AP BiologyUnit 1: Chemistry of Life1.6
AP Biology · Unit 1 · Topic 1.6

Nucleic Acids

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

DNA: covalent sugar-phosphate backbone outside, hydrogen-bonded base pairs inside. The order of bases is the message.

DNA and RNA are polymers of nucleotides, and their information is the order of the bases. Strong covalent bonds hold each strand together. Weak hydrogen bonds between bases let two strands pair up and come apart.

Key terms

  • Nucleotide — the monomer: a five-carbon sugar, a phosphate group, and a nitrogenous base.
  • Phosphodiester bond — the covalent bond linking one nucleotide's phosphate to the next nucleotide's sugar.
  • Complementary pairing — A pairs with T (U in RNA), and G pairs with C, held by hydrogen bonds.
  • Purines (A, G) have two rings. Pyrimidines (C, T, U) have one. Every base pair joins one of each.
  • Antiparallel — DNA's two strands run in opposite directions: one 5′→3′, its partner 3′→5′.
  • DNA vs RNA — DNA has deoxyribose and thymine and is usually double-stranded. RNA has ribose and uracil and is usually single-stranded.

How it works

  1. A phosphodiester bond joins the phosphate of one nucleotide to the sugar of the next. Repeat, and you get a sugar-phosphate backbone.
  2. Each sugar carries one base that sticks out from the backbone. The backbone repeats identically; only the order of bases varies.
  3. A strand has two different ends: a phosphate at the 5′ end, a sugar –OH at the 3′ end. New nucleotides attach only at the 3′ end.
  4. In DNA, two strands line up antiparallel. Their bases meet in the middle: A–T pairs with two hydrogen bonds, G–C pairs with three.
  5. The paired strands twist into a double helix. Each strand predicts its partner, so either can serve as a template for copying or repair.
  6. RNA is usually a single strand with no partner as backup. That suits it to short-lived working copies of genes.

Why it matters

Replication, transcription, and translation all build on this structure. Replication breaks the hydrogen bonds and uses each strand as a template. On data questions, two checks sort most unknowns. The sugar and base name the molecule: ribose and uracil mean RNA, deoxyribose and thymine mean DNA. Then compare partners: if A doesn't equal T, or G doesn't equal C, it can't be a DNA double helix.

Exam traps

TrapActually
The bases form the backbone of the strand.The backbone is alternating sugar and phosphate. Bases hang off the sugars and point inward, toward the partner strand.
Separating the two strands breaks covalent bonds.Hydrogen bonds between paired bases hold the strands together. Heat or enzymes break those and leave both backbones intact.
Same base counts means same genetic information.The information is the order of the bases. Two molecules with identical counts can encode completely different proteins.
C and T are purines; A and G are pyrimidines.Reversed. A and G are purines, with two rings. C, T, and U are pyrimidines, with one.
Worked example

A sample of double-stranded DNA from a plant is 18% cytosine. What percentage of its bases are guanine, adenine, and thymine?

  1. C pairs with G, so guanine is also 18%.
  2. Together G and C make up 36%, which leaves 100 − 36 = 64% for A and T.
  3. A pairs with T, so they split that evenly: adenine 32%, thymine 32%.

Guanine 18%, adenine 32%, thymine 32%.

Each base equals its partner, not its neighbor. A student who pairs A with C writes 18% for adenine, and one who splits 82% three ways gets about 27% each. The shortcut works only because the DNA is double-stranded; a single strand has no partner forcing the counts to match.

Now try it on real exam-style questions

Ten questions on Nucleic Acids, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 1.6 Nucleic Acids

Which molecule stores the hereditary information that a cell passes on to its daughter cells?

  1. A RNA
  2. B Protein
  3. C DNA
  4. D ATP
Show the answer

C — DNA stores hereditary information in its sequence of bases and is copied and passed on when a cell divides. RNA is tempting because it also carries genetic information, but in cells it serves as a short-lived working copy rather than the inherited record.

Question 2 · 1.6 Nucleic Acids

In double-stranded DNA, which base pairs with guanine on the opposite strand?

  1. A Adenine
  2. B Cytosine
  3. C Thymine
  4. D Uracil
Show the answer

B — Guanine pairs with cytosine, and adenine pairs with thymine, so each double-ringed purine is matched by a specific single-ringed pyrimidine. Adenine is wrong because two purines paired together would be too wide to fit inside the helix.

Question 3 · 1.6 Nucleic Acids

Which bond links each nucleotide to the next one along a single strand of DNA?

  1. A A hydrogen bond
  2. B A peptide bond
  3. C A glycosidic bond
  4. D A phosphodiester bond
Show the answer

D — Along one strand, the phosphate of one nucleotide is covalently joined to the sugar of the next by a phosphodiester bond, forming the sugar-phosphate backbone. Hydrogen bonds are the weaker bonds that hold the bases of two strands together, not the links within a strand.