Final answer:
The '+' symbols represent nucleotide matches in a sequence alignment, which are key components in scoring alignments in tools like BLAST, indicating potential biological relevance between the compared sequences.
Step-by-step explanation:
The "+" symbols in the middle row of a sequence alignment indicate nucleotide matches between the sequences being compared. In the context of alignments using Basic Local Sequence Alignment Tool (BLAST), when there's a nucleotide match between the query and the subject, a "+1" is added to the score of the alignment. Conversely, mismatches and gaps result in negative scores, reducing the overall alignment score.
In terms of sequence alignment, finding the highest scoring arrangement is key, as it signifies the best potential alignment under the scoring system employed. Introducing gaps may result in a better overall match, improving the alignment. The '+' symbol is therefore a positive indicator in this scoring method and helps in identifying regions of similarity that might be biologically relevant.