Aim for FiveBiology

Unit 1 · Topic 1.6

1.6 Nucleic Acids

DNA and RNA are nucleic acids: polymers of nucleotides that store and carry genetic information in the order of their bases. DNA is a double helix whose two strands run in opposite directions and pair A with T and C with G. RNA is usually single-stranded, uses ribose instead of deoxyribose, and has uracil in place of thymine.

Key terms

  • nucleotide
  • nitrogenous base
  • 5′ and 3′ ends
  • antiparallel
  • double helix
  • uracil

The nucleotide

Each nucleotide, the monomer of a nucleic acid, has three parts: a five-carbon sugar, a phosphate group and a nitrogenous base (a ring-shaped molecule containing nitrogen). The sugar is deoxyribose in DNA and ribose in RNA. The five bases are adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U). DNA uses A, G, C and T; RNA uses A, G, C and U.

Information is stored in the sequence of bases, much like meaning is stored in the order of letters in a word.

The backbone and its directions (5′ and 3′)

Nucleotides are joined by covalent bonds between the phosphate of one nucleotide and the sugar of the next. This makes a sugar-phosphate backbone with the bases sticking out to the side.

The carbons in the sugar are numbered, and two of them define the ends of a strand. One end has a free phosphate on the sugar's 5′ carbon (read 'five prime'), called the 5′ end. The other end has a free hydroxyl group (−OH) on the sugar's 3′ carbon, called the 3′ end. So every strand has a direction, written 5′ → 3′.

When a cell builds a nucleic acid, each new nucleotide can only be attached at the 3′ end of the strand being built. That's why we say strands grow in the 5′ → 3′ direction. This rule matters a lot for DNA replication and transcription in Unit 6.

DNA: an antiparallel double helix

DNA is two strands wound around each other in a double helix. The strands are antiparallel: they run in opposite directions, so one goes 5′ → 3′ and its partner runs 3′ → 5′ alongside it.

The bases on opposite strands pair up by hydrogen bonds, always in the same way: A pairs with T, and C pairs with G. A and G are larger, double-ring bases, while C, T and U are smaller, single-ring bases. So each pair has one large and one small base, which keeps the helix the same width all the way along. A G-C pair forms three hydrogen bonds and an A-T pair forms two, so DNA rich in G-C pairs holds its strands together more tightly.

Because the pairing is so predictable, the two strands are complementary: if you know one strand's sequence, you know the other's. It also means that in double-stranded DNA, the amount of A equals the amount of T, and the amount of G equals the amount of C.

DNA vs. RNA

You don't need to memorize the detailed chemical structures of the individual nucleotides. Know the three parts, the base-pairing rules, the 5′ and 3′ ends and the differences in the table.

FeatureDNARNA
SugarDeoxyriboseRibose
BasesA, T, G, CA, U, G, C
Usual structureDouble-stranded helixSingle-stranded
Base pairingA-T and C-GA-U and C-G (when RNA pairs with itself or with DNA)
Main roleLong-term storage of genetic informationCarrying and using information, such as mRNA in protein synthesis

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1

    Writing the complementary strand

    One strand of a DNA molecule reads 5′-ATTGCAGCT-3′. Write the complementary strand in the 5′ → 3′ direction.

    Show the solution
    1. Step 1: Pair each base: A↔T and C↔G. Going along the given strand, the partners are T A A C G T C G A.
    2. Step 2: Label the ends: the strands are antiparallel, so this complementary strand runs 3′-TAACGTCGA-5′ beside the original.
    3. Step 3: Rewrite it 5′ → 3′, as sequences are normally written, by reading it backward: 5′-AGCTGCAAT-3′.
    4. Step 4: Check: lining up 5′-ATTGCAGCT-3′ with 3′-TAACGTCGA-5′, every pair is A-T or C-G.

    Answer: 5′-AGCTGCAAT-3′ (the same strand as 3′-TAACGTCGA-5′)

  2. Example 2

    Using base-pairing rules on composition data

    A sample of double-stranded DNA is 22% adenine. What percentages of thymine, guanine and cytosine does it contain?

    Show the solution
    1. Step 1: A pairs with T, so in double-stranded DNA, %T = %A = 22%.
    2. Step 2: A + T together: 22% + 22% = 44%.
    3. Step 3: The rest is G + C: 100% − 44% = 56%.
    4. Step 4: G pairs with C, so they're equal: 56% ÷ 2 = 28% each.

    Answer: Thymine 22%, guanine 28%, cytosine 28%.

  3. Example 3

    When the rule doesn't apply (classic trap)

    A virus's genetic material is analyzed and found to contain 30% A, 18% U, 27% G and 25% C. Is this genetic material DNA or RNA, and is it single- or double-stranded?

    Show the solution
    1. Step 1: Look for thymine or uracil: there is U and no T, so it's RNA.
    2. Step 2: Check the pairing rule: in a double-stranded molecule, A would equal U and G would equal C. Here A = 30% but U = 18%, and G = 27% but C = 25%.
    3. Step 3: The trap: don't assume every sample follows A = T (or A = U). That rule only holds when every base has a partner on a second strand.
    4. Step 4: Since the amounts don't match, the bases aren't all paired, so the molecule is single-stranded.

    Answer: Single-stranded RNA: it contains uracil, and A ≠ U and G ≠ C.

Common mistakes

  • Writing the complementary strand in the same direction as the original. The strands are antiparallel; label the 5′ and 3′ ends every time.
  • Pairing A with U in DNA, or A with T in RNA. DNA uses thymine; RNA uses uracil.
  • Saying nucleotides are added to the 5′ end. New nucleotides always attach at the strand's 3′ end.
  • Applying A = T and G = C to single-stranded nucleic acids, where it doesn't have to hold.

On the exam

  • Expect questions that ask you to write or identify a complementary sequence, label 5′ and 3′ ends on a diagram, or calculate base percentages.
  • When comparing DNA and RNA, name the specific difference (sugar, base or number of strands), not just 'they're different.'

Connected topics

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Check yourself

4 questions on 1.6 Nucleic Acids. Pick an answer to see if you got it, and why.

Question 1 of 4

One strand of a DNA molecule has the sequence 5′-ATGCCA-3′. What is the sequence of the complementary strand, written 5′ to 3′?

Question 2 of 4Calculator allowed

A sample of double-stranded DNA is found to be 30 percent adenine. What percent of the bases in the sample are guanine?

Question 3 of 4

A molecule isolated from a cell is single-stranded and contains the base uracil. Which other feature would this molecule most likely have?

Question 4 of 4

Researchers add a nucleotide to the end of a growing nucleic acid strand in a test tube. According to the rule for nucleic acid synthesis in cells, to which end of the strand is the new nucleotide attached?

0 of 4 answered