DNA Sequencing: Definition, Methods & Applications

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  • 0:04 What Is DNA Sequencing?
  • 0:35 DNA Replication
  • 1:59 DNA Sequencing
  • 4:12 Uses for DNA Sequencing
  • 5:15 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

DNA sequencing allows scientists to know the exact message coded in DNA. In this lesson, we'll talk about how DNA replication is the basis for sequencing, how scientists do DNA replication in the lab, and important uses for it in research and everyday life.

What Is DNA Sequencing?

Do you ever wonder what really makes you you? Are you interested in understanding human disease, personalized drugs, or forensic science? If so, this lesson is for you. What all of these things have in common is DNA sequencing, a technique scientists use to find the exact message in a DNA molecule. This is important because DNA codes for every single part of us. From our hair to our personality, DNA makes us who we are.

DNA Replication

DNA sequencing is based on a process called DNA replication, which naturally occurs in cells to create two copies of DNA from one original copy. During DNA sequencing, scientists also need to create more DNA, so we model this process in the lab.

DNA is made of two strands of building blocks called nucleotides, which are twisted together in a double helix shape. The outside of the DNA is called the sugar phosphate backbone and holds the strands in place. There are four types of nucleotides, and the order of them in a piece of DNA determines the message it holds. The four types of nucleotides are adenine (A), guanine (G), cytosine (C), and thymine (T). When we talk about DNA, scientists will often refer to these nucleotides using just their letters: A, G, C, or T. Since there are two strands, the nucleotides on either strand match up, like partners. A always partners with T, and G always partners with C. Think of it like puzzle pieces. A is a perfect fit for T, and G is a perfect fit for C.

During DNA replication, the two strands separate, and a special protein called DNA polymerase comes in and adds new nucleotides. Now each double helix has one original strand and one new strand.

DNA Sequencing

Remember, we said that DNA sequencing is a technique scientists use to find the exact message in a DNA molecule. They do this by determining the sequence of nucleotides in a particular sample of DNA.

During DNA sequencing, scientists combine a sample of DNA they would like to sequence with A, G, C, and T, along with additional nucleotides called dideoxynucleotides in a test tube. These nucleotides are like A, G, C, and T, but they have something different about them as they are each marked with a different fluorescent color.

They then add a special kind of DNA polymerase called taq polymerase. This process is called PCR, or polymerase chain reaction, and is used to make many copies of a DNA sample in the lab. It's like making 30 photocopies of a worksheet for class, each one exactly the same as the first. Scientists use the right temperatures to separate the double helix to allow DNA polymerase to add new nucleotides and form two new strands of DNA.

When DNA polymerase adds in these fluorescent, color-coded dideoxynucleotides, the DNA can no longer add new nucleotides, the copying stops, and the scientists can see the sequence of nucleotides. It would be as if each time you copied a worksheet, the copier stopped at a different sentence, which is a strand of DNA in our case.

The sample is run through a machine that detects each of the colors and the size of the DNA piece. Smaller pieces migrate faster through the machine, so they are read first. The machine then creates a sequence of nucleotides based on which size fragment contains which nucleotides. The sequence shows up as peaks of each color on a graph, and the machine produces the sequence in letters of A, G, T, and C. The machine generates an electropherogram, or graph of a DNA sequence. Each peak corresponds to one nucleotide.

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