Final answer:
The ORF might extend beyond the sequenced region, indicating that the sequencing did not cover the full length of the gene. cDNA libraries, reflecting only expressed genes minus introns, along with possible alternative splicing or mRNA degradation, could also explain why only part of the ORF is present in the sequenced DNA.
Step-by-step explanation:
The DNA sequence you're analyzing contains the start of the open reading frame (ORF), but is missing the end. The ORF might extend beyond the sequenced region. This implies that the sequencing coverage did not span the entire length of the ORF. Looking at the nature of cDNA libraries and mRNA sequencing can offer further insight. cDNA libraries represent the transcriptome, which consists only of the expressed genes of a particular cell at a certain time, excluding non-translated regions such as introns.
During mRNA processing, introns are removed, and exons are spliced together to form the final mRNA transcript. This mRNA is reverse-transcribed into cDNA, which is then used to construct the cDNA library. If the full length of the mRNA was not reverse-transcribed or the cDNA was partially cloned, the sequenced DNA could be missing portions of the ORF. Alternative splicing may have also resulted in a shorter transcript that was subsequently cloned into the cDNA library. Finally, post-transcriptional regulations such as mRNA degradation could explain discrepancies between the length of the cloned DNA and the actual length of mRNA encoding the ORF.