Key concepts in molecular biology

Essentials of Chemical Biology Andrew Miller and Julian Tanner 2008 John Wiley Sons, Ltd 

if eukaryotic gene structure is so complicated, how can we expect to obtain functionally useful genes for further molecular biology manipulation The answer to this question lies with mature mRNA transcripts. Such mature transcripts from eukaryotes can be viewed as the functional equivalent of a primary mRNA transcript from a prokaryote. Mature mRNA transcripts contain processed intron-free nucleotide sequences, which harbour the complete genetic code for a protein of interest. 

Mature mRNA transcripts (sense strand) from eukaryotic cells can be purified and then reverse transcribed, with the assistance of a reverse transcriptase enzyme (from Moloney murine leukemia virus, MMLV), into complementary DNAs (cDNAs) that will anneal with the mRNA transcripts by Watson-Crick base pairing to give anti-parallel DNA/RNA duplexes or double helices. The poly(A) tail in each mature mRNA transcript is actually a usefiil handle for each reverse transcriptase reaction. Thereafter, DNA/RNA duplexes must be broken down with the assistance ofRNAse enzymes (specific for the hydrolysis of RNA phospho diester links) and a sense strand of DNA constructed instead on each cDNA single strand so that equivalent, more stable antiparallel DNA/DNA duplexes are generated instead, with the assistance of a DNA polymerase enzyme. In this instance, the poly(T) tail in each cDNA molecule turns out to be important for the DNA polymerase reaction  

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