Q.35 Genomic DNA isolated from a bacterium was digested with
a restriction enzyme that recognizes a 6-base pair (bp) sequence.
Assuming random distribution of bases, the average length (in bp) of the fragments generated is ________
Average DNA Fragment Length from 6-bp Restriction Enzyme: 4096 bp Explained
Restriction enzymes cut DNA at specific recognition sites, and for a 6-bp sequence with random base distribution, the average fragment length is 4096 bp.
Calculation Method
Restriction enzymes recognize specific sequences; a 6-bp site like GAATTC occurs every 46 bases in random DNA, as each position has 4 possibilities (A, T, C, G). Thus, 46 = 4096 bp gives the average distance between cuts, equaling average fragment size. This assumes equal base frequencies and linear genomic DNA.
Why 4096 bp?
The probability of the exact 6-bp match is (1/4)6 = 1/4096, so cuts happen every 4096 bp on average. For comparison, a 4-bp cutter averages 44 = 256 bp fragments. Real bacterial genomes approximate this under random distribution, though slight base biases exist.
Common Options Explained
| Option | Length (bp) | Explanation |
|---|---|---|
| 256 | 44 | Correct for 4-bp recognition site, not 6-bp; too frequent cuts. |
| 1024 | 45 (approx.) | Matches 5-bp site; underestimates for 6-bp enzyme. |
| 4096 | 46 | Correct: Exact average for 6-bp site with random bases. |
| 8192 | 47 (approx.) | For 7-bp site; overestimates fragment size. |
