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3.40: Nucleic Acid Classification

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Did you ever see a spiral staircase like the one in the opening image? The steps connect the two sides of the staircase, which winds around a central axis. A spiral staircase can be used as a model for a DNA molecule. DNA is a nucleic acid.

Q: DNA is such a well-known molecule that you probably know something about it already. What do you know about DNA, and why do you think this molecule is so “famous”?

A: You many know that DNA contains genes. It is so “famous” because it explains how we inherit traits from our parents and pass them on to our offspring.

What Are Nucleic Acids?

Nucleic acids are one of four classes of biochemical molecules. (The other three classes are carbohydrates, proteins, and lipids.) Nucleic acids include RNA (ribonucleic acid) as well as DNA (deoxyribonucleic acid). Both types of nucleic acids contain the elements carbon, hydrogen, oxygen, nitrogen, and phosphorus.

Q: Which of the elements in DNA is not found in any other class of biochemical compounds?

A: All biochemical compounds contain carbon, hydrogen, and oxygen; and proteins as well as nucleic acids contain nitrogen. Phosphorus is the only element that is found in nucleic acids alone.


Nucleic acids consist of chains of small molecules called nucleotides, which are held together by covalent bonds. The structure of a nucleotide is shown in the Figure below . Each nucleotide consists of:

  1. a phosphate group, which contains phosphorus and oxygen (PO 4 ).
  2. a sugar, which is deoxyribose (C 5 H 8 O 4 ) in DNA and ribose (C 5 H 10 O 5 ) in RNA.
  3. one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA, the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the other three bases are the same.

The Double Helix

RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains of DNA form hydrogen bonds with each other. Hydrogen bonds are relatively weak bonds that form between a positively charged hydrogen atom in one molecule and a negatively charged atom in another molecule. Hydrogen bonds form only between adenine and thymine, and between guanine and cytosine. These bonds hold the two chains together and give DNA is characteristic double helix, or spiral, shape.

You can see the shape of the DNA molecule in the Figure below . Sugars and phosphate groups form the “backbone” of each chain of DNA. The bonded bases are called base pairs. Determining the structure of DNA was a huge scientific breakthrough. You can read the interesting story of its discovery and why it was so important at the URL below.


Q: Compare the structure of DNA to a spiral staircase. What part of the molecule do the stair steps represent?

A: The steps represent the base pairs.

Functions of Nucleic Acids

DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in the nucleotide chains of DNA. RNA copies and interprets the genetic code in DNA and is also involved in the synthesis of proteins based on the code. You can watch these events unfolding at the following URL. http://www.youtube.com/watch?v=NJxobgkPEAo&feature=related

Q: DNA is found only in the nucleus of cells, but proteins are synthesized in the cytoplasm of cells, outside of the nucleus. How do you think the instructions encoded in DNA reach the cytoplasm so they can be used to make proteins?

A: After RNA copies the instructions in DNA, it carries them from the nucleus to a site of protein synthesis in the cytoplasm, where the instructions are translated into a protein.


  1. Nucleic acids are one of four classes of biochemical molecules. Nucleic acids include RNA (ribonucleic acid) and DNA (deoxyribonucleic acid).
  2. Nucleic acids consist of chains of small molecules called nucleotides, which are held together by covalent bonds.
  3. RNA is made of one nucleotide chain, and DNA is made of two. The two chains of DNA are held together by hydrogen bonds in the shape of a double helix.
  4. DNA contains the genetic code, which instructs cells how to make proteins. RNA copies the genetic code in DNA and helps synthesize proteins based on the code.


  • nucleic acid : Biochemical compound (either DNA or RNA) that contains oxygen, nitrogen, and phosphorus in addition to carbon and hydrogen; consists of one or two long chains of smaller molecules called nucleotides.


Watch the video about DNA at the following URL, and then answer the questions below.


  1. What are the complementary bases in DNA?
  2. Assume that one of the nucleotide chains in a DNA molecule has the following sequence of bases: ATCGGCTAAC. What is the sequence of bases on the other nucleotide chain of the molecule?


  1. What are nucleic acids? What are the two types of nucleic acids?
  2. Describe the structure of a nucleotide.
  3. What holds nucleotides together in a nucleic acid?
  4. Compare and contrast the structure and functions of DNA and RNA.

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Difficulty Level:



7 , 8

Date Created:

Nov 14, 2012

Last Modified:

Jul 29, 2014
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