You have constructed four different libraries: a genomic library made from DNA isolated from human brain tissue, a genomic library made from DNA isolated from human muscle tissue, a human brain cDNA library, and a human muscle cDNA library. Would the sequences contained in each library be expected to overlap completely, partially, or not at all with the sequences present in each of the other libraries?

Key Concepts

Genomic Library vs. cDNA Library

A genomic library contains fragments of genomic DNA, representing the entire genome of an organism, including both coding and non-coding regions. In contrast, a cDNA library is constructed from complementary DNA synthesized from mRNA, thus representing only the expressed genes in a specific tissue at a given time. This fundamental difference means that genomic libraries will contain sequences from all genes, while cDNA libraries will only include sequences from actively expressed genes in the respective tissues.

Tissue-Specific Gene Expression

Different tissues express different sets of genes, a phenomenon known as tissue-specific gene expression. For example, genes that are active in brain tissue may not be expressed in muscle tissue and vice versa. This means that while there may be some overlap in the genomic libraries (as they represent the entire genome), the cDNA libraries are likely to show less overlap due to the unique expression profiles of brain and muscle tissues.

Sequence Overlap

Sequence overlap refers to the extent to which the DNA sequences in one library are found in another. In the case of the genomic libraries, there will be a significant amount of overlap since both libraries are derived from the same organism's genome. However, the cDNA libraries will have limited overlap with each other and with the genomic libraries, as they only include sequences from genes that are expressed in the specific tissues, leading to partial or no overlap depending on the genes expressed in brain versus muscle.