizes of the fragments left after a restriction digest do not depend on the total size of the genome; they depend on the sequence of the genome and the frequency with which the restriction enzyme recognition site is found in the genome. Choices (a) and (b) are true: as a limiting case, think of what would happen if a fragment the size of the entire genome were inserted into the bacterial vector. You would have to screen only one colony to find the clone that hybridized to your probe, but it would be very difficult to find out where on the insert your gene of interest lay. Choice (d) is true: the larger the gene you are seeking, the more likely it is that there will be a restriction fragment in the gene (or that the gene will be broken if the DNA was fragmented by random shearing), and hence the less likely it is that the entire gene will be found in one clone.