Study Guide

AHL: Nucleic acid structure

IB Biology HLΒ· Theme D, D1: Nucleic acids (AHL)Β· 15 min read

1. Nucleotide Structureβ˜…β˜…β˜†β˜†β˜†β± 5 min

πŸ“˜ Definition

Nucleotide

The monomer subunit of all nucleic acids (DNA and RNA), consisting of three covalently bonded components: a pentose (5-carbon) sugar, a nitrogenous base, and a phosphate group.

Example:

A DNA adenine nucleotide contains deoxyribose sugar, adenine base, and one phosphate group.

Carbons in the pentose sugar are numbered 1' to 5' (the prime symbol distinguishes them from carbons in the nitrogenous base). Nitrogenous bases are divided into two groups based on structure.

  • Purines (double-ring): Adenine (A) and Guanine (G) found in both DNA and RNA

  • Pyrimidines (single-ring): Cytosine (C) found in both; Thymine (T) only in DNA; Uracil (U) only in RNA

  • Sugar difference: DNA uses deoxyribose (hydrogen at 2' carbon), RNA uses ribose (hydroxyl at 2' carbon)

πŸ“ Worked Example

Draw and label a single DNA nucleotide, identifying 3' and 5' carbons.

  1. 1
    1. Draw the 5-carbon deoxyribose sugar ring, then number each carbon:
  2. 2
    • 1' carbon: bonds to the nitrogenous base
  3. 3
    • 2' carbon: has only a hydrogen atom (no hydroxyl, distinguishing it from RNA)
  4. 4
    • 3' carbon: has a free hydroxyl (-OH) group
  5. 5
    • 5' carbon: extends outside the ring, bonds to the phosphate group
  6. 6
    1. Add the phosphate group covalently bonded to the 5' carbon, and a nitrogenous base bonded to the 1' carbon.
  7. 7

    Result: A correctly labeled nucleotide has a free 3' hydroxyl and 5' phosphate group, ready to bond into a nucleic acid strand.

2. Polymerization and Strand Directionalityβ˜…β˜…β˜…β˜†β˜†β± 5 min

πŸ“˜ Definition

Phosphodiester Bond

A covalent bond formed between the 5' phosphate group of one nucleotide and the 3' hydroxyl group of the adjacent nucleotide in a growing strand, formed by a condensation reaction.

The sugar-phosphate backbone forms the negatively charged outer structure of the nucleic acid strand, with nitrogenous bases pointing inward (in double-stranded DNA) to form hydrogen bonds.

πŸ“ Worked Example

Explain why the sequence 5'-ATGC-3' has a clear directionality, and identify the functional groups at each end.

  1. 1
    1. The 5' end of this sequence (the adenine nucleotide) has a free phosphate group attached to its 5' carbon, which is not bonded to another nucleotide.
  2. 2
    1. The 3' end of this sequence (the cytosine nucleotide) has a free hydroxyl group attached to its 3' carbon, which is available to form a new phosphodiester bond with another nucleotide.
  3. 3
    1. By convention, all nucleic acid sequences are written 5' to 3' because that is the direction they are synthesized and read in cells.

3. DNA Double Helix Structureβ˜…β˜…β˜…β˜†β˜†β± 4 min

Native DNA exists as a double-stranded right-handed helix, held together by hydrogen bonds between complementary nitrogenous bases from opposite strands. The two strands have opposite orientation, called antiparallel.

πŸ“˜ Definition

Antiparallel Orientation

The two strands of DNA run in opposite 5'β†’3' directions, which allows complementary base pairs to align correctly for hydrogen bonding.

πŸ“ Worked Example

One DNA strand has sequence 5'-GATCAG-3'. Write the complementary strand with correct orientation.

  1. 1
    1. Apply complementary base pairing to each base: G→C, A→T, T→A, C→G, A→T, G→C. This gives the sequence C T G A T C aligned 3'→5' relative to the original strand.
  2. 2
    1. Reverse the sequence to write it in standard 5'β†’3' orientation, since complementary strands are antiparallel.
  3. 3
    1. Final complementary strand sequence: 5'-CTGATC-3'

4. Structural Differences Between DNA and RNAβ˜…β˜…β˜†β˜†β˜†β± 3 min

DNA and RNA have distinct structural differences that reflect their different functions in cells: DNA stores long-term genetic information, while RNA transfers genetic information for protein synthesis and has catalytic roles.

Feature

DNA

RNA

Pentose sugar

Deoxyribose (2' H)

Ribose (2' OH)

Nitrogenous bases

A, T, C, G

A, U, C, G (U replaces T)

Typical structure

Double-stranded double helix

Single-stranded, folded 3D shape

Length

Very long (whole genome)

Short (single gene copy)

βœ“ Quick check

Test your understanding

  1. Which feature is unique to RNA?

    • Adenine

    • Deoxyribose

    • Uracil

    • Phosphate group

    Reveal answer
    Uracil β€”

    Correct! Uracil replaces thymine in RNA, and is not found in DNA. Deoxyribose is unique to DNA.

5. Common Pitfalls

Wrong move:

Claiming deoxyribose is missing oxygen at the 3' carbon, not 2'.

Why:

The missing oxygen is at the 2' carbon; 3' still has a hydroxyl required for bond formation.

Correct move:

Deoxyribose has a hydrogen at 2', ribose has a hydroxyl at 2'; both have a hydroxyl at 3'.

Wrong move:

Writing the complementary DNA sequence in the same 5'β†’3' orientation as the original strand.

Why:

DNA strands are antiparallel, so the complementary strand must run in the opposite direction.

Correct move:

Generate complementary bases, then reverse the sequence to get the correct 5'β†’3' orientation.

Wrong move:

Mixing up purines and pyrimidines: claiming pyrimidines are double-ring.

Why:

Purines are larger double-ring structures, pyrimidines are smaller single-ring.

Correct move:

Mnemonic: Pure As Gold (Purines: Adenine, Guanine) = double ring; CUT the Py (Cytosine, Uracil, Thymine = Pyrimidines) = single ring.

Wrong move:

Claiming RNA is always single-stranded so it never forms base pairs.

Why:

Most RNA forms intramolecular base pairs to create functional 3D shapes (e.g., tRNA).

Correct move:

RNA is typically single-stranded, but can form complementary base pairs within the strand or with other nucleic acids.

6. Quick Reference Cheatsheet

Key Concept

Core Fact

Nucleotide components

5-carbon sugar + nitrogenous base + phosphate

Deoxyribose vs ribose

Deoxy = 2' H; Ribose = 2' OH

Complementary base pairing

A-T (2 H bonds), G-C (3 H bonds)

Strand direction

Synthesized 5'β†’3', free OH at 3' end

DNA structure

Antiparallel double helix

R

N

A

u

n

i

q

u

e

f

e

a

t

u

r

e

s

U

r

a

c

i

l

r

e

p

l

a

c

e

s

t

h

y

m

i

n

e

,

r

i

b

o

s

e

s

u

g

a

r

7. Frequently Asked

Why is deoxyribose called deoxyribose?

It lacks an oxygen atom at the 2' carbon, compared to ribose (the sugar in RNA) which has a hydroxyl group at this position.

Why is DNA antiparallel?

Antiparallel orientation allows complementary nitrogenous bases to align correctly and form the hydrogen bonds that hold the double helix together.

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2025 Β· Paper 1

    Compare DNA and RNA structure

  • 2024 Β· Paper 2

    Draw and label a DNA nucleotide

  • 2023 Β· Paper 1

    Directionality of nucleic acid strands

Going deeper

What's Next

Nucleic acid structure is the foundation for all genetic processes in IB Biology. The directionality and base pairing rules you learned here directly explain how DNA is replicated accurately, how genetic information is transcribed into RNA, and how mutations arise from changes to nucleotide sequence. Mastery of this topic is required to answer almost all genetics questions on IB Biology HL exams, and supports understanding of continuity and change across generations.