Taq I has a recognition site of:
5' --- TCGA --- 3'

3' --- AGCT --- 5'

And makes the cut:
5' --- T CGA --- 3'

3' --- AGC T --- 5'

How many DNA fragments were created when TaqI was introduced in the sequence below?

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 C G A --- 3'

Question

Taq I has a recognition site of:
5' --- TCGA --- 3'

3' --- AGCT --- 5'

And makes the cut:
5' --- T      CGA --- 3'

3' --- AGC      T --- 5'

How many DNA fragments were created when TaqI was introduced in the sequence below?

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 C G A --- 3'

BlackTom AI · Accepted Answer

TaqI recognizes the 4‑base sequence TCGA and cleaves between the T and C on the top strand (and correspondingly on the bottom strand), so each recognition site yields one double‑stranded break. For a circular DNA molecule, the number of fragments produced equals the number of such double‑stranded cuts (for linear DNA it would be cuts + 1).

Step‑by‑step identification of sites in the given sequence (spaces removed):
TACATGTCGAAAATCGCGTATCGACCCGATGCTACATCGATCGA
Indexing the bases 1→44, scan for the exact motif "TCGA":
- positions 7–10: bases 7 T, 8 C, 9 G, 10 A → TCGA (site 1)
- positions 21–24: bases 21 T, 22 C, 23 G, 24 A → TCGA (site 2)
- positions 37–40: bases 37 T, 38 C, 39 G, 40 A → TCGA (site 3)
- positions 41–44: bases 41 T, 42 C, 43 G, 44 A → TCGA (site 4)

That gives four distinct TCGA recognition sites total. Each site will be cleaved once by TaqI, producing four double‑stranded breaks. Because the typical restriction‑mapping context for such problems assumes a circular molecule unless otherwise stated, four cuts on a circular DNA molecule produce four fragments (cut number = fragment number on circular DNA).

Why the other answers are incorrect:
- "5" would be the fragment count if the DNA were linear and had four cuts (linear fragments = cuts + 1). The problem’s intended context (restriction mapping) treats the sequence as circular, so 5 is not correct here.
- "3" or "2" would imply only 2 or 3 recognition sites/cuts; the explicit scanning above shows four TCGA sites, so those options are inconsistent with the sequence.

Therefore the correct number of DNA fragments produced by TaqI in this sequence is 4, matching the provided answer.

类似问题

A molecule that cuts DNA at a specific site is called a:

Which condition would lead to a likely increase in the number of fragments produced during a restriction endonuclease (RE) digestion reaction?

When choosing restriction enzymes for insert verification in molecular cloning, why is it preferable to select an enzyme that makes multiple cuts in the gene sequence?

Which of the following is responsible for detecting sequence variations in DNA during RFLP analysis?

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