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Notes for the Instructor

This lesson takes students through the basic steps involved in DNA profiling, imitating the process using DNA strands made from paper. They simulate the gel electrophoresis process of DNA profiling. Lastly, they will examine DNA profiles to identify the source of the blood evidence.

DNA. Deoxyribonucleic acid. A unique piece of evidence. DNA is located in the nucleus, or center, of most cells. It contains coded information about how each person is made. Each cell has 46 chromosomes: 23 from the mother and 23 from the father. These chromosomes contain groups of coded information called genes. This is DNA.

DNA is a coiled structure, like a long, twisted ladder. The ladder has two rails, nucleotides, which support the rungs. It is the order of the paired bases, on the rungs, that determine genetics. In this lesson, students will use only a piece of a single strand of DNA (one half of the paired bases on the rungs).

Although humans share 99.9% of DNA, it is still very unique. Except for identical twins, no two people have the exact same DNA, or genetic makeup. DNA is often found at crime scenes in blood, hair, and skin. All of these can be tested to determine genetic information. Because of its unique qualities, only a very small sample is needed in order to complete a DNA profile. DNA profiling is separating a person’s unique individual qualities from those that we all share as human beings. In profiling, DNA is mixed with a chemical that cuts it into sections.

A sample of DNA can be obtained using a cotton swab and storing it in a clean tube. The sample is kept cold to ensure that it does not deteriorate before testing. For testing, electricity, dyes and a gel box are used in a laboratory. During testing, the DNA must first be removed from the nucleus of the cell. Next, strands of the DNA are separated from the rest of the cell. These are cut into smaller pieces. The smaller pieces are combined with radioactive DNA, which will allow scientists to track it later.

The DNA sections are put at the end of a gel box. An electrical current runs through the gel box and causes the sections of DNA to move down the box. The shorter sections of DNA move farther and faster than the longer ones. When the electricity is shut off, the DNA pieces have separated from each other into bands according to size. This is called gel electrophoresis. X-rays can now be taken of the DNA pieces to record its makeup.

The activities in this lesson address Next Generation Science Standards practices of Asking Questions and Defining Problems, Planning and Carrying Out Investigations, and Engaging in Argument from Evidence as well as disciplinary core idea LS3.B. In addition, they address Common Core State Standards CCSS.ELA-Literacy.CCRA.SL.1 and CCSS.ELA-Literacy.CCRA.SL.2. See the Standards Matrix included in the appendix for more detailed information.