Test tubes in a laboratory

Test tubes in a laboratory (nicolas, iStockphoto

DNA Extraction

Let's Talk Science
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6.8
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Summary

Learn the basics of how DNA can be extracted from cells.

DNA analysis can be used in many ways. For example, it can be used for matching crime scene samples, testing for genetic diseases or identifying a new species. But the first step of DNA analysis is the extraction of DNA from the sample that’s going to be studied. 

Extracting Strawberry DNA (2014) by the Sci Guys (5:36 min.).

DNA samples from humans and many other animals are often extracted from blood or skin cells. The easiest way to collect skin cells from humans is to brush the inside of the cheek with a cotton swab. This is called a buccal swab. “Buccal” is a term meaning cheek or mouth. 

DNA samples from small animals (like insects) or from plants can be extracted from small tissue samples. 

DNA analysis requires pure DNA. DNA is located in cells. But there are lot of other materials in cells, too. For example, cells also contain membranes and proteins. To get pure DNA, technicians must separate and remove these unwanted materials first. 

Graphic decomposition of a chromosome to the base pair of the DNA
Graphic decomposition of a chromosome to the base pair of the DNA (Source: Let’s Talk Science, adapted from illustration by KES47 via Wikimedia Commons).

Did you know?

Human DNA is over 60% identical to banana DNA!

The basic steps for extracting DNA are the same no matter what the cell type.

1. Cell lysis

The word lysis means “to separate.” In a cell, lysis occurs when membranes are broken apart. Cells have an outer membrane called the cell membrane. They also have an inner membrane that surrounds the DNA. This inner membrane is called the nuclear membrane. Cell membranes are made up of two layers of lipids (fat molecules) and proteins

Chemical lysis is one way to break apart the cell membrane and nuclear membrane. In chemical lysis, a lab technician adds a  detergent to the cell. (2) The detergent separates the lipid molecules, causing the membrane to break down. (Detergents clean dishes in the same way. They bind to grease (lipids), which makes it easier to wash the grease particles off with water.) 

Physical lysis can also help break down cell membranes. This involves grinding or blending the cells, which breaks their membranes. Chemical lysis is sometimes combined with physical lysis.

Artist’s interpretation of the gradual lysis of cell and nuclear membranes
Artist’s interpretation of the gradual lysis of cell and nuclear membranes (Source: r_Microbe via iStockphoto).

2. Precipitation of DNA

After the cell and nuclear membranes are broken down, the lipid molecules must be removed. A lab technician adds a highly concentrated salt solution. This causes the detergent and other cellular debris, such as proteins, to precipitate. To precipitate means to form a solid that separates from the solution.

The DNA remains dissolved in the liquid solution. It can be removed from the cellular debris by centrifugation. In centrifugation, the liquid solution spins at high speed so that the precipitate collects as a pellet at the bottom of a tube. The DNA, which is still dissolved in the liquid, can be moved to a new sample tube. Alternatively, the precipitate can be filtered out of the solution. That way, the liquid containing the DNA is left behind .

DNA in the nucleus is wrapped around proteins called histones. This helps organize the DNA into chromosomes. To remove the histone proteins, a protease can be added. A protease is an enzyme that breaks down proteins. 

DNA extraction from a strawberry
DNA extraction from a strawberry. The floating white inside the test tube indicate the DNA has completely separated (Source: Joo Nath [CC BY-SA 4.0] via Wikimedia Commons).

3. Removal of the DNA

The DNA now needs to be removed from the liquid solution. DNA is soluble in water. That means it can dissolve in water. However, it is not soluble when alcohol and salt are present. Lab technicians can add ethanol or isopropyl alcohol (rubbing alcohol) so that the DNA clumps and form a visible white precipitate. It’s important to use cold alcohol because it allows a larger amount of DNA to be extracted. If the alcohol is too warm, it may cause the DNA to denature [bold], or break down. 

During centrifugation, the DNA condenses into a pellet. When the alcohol is removed, relatively pure DNA will be left behind!

DNA precipitate in sample tubes
The clumpy, white globs of DNA can be seen in the sample tubes (Source: Ipseidem [CC BY 4.0] via Wikimedia Commons).

Did you know?

The oldest DNA samples ever recovered were about 800 000 years old!

References

Alaska BioPREP Virtual Textbook. (n.d.). The basics of DNA extraction. University of Alaska Fairbanks.

Bio Technology Notes. (n.d.). 3 main steps involved in the extraction of DNA.

ScienceDirect. (2019). DNA extraction - an overview.