DNA sequencing allows us to determine the exact genes that make up an individual. It can be used to screen for certain diseases and in crime scene analysis. Learn more about it in this lesson.

The Importance of DNA Sequencing

You probably already know that your DNA determines everything that happens in your body and that changes in DNA can lead to many different diseases and disorders. Have you ever wondered how exactly we know what genes are encoded in DNA and how they vary from person to person? Humans have 23 chromosomes that each contain many genes. The complete set of all the genes on all the chromosomes is known as the human genome.

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Although there are many genes that are exactly the same in all people, there are also differences that make each person unique. You have a genome that is different from everyone else’s! It determines your hair color, how tall you are, and what diseases you are more likely to develop, among many other things.

In this lesson, you’ll learn about the methods that are used to sequence DNA. You will also learn about how the entire human genome was sequenced and about how DNA sequencing is used today.

Structure of DNA

DNA is made of only four nucleotide bases, thymine (T), adenine (A), guanine (G), and cytosine (C). Each gene is simply a specific sequence of these four bases. In a chromosome, two long strands of DNA bind to each other and form a coil that looks like a double helix. There are 23 distinct chromosomes in each of your cells, each one containing many genes. DNA sequencing is the method of determining the exact sequence of nucleotide bases in a chromosome.

Human Genome Project

Although we have known about the structure of DNA for a long time, it wasn’t until relatively recently that we were able to sequence the entire human genome and know all the genes that are contained in a human like you! In 1990, a massive international research project called the Human Genome Project to sequence the entire human genome was proposed and funded by the United States government. Scientists all over the world worked on this for over ten years, and finally, in 2003, it was declared to be completed.

Several donors, both male and female, donated blood and other tissues over the years to this project so the genome could be determined. Of course, each person has genes that are unique, so even though we now know the locations of all the genes in a human genome, those genes may differ from person to person. Mapping those differences is something that scientists are working on now.

How Is DNA Sequenced?

How exactly did scientists first sequence the human genome? The first method used to sequence DNA was developed by Fred Sanger in the late 1970s, and this basic method was used to complete the Human Genome Project in the 1990s and is still used today. The Sanger method is a method of DNA sequencing that incorporates chain terminating nucleotide bases that will stop the copying of DNA when it gets to a particular base.

To sequence DNA using Sanger’s method, you must first separate the two-paired DNA strands. Then, you make copies of each piece of DNA but include special nucleotide bases so that the copy will stop when it gets to a certain base (either A, T, C, or G). By doing this over and over for all the bases, you can identify exactly where in the genome each base pair is located. To get the entire sequence, you then have to fit all these little copies together, like a jigsaw puzzle.

Now, this process has been improved on by using a method called dye terminator sequencing. Using this method, each base is tagged with a special color of fluorescent dye. Then, a special sequencing machine is used to automatically read the DNA sequence by reading the order of the colors. Dye terminator sequencing has allowed DNA sequencing to be automated and as a result, has sped up the process tremendously and made it much cheaper. This is the way that most DNA is sequenced today.

In addition to the Sanger method, there are several next generation sequencing methods that allow many DNA fragments to be sequenced at the same time. This makes DNA sequencing both cheaper and faster.

Applications of DNA Sequencing

In medicine, DNA sequencing can be used to identify people who are at high risk for certain diseases or who have certain genetic conditions. For example, by sequencing the human genome, we were able to identify a specific mutation in the BRCA gene that leads to a very high risk of breast and ovarian cancer in women.

Women can now have a blood sample tested for this gene mutation and know immediately if it is present in their genome. Identifying this one gene mutation has saved thousands of lives already, and there are many other gene mutations that have been identified as disease risk factors. More and more of these important gene mutations are being discovered every day.

DNA sequencing is also used in crime scene analysis. DNA samples taken from the scene of a crime can be taken and sequenced. Then, the sequence can be compared to DNA taken from suspects and if the sequences match, you know you have the right person! This has revolutionized the way we solve crimes and has exonerated people who have been in prison for decades for crimes they didn’t commit.

In addition to these well-known applications, DNA sequencing has also been used to identify a person’s ancestry. Simply by submitting a small sample of your DNA, you can quickly find out where in the world your ancestors came from.

Lesson Summary

DNA sequencing is the process of determining the exact sequence of bases in a piece of DNA. It can be used to sequence the entire human genome, which contains all the genes in a person, or shorter strands of DNA. DNA sequencing was originally performed using the Sanger method, but newer technologies, like dye terminator sequencing and other next generation sequencing methods, are faster and cheaper to use now. DNA sequencing can be used to identify your risk of developing certain diseases, to determine your ancestry, and in crime scene analysis.

Learning Outcomes

You’ll have the ability to do the following after this lesson:

  • Define DNA sequencing
  • Recall what the Human Genome Project was
  • Describe the Sanger method, dye terminator sequencing and next generation sequencing methods
  • Explain how DNA sequencing is used in everyday life