You have isolated another cDNA clone of the CRABS CLAW gene from a cDNA library constructed using the vector shown in Problem 18. The cDNA was directionally cloned using the EcoRI and XhoI sites. You sequence the recombinant plasmid using primers complementary to the T7 and T3 promoter sites flanking the MCS (the positions of these sequences are shown in the figure in Problem 18). The first 30 to 60 bases of sequence are usually discarded since they tend to contain errors. Will the long stretch of T residues in the T3 sequence exist in the genomic sequence of the gene?

Key Concepts

cDNA and Genomic DNA

cDNA (complementary DNA) is synthesized from mRNA through reverse transcription and represents only the expressed genes, lacking introns. In contrast, genomic DNA contains both coding (exons) and non-coding regions (introns) of the entire genome. Understanding the difference is crucial for determining whether sequences found in cDNA, like the T3 promoter, will also appear in the genomic DNA.

Promoter Sequences

Promoter sequences are regions of DNA that initiate transcription of a gene. The T3 promoter is a specific sequence recognized by RNA polymerase for transcription initiation. When analyzing cDNA, it is important to recognize that promoter sequences are typically not present in the mature mRNA, as they are upstream of the coding region and are not transcribed into RNA.

Poly-A Tail and T Residues

The long stretch of T residues in the T3 sequence often corresponds to the poly-A tail added to the 3' end of eukaryotic mRNA during processing. This tail is not part of the genomic sequence but is crucial for mRNA stability and translation. Therefore, while the T3 sequence may contain T residues, these are not indicative of a corresponding sequence in the genomic DNA.