What you will learn

• Nucleic acids and their structure
• Role of nucleic acids

[Figure 1]

You may have heard that something is “encoded in your DNA.” What does that mean?

Nucleic acids. Essentially the “instructions” or the “code” of life. Deoxyribonucleic acid, or DNA, carries the unique code to make the proteins that give you your traits. Half of this code comes from your biological mother and a half from your biological father. Therefore, every person that has ever lived – except for identical twins – has his or her own unique sequence of code – or instructions – or DNA.

Nucleic Acids

A nucleic acid is an organic compound, such as DNA or RNA, that is built of monomers called nucleotides. Many nucleotides bind together to form a chain called a polynucleotide. The nucleic acid DNA (deoxyribonucleic acid) consists of two polynucleotide chains. The nucleic acid RNA (ribonucleic acid) consists of just one polynucleotide chain. Adenosine Triphosphate (ATP) is another important nucleic acid. ATP is described as the “energy currency” of the cell or the “molecular unit of currency.” One molecule of ATP contains three phosphate groups.

Structure of Nucleic Acids

Each nucleotide consists of three smaller molecules as shown in the Figure below:

1. sugar
2. phosphate group
3. nitrogen base

If you look at the Figure below, you will see that the sugar of one nucleotide binds to the phosphate group of the next nucleotide. These two molecules alternate to form the backbone of the nucleotide chain. This backbone is known as the sugar-phosphate backbone.

[Figure 2]

The nitrogen bases in a nucleic acid stick out from the backbone. There are four different types of bases: cytosine (C), adenine (A), guanine (G), and either thymine (T) in DNA, or uracil (U) in RNA. In DNA, bonds form between bases on the two nucleotide chains and hold the chains together. Each type of base binds with just one other type of base: cytosine always binds with guanine, and adenine always binds with thymine. These pairs of bases are called complementary base pairs.

The binding of complementary bases allows DNA molecules to take their well-known shape, called a double helix, which is shown in the Figure below. A double helix is like a twisted ladder. The double helix shape forms naturally and is very strong, making the two polynucleotide chains difficult to break apart.

[Figure 3]

Roles of Nucleic Acids

DNA is also known as the hereditary material or genetic information. It is found in genes, and its sequence of bases makes up a code. Between ‘starts’ and ‘stops’, the code carries instructions for the correct sequence of amino acids in a protein. DNA and RNA have different functions relating to the genetic code and proteins. Like a computer code, DNA contains the genetic instructions for the correct sequence of amino acids in proteins. RNA uses the information in DNA to assemble the correct amino acids and help make the protein. The information in DNA is passed from parent cells to daughter cells whenever cells divide. The information in DNA is also passed from parents to offspring when organisms reproduce. This is how inherited characteristics are passed from one generation to the next.

Summary

• DNA, RNA, and ATP are nucleic acids. Polynucleotides like DNA and RNA are built of monomers called nucleotides.
• The bases of DNA are adenine, guanine, cytosine, and thymine. In RNA, thymine is replaced by uracil.
• In DNA, A always binds to T, and G always binds to C.
• The shape of the DNA molecule is known as a double helix.
• DNA contains the genetic instructions for the correct sequence of amino acids in proteins. RNA uses the information in DNA to assemble the correct amino acids and help make the protein.

Review

1. Identify the three parts of a nucleotide.
2. What is DNA?
3. What are complementary base pairs? Give an example.
4. Describe the shape of DNA.
5. How are DNA and RNA related to proteins?

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