Biotechnology for Health (Biomedical Innovations)

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

Interpret a gel against a ladder

Use a DNA ladder of known sizes to estimate how big an unknown DNA band is from how far it traveled.

Builds on (2 levels back)inferred · high confidence

DNA carries a negative charge: Gel electrophoresis works because DNA is pulled toward the positive end; you need to know DNA is negatively charged to know which way it moves.
Estimating with a known scale: Reading an unknown band means comparing it to known ladder sizes, the same skill as reading a value off a labeled scale.

Prerequisites are inferred: pending teacher review.

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

Smaller DNA fragments travel farther (lower on the gel); larger fragments stay near the wells. A ladder of known sizes lets you estimate an unknown band by which rungs it sits between.

Step 1: Link size to distance

The gel is a mesh. Small fragments slip through quickly and travel far (ending up LOW on the gel). Large fragments get caught and stay HIGH near the wells.

Step 2: Use the ladder

The ladder lane has bands at known sizes. To size an unknown band, find which two ladder rungs it falls between.

Step 3: Give a range

If the unknown sits below the 500 bp rung but above the 250 bp rung, its size is between 250 and 500 base pairs: that is your estimate.

Practice

Use the gel. The unknown lane has one band (band X). Compared to the ladder, what is the best size estimate for band X?

Reviewed

Gel with two lanes. Wells are at the top. The ladder lane shows four labeled bands from top to bottom: 1000 bp, 500 bp, 250 bp, 100 bp (larger fragments stay near the top, smaller fragments travel farther down). The unknown lane has one band, called band X, that sits below the 500 bp band and above the 250 bp band. Its size is not printed.

A.Greater than 1000 bp
B.Between 250 and 500 bp
C.Exactly 100 bp
D.Between 500 and 1000 bp

Show the worked solution ▾

Answer: B. Between 250 and 500 bp

Step 1: Find band X: Band X sits below the 500 bp ladder rung and above the 250 bp ladder rung.
Step 2: Read the bracket: A band between those two rungs is between 250 and 500 base pairs in size.

Why it's right: Band X migrated farther than the 500 bp band but not as far as the 250 bp band, so its size is between 250 and 500 bp.

Why the others miss:

A: Greater than 1000 bp would sit at or above the top ladder rung, but band X is well below it.
C: 100 bp is the farthest, lowest rung; band X is much higher than that.
D: Between 500 and 1000 bp would sit above the 500 bp rung, but band X is below it.

Aligned to BI 6.1: estimating size from a gel · reading level ~grade 9

Where you'd see this

A lab worker reports an unknown fragment as 'about 250–500 bp' by reading which ladder rungs it falls between.

Careers that use this

Forensic DNA Analyst: Sizes DNA bands against a ladder to compare evidence samples.

Video library

Watch: Interpret a gel against a ladder

Gel Electrophoresis

Amoeba Sisters · ~7 min

Guided notes

Fill these in as you work through the lesson.

Big idea: In gel electrophoresis, DNA is negative and moves toward the positive end. Smaller pieces slip through the gel faster and travel farther, so position tells you size when you compare to a ladder of known sizes.

Key terms: write the meaning

Gel electrophoresis (sorts DNA by size using charge):
DNA ladder (lane of known sizes to compare against):
Base pairs (the unit DNA length is counted in):
Migrate (to travel through the gel):

The rule

DNA is charged, so it moves toward the end. fragments travel farther, so a band low on the gel is than a band near the wells.

Check yourself

Which way does DNA move in the gel, and why?
If band A is higher (nearer the wells) than band B, which one is larger?
How does a ladder let you put a number on an unknown band's size?

Work one example

An unknown band sits between the 500 bp and 250 bp rungs of the ladder. Give a size range for it and explain, using how far it traveled, why your range makes sense.