Final answer:
cDNA often lacks terminal mRNA sequences because of incomplete reverse transcription and mRNA processing, such as splicing which removes introns.
Step-by-step explanation:
cDNA is often lacking in terminal sequences of the template mRNA because the reverse transcription process is not always fully completed at the ends of the mRNA molecules. This is partially due to the way mRNA is transcribed and processed. RNA polymerase may stall or fall off the DNA during the transcription of terminal sequences, which can be influenced by structures such as the hairpin loop in Rho-independent termination.
Differences in cDNA and mRNA arise because cDNA is synthesized from mature, processed mRNA which does not include introns and some regulatory sequences. These mature mRNAs are spliced, meaning non-coding regions such as introns are removed before the mRNA leaves the nucleus. Hence, when reverse transcriptase synthesizes cDNA, the resulting sequence reflects only the exons or coding regions of the mRNA and not the complete genomic sequence.