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. Which of these would have the greatest diversity of sequences?

Verified step by step guidance

1. Understand the difference between genomic and cDNA libraries: Genomic libraries contain DNA fragments that represent the entire genome of an organism, including both coding (exons) and non-coding regions (introns, regulatory sequences, etc.). cDNA libraries, on the other hand, are made from mRNA and represent only the expressed genes (exons) in a particular tissue at a specific time.
2. Consider the source of the DNA: Both the genomic libraries (from brain and muscle tissue) will contain the same genomic DNA, as all cells in an organism have the same DNA. Therefore, the diversity of sequences in genomic libraries is not dependent on the tissue type.
3. Evaluate the cDNA libraries: cDNA libraries are tissue-specific and reflect the genes that are actively expressed in that tissue. Therefore, the diversity of sequences in cDNA libraries depends on the variety of genes expressed in the specific tissue from which the mRNA was isolated.
4. Compare the diversity: Since genomic libraries contain the entire genome, they inherently have a greater diversity of sequences compared to cDNA libraries, which only contain sequences from expressed genes.
5. Conclude which library has the greatest diversity: The genomic libraries, whether from brain or muscle tissue, will have the greatest diversity of sequences because they include all genomic DNA, not just the expressed genes.

Key Concepts

Genomic Library

A genomic library is a collection of DNA fragments that represent the entire genome of an organism. It includes both coding (exons) and non-coding (introns and regulatory regions) sequences. Since it encompasses all genetic material, a genomic library typically has a high diversity of sequences, reflecting the complete genetic makeup of the source tissue.

cDNA Library

A cDNA library is constructed from complementary DNA synthesized from mRNA transcripts. This type of library represents only the expressed genes in a specific tissue at a given time, thus it reflects the functional aspects of the genome. Consequently, a cDNA library has less diversity compared to a genomic library, as it excludes non-coding regions and genes not actively expressed in the tissue.

Tissue-Specific Expression

Tissue-specific expression refers to the phenomenon where certain genes are expressed in specific tissues while remaining silent in others. This results in different cDNA libraries from various tissues (like brain and muscle) containing distinct sets of sequences. Therefore, while both genomic libraries have high diversity, the cDNA libraries will show less diversity due to their focus on the genes expressed in the respective tissues.