Biotechnology for Health (Biomedical Innovations)

Unit 6: Problem 6: Molecular Biology in ActionBI 6.1Biomedical Innovation: transformation & gel electrophoresis

Read a restriction digest

Count cut sites on a piece of DNA and work out how many fragments a restriction enzyme will produce.

Builds on (2 levels back)inferred · high confidence

Restriction enzymes cut DNA at specific sites: A restriction digest is DNA cut by an enzyme at set spots; you need to know the enzyme cuts only where its target sequence appears.
Linear vs. circular DNA: How many fragments a cut makes depends on whether the DNA is a straight piece or a closed loop, so you must tell the two shapes apart.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

On a circular plasmid, N cut sites give N fragments; on a linear piece, N cut sites give N+1 fragments, because the loop has no free ends.

Step 1: Circular: cuts equal pieces

A circular plasmid is a closed loop. The first cut opens it into one long strand; each additional cut adds one more piece. So N cuts give exactly N fragments.

Step 2: Linear: one extra piece

A linear piece already has two ends. Cutting it once makes 2 pieces, twice makes 3, and so on. So N cuts give N+1 fragments.

Step 3: Check the shape first

Always ask 'is this DNA a loop or a straight piece?' before you count: the same number of cuts gives different fragment counts.

Practice

A circular plasmid has 3 cut sites for one restriction enzyme. After a complete digest, how many DNA fragments are produced?

Reviewed

A.2 fragments
B.3 fragments
C.4 fragments
D.6 fragments

Show the worked solution ▾

Answer: B. 3 fragments

Step 1: Note the shape: The DNA is a circular plasmid: a closed loop with no ends.
Step 2: Apply the rule: For a circle, fragments = cuts. With 3 cut sites, 3 cuts give 3 fragments.

Why it's right: A circular plasmid cut at 3 sites yields 3 fragments, because a closed loop produces as many pieces as cuts.

Why the others miss:

A: 2 would be the count for a circle cut twice, but there are 3 cut sites.
C: 4 (N+1) is the LINEAR rule; this DNA is circular, so it is N, not N+1.
D: 6 is double the cut count; cutting does not double the number of pieces.

Aligned to BI 6.1: fragment count (circular) · reading level ~grade 9

Where you'd see this

Before running a gel, a scientist predicts the number of bands by counting cut sites and checking whether the DNA is linear or circular.

Careers that use this

Genetic Engineer: Predicts fragment counts from a plasmid map to plan a cloning step.

Video library

Watch: Read a restriction digest

DNALC Short: Restriction Enzymes

DNA Learning Center · ~3 min

Guided notes

Fill these in as you work through the lesson.

Big idea: A restriction enzyme cuts DNA at its specific sites. On a straight (linear) piece, N cuts make N+1 fragments; on a closed loop (circular plasmid), N cuts make N fragments.

Key terms: write the meaning

Restriction digest (DNA cut by an enzyme at set sites):
Cut site (where the enzyme's target sequence appears):
Fragment (one piece of DNA after cutting):
Circular plasmid (a closed DNA loop with no ends):

The rule

On a LINEAR piece of DNA, N cuts give fragments. On a CIRCULAR plasmid, N cuts give fragments, because the loop has no .

Check yourself

Why does a linear piece give one more fragment than the number of cuts?
Why does a circular plasmid give exactly as many fragments as cuts?
If you cut a circular plasmid only once, how many fragments do you get and what shape are they?

Work one example

A circular plasmid has 3 cut sites for an enzyme. Work out how many fragments the digest makes, and explain how your answer would change if the same DNA were a straight linear piece with 3 cut sites.