Nucleic Acids

Essential Question

How do nucleic acids store information?

Every cell contains instructions for building its proteins, and those instructions are stored in nucleic acids. DNA and RNA are polymers built from only four kinds of monomers each, yet the order of those monomers can store an enormous amount of information. In this lesson, you’ll learn how nucleotides are built, how the two strands of DNA pair up, and how DNA differs from RNA. Unit 9 returns to DNA in more depth.

DNA and RNA

• Nucleic acids store and carry genetic information
• DNA is the long-term storage of instructions
• RNA carries information and helps build proteins
• Both are polymers of nucleotides

Nucleic acid	Full name	Main job
DNA	Deoxyribonucleic acid	Stores the instructions for building proteins
RNA	Ribonucleic acid	Copies and carries those instructions

The two kinds of nucleic acids are DNA and RNA. DNA stores the instructions for building the proteins of an organism, and it is passed on from parent cells to new cells. RNA copies parts of those instructions and carries them to where proteins are built. Nucleic acids contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. Because they contain phosphorus, a scientist can tell them apart from proteins, which contain nitrogen but usually not phosphorus.

Nucleotides

• Nucleotide: the monomer of a nucleic acid
• Each has a phosphate group, a sugar, and a nitrogen base
• DNA bases: A, T, G, C
• RNA bases: A, U, G, C

The monomer of a nucleic acid is a nucleotide. Each nucleotide has three parts: a phosphate group, a five-carbon sugar, and a nitrogen-containing base. The phosphate and sugar are the same in every nucleotide of a given kind of nucleic acid. What differs is the base. DNA has four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). RNA has uracil (U) in place of thymine.

The Double Helix

• A DNA molecule has two strands
• Each strand has a sugar-phosphate backbone
• Bases in the middle pair up by hydrogen bonds
• A pairs with T; G pairs with C

Nucleotides join into a chain by dehydration synthesis, with the phosphate of one nucleotide bonding to the sugar of the next. This makes a sugar-phosphate backbone. DNA is made of two of these strands that twist around each other in a double helix. The bases face each other in the middle and are held together by hydrogen bonds. In DNA the pairing is specific: adenine pairs with thymine (A with T), and guanine pairs with cytosine (G with C). Each A to T pair has two hydrogen bonds and each G to C pair has three.

Base-Pairing Rules in the Data

• In double-stranded DNA, the amount of A equals the amount of T
• The amount of G equals the amount of C
• A + T + G + C = 100%
• The ratio of A to G can differ between species

The graph shows sample base composition of DNA from two sources. In both, the percent of A equals the percent of T, and the percent of G equals the percent of C. This is what the base-pairing rule predicts, because every A in one strand is paired with a T in the other strand, and every G with a C. The two sources have different amounts of A and G, however. This means that the proportions of bases can differ from one species to another, but the pairing rules stay the same.

DNA Compared with RNA

• DNA has deoxyribose; RNA has ribose
• DNA has thymine; RNA has uracil
• DNA is usually double-stranded; RNA is usually single-stranded
• Both are made of nucleotides

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Two (double helix)	One
Main job	Stores instructions	Carries instructions, helps build proteins

DNA and RNA are alike because both are polymers of nucleotides, but they differ in three ways that you can use to tell them apart. A molecule with uracil is almost certainly RNA, and a molecule with thymine is almost certainly DNA. A molecule with ribose is RNA, and a molecule with deoxyribose is DNA. A double-stranded molecule is usually DNA.

Information in the Sequence

• The order of bases is the information
• Three bases in a row can code for one amino acid
• The sequence of DNA determines the sequence of a protein
• The sequence of a protein determines its shape and function

Level	What is in order	Result
DNA	Sequence of nucleotides	Instructions
Protein	Sequence of amino acids	Shape and function

Just as the letters of the alphabet carry meaning in the order they are written, the four bases of DNA carry information in the order they appear. The order of bases in a gene determines the order of amino acids in a protein, and as you saw in the last lesson, the order of amino acids determines the protein’s shape and function. This is how the structure of DNA determines the structure of proteins. Unit 9 covers how the cell reads DNA to build proteins.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Using the Base-Pairing Rules

In Source 1 of the graph, A makes up 31% of the bases. (a) What percent is T? (b) What percent are G and C together, and what percent is each? (c) A DNA molecule has 1,000 base pairs, and 20% of its bases are G. How many nucleotides does it have, and how many are C?

Show the solution

1. (a) T: A pairs with T, so T = 31%.
2. (b) G and C: A + T = 62%, so G + C = 100% − 62% = 38%. G equals C, so each is 38% ÷ 2 = 19%. This matches the graph.
3. (c) Nucleotides: 1,000 base pairs means 2 × 1,000 = 2,000 nucleotides. G = 20%, so C = 20%, and C = 0.20 × 2,000 = 400.
4. Answer: (a) 31%. (b) 38% together, 19% each. (c) 2,000 nucleotides and 400 C.

Example 2: Writing the Complementary Strand

One strand of DNA has the sequence A T G C C T A G. (a) Write the sequence of the complementary strand. (b) How many hydrogen bonds hold the two strands together along this stretch? An A to T pair has 2 and a G to C pair has 3.

Show the solution

1. (a) Complementary strand: pair each base. A to T, T to A, G to C, C to G, C to G, T to A, A to T, G to C. The complementary strand is T A C G G A T C.
2. (b) Hydrogen bonds: there are 4 A to T pairs (positions 1, 2, 6, and 7), which gives 4 × 2 = 8 bonds. There are 4 G to C pairs (positions 3, 4, 5, and 8), which gives 4 × 3 = 12 bonds. The total is 8 + 12 = 20 hydrogen bonds.
3. Answer: (a) T A C G G A T C. (b) 20 hydrogen bonds.

Example 3: DNA or RNA?

Two molecules were isolated from a cell. Molecule 1 has two strands, contains thymine, and its sugar is deoxyribose. Molecule 2 has one strand, contains uracil, and its sugar is ribose. Identify each molecule and state the evidence.

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Two	One

Show the solution

1. Molecule 1: DNA. Evidence: two strands, thymine, and deoxyribose all match the DNA column.
2. Molecule 2: RNA. Evidence: one strand, uracil, and ribose all match the RNA column.
3. Reasoning: uracil is the usual base in RNA and thymine is the usual base in DNA, so the base alone is good evidence, and the sugar and the number of strands support it.
4. Answer: Molecule 1 is DNA. Molecule 2 is RNA.

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Practice

Write complementary strands, use the base-pairing rules to find percents, read a graph of base composition, and compare DNA with RNA. Try it on your own first, then check your work with the answer key.

• Nucleic Acids Practice Worksheet (PDF)
• Nucleic Acids Practice Answer Key (PDF)

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Lesson Slides

• 5.5 Nucleic Acids Slides (PDF)

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Big Ideas

• Nucleotides (phosphate, sugar, base) are the monomers of nucleic acids
• DNA stores instructions; RNA carries them
• A pairs with T (or U in RNA); G pairs with C
• The order of bases is the information
• DNA and RNA differ in sugar, bases, and number of strands

Back to the essential question: nucleic acids store information in the order of their bases. The two strands of DNA are held together by pairing A with T and G with C, which lets a cell copy the information accurately, and the sequence of bases determines the sequence of amino acids in proteins. Next, you’ll see how enzymes, which are proteins, speed up the reactions of life.

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