Which DNA sequence is common across species and typically codes for proteins?
Expressed Sequence Tags are short sequences derived from cDNA clones representing expressed genes. The 5' EST contains coding portion and is relatively conserved across species because protein-coding constraints limit divergence, making it useful for ortholog identification and comparative genomics. The 3' EST corresponds more to 3' UTR, less conserved and more species-specific. SNPs represent single-base variations and VNTRs are tandem repeat copy-number variants, neither inherently coding nor necessarily conserved. Hence 5' ESTs provide cross-species coding information and functional annotation potential.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.