What Is DNA?

Deoxyribonucleic acid (DNA) is a chemical found in the nucleus of cells and carries the ‘instructions’ for the development and functioning of living organisms. It is often compared to a set of blueprints since it contains the instructions needed to build cells. These instructions are divided into segments along a strand of DNA and are called genes. Genes are a DNA sequence that code for the production of a protein and control hereditary characteristics such as eye color or personality behaviors. Proteins determine the type and function of a cell, so a cell knows whether it is a skin cell, a blood cell, a bone cell, etc, and how to perform its appropriate tasks. Other DNA sequences are responsible for structural purposes or are involved in the regulation and use of genetic information.

Structure of DNA

DNA Structure and Nucleotide

The structure of DNA can be compared to a ladder. It has an alternating chemical phosphate and sugar backbone, making the ‘sides’ of the ladder. (Deoxyribose is the name of the sugar found in the backbone of DNA.) In between the two sides of this sugar-phosphate backbone are four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). (A grouping like this of a phosphate, a sugar, and a base makes up a subunit of DNA called a nucleotide.) These bases make up the ‘rungs’ of the ladder, and are attached to the backbone where the deoxyribose (sugar) molecules are located.

The chemical bases are connected to each other by hydrogen bonds, but the bases can only connect to a specific base partner – adenine and thymine connect to each other and cytosine and guanine connect to each other. The arrangement of these bases is very important as this determines what the organism will be – a plant, an animal, or a fungus. This is called genetic coding. For example, one side of DNA could have the genetic code of AAATTTCCCGGGATC. Its complementary side would then have to be TTTAAAGGGCCCTAG.

Even though the shape of DNA is often described as a ladder, it is not a straight ladder. It is twisted to the right, making the shape of the DNA molecule a right-handed double helix. This shape allows for a large amount of genetic information to be ‘stuffed’ into a very small space. In fact, if you lined up each molecule of DNA in one cell end to end, the strand would be six feet in length.

DNA Replicates Itself

Click for Larger View

Before a cell can divide and make a new cell, it must first duplicate its DNA. This process is called DNA replication. When it is time to replicate, the hydrogen bonds holding the base pairs together break, allowing the two DNA strands to unwind and separate. The specific base pairing provides a way for DNA to make exact copies of itself. Each half of the original DNA still has a base attached to its sugar-phosphate backbone. A new strand of DNA is made by an enzyme called DNA polymerase. It reads the original strand and matches complementary bases to the original strand. (The sugar-phosphate backbone comes with the new bases.) New strands attach to both sides of the original DNA, making two identical DNA double helices composed of one original and one new strand.

Please note that the above explanation of DNA replication is highly simplified. To see a more detailed, animated explanation of the structure of DNA and the replication process, click here.

How DNA Is Used

DNA graphicAll living things – plants, animals, and humans – pass DNA from parents to offspring in the form of chromosomes. In humans, 23 chromosomes are passed on from the mother and 23 chromosomes are passed on from the father, giving the child 46 chromosomes. Chromosomes carry genes from the parents, but not all the genes of a parent are sent along. For each child, different sets of genes are passed on from the parents, resulting in unique DNA for each child. This means that even though the genetic code for all human beings is 99.9% identical, no one has the exact same DNA code except in the case of true identical twins.

Knowing this, DNA can be used to identify people in a variety of situations. DNA is often used to solve crimes by identifying victims and suspects while at the same time ruling out innocent people as possible suspects for a crime. It is also used to prove or disprove family relationships, identify missing persons, and identify the victims of catastrophes who are no longer physically identifiable. And since DNA can be found in a variety of human tissues and fluids such as hair, urine, blood, semen, skin cells, bones, teeth, and saliva, it greatly aids in identification when other methods, such as fingerprints and teeth structure, are no longer usable.

The medical field also uses DNA. Now that doctors at least partially understand how DNA works, modern medicine has made advances in identifying diseases and finding cures. Many diseases, like cystic fibrosis, are hereditary diseases, meaning they are passed on from parent to offspring. By looking at the DNA of an individual, doctors can determine what the disease is or how susceptible a person or their children are to having a particular disease. Doctors also study how cells with damaged DNA multiply to help them find cures or treatments for diseases such as cancer and tumors.

But knowledge of DNA is not just used in humans. Food scientists use DNA information to improve crops and develop new food sources. Plant breeders select plants that produce high yields of food, are resistant to pests, and tolerate environmental stresses better than similar plant varieties. This is especially important in areas that have poor growing conditions and/or the area has a large population to feed. However, there has been growing debate on whether or not these genetically modified food sources are safe and healthy for human consumption.


DNA Science Project

Build a DNA Model

To help further understand how DNA is structured, build a model of it. This is a simplified model of DNA, but it will still give you the general idea of how the sugars, phosphate groups, and bases all connect together to make the famous double helix shape of DNA. You can make a model out of a variety of materials. Here’s how you can do it with candy.

What You Need:

  • Red and black hollow licorice sticks
  • Gummy bears
  • String
  • Toothpicks
  • Small white marshmallows

What You Do:

  1. Cut the red and black licorice sticks into one inch strips.
  2. Make two equal lengths of licorice strands by threading the pieces of licorice onto the string, alternating the red and black pieces.
  3. Gather together four different colors of gummy bears, the marshmallows, and the toothpicks.
  4. Pair two colors of the gummy bears together and then pair two other colors together. For example, red and orange gummy pairs could be paired together, and green and yellow ones be paired together.
  5. Take a gummy bear and thread it onto the toothpick. Thread the marshmallow onto the toothpick so that it is in the center of the toothpick and next to the gummy bear. Thread the complementary gummy bear onto the toothpick so that it is next to the marshmallow. You should now have a toothpick with a gummy bear-marshmallow-gummy bear centered on it.
  6. Repeat step five to make more gummy bear-marshmallow toothpicks, making sure the gummy bears are matched with their complementary colors. Make as many of these toothpicks as you have red pieces on one of your licorice strands.
  7. Take one strand of licorice and start attaching the gummy bear-marshmallow toothpicks to it, connecting one of these toothpicks at each of the red pieces on the strand. Then, take the second licorice strand and connect it to the other side of the toothpicks. Again, connect the toothpicks to the red pieces of licorice. You should end up with a ‘ladder’ with the red and black licorice stands making the sides of the ladder and the gummy bear-marshmallow toothpicks making the rungs of the ladder.
  8. Hold your candy ladder up and turn the top counterclockwise to add twists to the ladder.

What Happened:

You have just made a candy model of a strand of DNA. The red licorice represents the sugar deoxyribose, the black licorice represents the phosphate groups, and together they represent the sugar-phosphate backbone of DNA.

The gummy bears represent the bases that make the code of DNA. The four different colors are used to represent the four different bases found in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). It doesn’t really matter in your model how much of a base you use or where it is placed in the strand, but it is important that bases are paired up correctly: A with T and G with C. (In real DNA the order does matter as that determines what type of organism it is and how functional it will be.)

The marshmallow in between the gummy bears represents the hydrogen bonds connecting the bases. This is the point at which the DNA strands break apart during replication and where the new strand connects to the original strand. Twisting the ladder at the top in a counterclockwise direction gives the DNA model its true shape: a right-handed double helix.