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    Home / Science lessons / Learn about DNA
    • Learn about DNA

      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

      All 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.

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