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# Size Estimation

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Gel electrophoresis is a technique used by molecular biologists to analyze samples of DNA or proteins. Forensic technicians use it to identify blood, hair and other evidence from crime scenes to solve murders and other crimes. Evolutionary biologists use gel electrophoresis to see evolutionary links between different species, while geneticists can use it to determine familial relationships or to look for problematic genes.

The process for electrophoresis is complicated, but it boils down to separating molecules based on their size. In the case of DNA gel electrophoresis, samples of DNA are cut into smaller parts by proteins called restriction enzymes. Restriction enzymes cut DNA at specific points. This is important in forensics because restriction enzymes will cut different DNA at different points, so every human will have their DNA cut at different points, thus allowing scientists to use electrophoresis to pinpoint DNA samples to one specific person.

Once the DNA is digested by the restriction enzymes, the sample is then loaded into a gel sheet. Electricity is then run through this gel. The electricity will separate the DNA within the gel based on size. Smaller samples will travel farther away from the origin of the sample, as the larger samples. DNA is made up of different combinations of bases paired to one another. Size of DNA molecules is measured in base-pairs. The more base pairs, the larger the sample. The DNA is then stained, leaving different bands of DNA in different spots for each sample.

When running unknown samples, a sample known as the standard is also run. The standard is made up of pieces of DNA with known sizes. The sizes of other samples are then compared to the standard to determine the length of each of the pieces of the unknowns.

The following diagram, chart, and graph are representative of a fictional crime case. DNA was found at the scene of a crime. The crime scene DNA was then compared to the victim’s DNA along with the two suspects of the crime. These samples were run in the same gel with a DNA Standard Marker for comparison.

Distance Bands Traveled From Origin

Standard Crime Scene 1 Suspect 1 Suspect 1 Victim
1 18 29 26 29 8
2 23 33 29 33 18
3 28 34 30 34 25
4 34 35 32 35 30
5 45 47 36 47 32
6 47 48 48 34

Standard

Distance (mm) Size of Fragment in Base Pairs (bp)
18 23109
23 9416
28 6557
34 4361
45 2328
47 2027

Based on the chart and graph, what is the size of the DNA fragment in base pairs for Band 2 of suspect 1?

A

Approximately $7,000 \ bp$

B

Approximately $100,00 \ bp$

C

Approximately $30 \ bp$

D

Approximately $70,000 \ bp$