Restriction enzymes function by locating sequences of DNA known as recognition sites which identify locations where DNA will be cut. Each restriction enzyme has a unique recognition site.

The recognition site of the restriction enzyme Taq I (Thermus aquaticus) is:

5' ---T C G A --- 3'

How many recognition sites does this DNA sequence contain?

5' --- T A C A T G T C G A A A A T C G C G T A T C G A C C C G A T G C T A C A T C G A T --- 3'

Question

Restriction enzymes function by locating sequences of DNA known as recognition sites which identify locations where DNA will be cut. Each restriction enzyme has a unique recognition site.

The recognition site of the restriction enzyme Taq I (Thermus aquaticus) is:

5' ---T C G A --- 3'

How many recognition sites does this DNA sequence contain?

5' --- T A C A T G T C G A A A A T C G C G T A T C G A C C C G A T G C T A C A T C G A T  --- 3'

BlackTom AI · Accepted Answer

Reiterating the question in your own words can help set the task: we’re looking for every occurrence of the recognition sequence for TaqI, which is TCGA, within the given DNA strand, counting how many times this exact four-base sequence appears.
Option 1: '1' — This claims TCGA appears only once. To evaluate, scan the long sequence for the exact four-base motif T-C-G-A in consecutive positions. When you check carefully, you can find TCGA at multiple starting points, so claiming a single occurrence ignores other matches and is misleading.
Option 2: '2' — This asserts there are exactly two TCGA sites. While it’s plausible to miss a site when scanning by eye, the provided sequence actually contains three distinct, non-overlapping (and also some overlapping possibilities, depending on indexing) instances of TCGA. Therefore, stating two would undercount, indicating a counting or pattern-recognition error.
Option 3: '4' — This states there are four TCGA sites. If you methodically slide a four-base window along the entire sequence from 5' to 3' and check each group of four consecutive nucleotides, you should note that there are three positions where TCGA occurs. This option overcounts by treating some partial overlaps or misreading nearby bases as separate sites, which is incorrect.
Option 4: '3' — This claims there are exactly three TCGA sites. By examining the sequence and marking each occurrence of the exact motif T-C-G-A, you identify three distinct starting positions where TCGA appears. This aligns with a careful, base-by-base scan of the entire strand, ensuring no additional TCGA motifs are overlooked or double-counted.

Takeaway across options: The reliability of the count hinges on checking every possible starting position for a four-nucleotide window and matching the exact order T-C-G-A. Any deviation in the four bases or a shift in the starting point results in a non-match, so precise counting is essential. Miscounts typically arise from missing overlapping opportunities or misreading the sequence, which leads to incorrect tallies such as 1, 2, or 4 instead of the true count.

APBIO142-030 with labs 21.01 to 21.03 - Video Quiz

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