IB Biology HL A1.2 Nucleic acids Questions

Practise IB Biology HL A1.2 with questions on nucleotide structure, DNA and RNA, complementary pairing and the information stored in sequences.

97 questions in this syllabus scope, with question parts counted together.

Exam analysis

HL · 2010–2025

Papers with this topic41 / 117in the analysed sample

Latest appearanceNovember 2025

Tagged question parts53

Paper types5

Most tested objectives

A1.2.13 (HL)—Structure of a nucleosome13 papers
A1.2.6—DNA as a double helix12 papers
A1.2.14 (HL)—Hershey-Chase experiment8 papers
A1.2.2—Components of a nucleotide4 papers
A1.2.3—Sugar-phosphate bonding3 papers

Common question formats

Diagram interpretation
Definition or recall
Structured response
Data analysis
Calculation
Comparison
Graph interpretation

Recent exam appearances

November 2025Paper 2 HL · TZ1Q7(a)[ 4 ]

November 2025Paper 1B HL · TZ1Q1(a)(i), 1(b)[ 4 ]

November 2025Paper 1A HL · TZ3Q1[ 1 ]

117 papers analysed · Tagging incomplete

Exam points

Draw and label DNA or RNA nucleotides, then show how covalent sugar-phosphate bonds form a polynucleotide.
Compare DNA and RNA using their sugars, bases, strand structure and complementary pairing.
Explain antiparallel DNA double-helix structure using purine–pyrimidine pairing, hydrogen bonds and 5′/3′ directionality.
Use complementary sequences and genetic-code evidence to infer base-sequence diversity, conserved information and DNA/RNA relationships.
Interpret X-ray diffraction and Hershey–Chase evidence to justify DNA as genetic material and support the double-helix model.
Describe nucleosomes as DNA wrapped around histone octamers and explain their roles in chromosome packing and transcription regulation.

Covered syllabus points

A1.2.1—DNA as genetic material A1.2.2—Components of a nucleotide A1.2.3—Sugar-phosphate bonding A1.2.4—Bases form the basis of a code A1.2.5—RNA as a polymer A1.2.6—DNA as a double helix A1.2.7—Differences between DNA and RNA A1.2.8—Complementary base pairing A1.2.9—Diversity of DNA base sequences A1.2.10—Conservation of genetic code A1.2.11 (HL)—Directionality of RNA and DNA A1.2.12 (HL)—Purine-to-pyrimidine bonding and helix stability A1.2.13 (HL)—Structure of a nucleosome A1.2.14 (HL)—Hershey-Chase experiment A1.2.15 (HL)—Chargaff's data

Exam points

State that DNA is the genetic material of living organisms and occurs in chromosomes, mitochondria and chloroplasts.
Recognise that some viruses use RNA as genetic material and are not considered living organisms.

Exam points

Identify the phosphate group, pentose sugar and nitrogenous base that make up a nucleotide.
Distinguish deoxyribose in DNA from ribose in RNA, and classify A/G as purines and C/T/U as pyrimidines.

[Maximum number: 7]

The graph shows results of an experiment by Hershey and Chase in 1952, in which bacteria were infected with a mixture of virus particles labelled with ${ }^{32} \mathrm{P}$ or ${ }^{35} \mathrm{~S}$ isotopes. A suspension of the infected bacteria was agitated with a blender, and samples collected from the suspension were centrifuged to record the percentage of isotope remaining directly outside of the cells.

(a)

Explain how this data provides evidence that DNA is the genetic material of cells.

[ 3 ]

a. phosphorous/P is present in DNA AND sulphur/S in proteins/amino acids;

b. lower percentage of ${ }^{32} \mathrm{P}$ remaining outside the cells show that ${ }^{32} \mathrm{P}$ was (injected) in the bacteria / vice versa;

c. since phosphorus/P is a component of DNA therefore DNA is the genetic material / OWTTE;

d. high percentage of ${ }^{35} \mathrm{~S}$ remaining outside the cells shows that ${ }^{35} \mathrm{~S}$ was not (injected) in the bacteria / vice versa;

e. (since sulphur/S is component of protein), therefore protein is not genetic material;

b. and d. must indicate radioactive /isotopic P or S

(b)

Draw a labelled diagram to show the structure of a DNA nucleotide.

[ 3 ]

(c)

List the DNA nucleotides that are purines and pyrimidines.

Purines:

Pyrimidines:

[ 1 ]

Exam points

Explain how condensation reactions link nucleotides through sugar-phosphate bonds to form a continuous covalent backbone.
Describe how bases project from the sugar-phosphate backbone and carry sequence information.

[Maximum number: 4]

RNA molecules play numerous roles in gene expression and so contribute to variation.

Draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA.

a. Ribose drawn as pentagon and labelled sugar/ribose.
b. Base drawn with correct link to $C_{1}$ of ribose and labelled base/nitrogenous base.
c. Phosphate drawn with correct link to $C_{5}$ of ribose and labelled P/phosphate.
d. Two (or more) ribonucleotides drawn with correct link $C_{3}$ to $C_{5}$ .

official ribonucleotide diagram answer

[4 max]

No public sample questions are available for this syllabus point yet.

Exam points

Describe DNA as a double helix formed by two antiparallel polynucleotide strands.
Explain complementary base pairing by hydrogen bonds: A–T and G–C.
Draw DNA bases attached to sugars rather than to phosphate groups.

[Maximum number: 1]

Which of the following describes the molecular structure of DNA?

I. Complementary base pairing occurs within the molecule
II. Contains deoxyribose
III. Contains thymine

I and II only

I and III only

II and III only

I, II and III

Exam points

Compare the usual strand number and molecule length of DNA and RNA: DNA is usually double-stranded, while RNA is usually single-stranded and much shorter.
Distinguish DNA deoxyribose and thymine from RNA ribose and uracil.

[Maximum number: 1]

What usually distinguishes DNA from RNA?

DNA

RNA

strands are symmetrical

strands are antiparallel

contains adenine

contains cytosine

pentoses linked to phosphates

pentoses linked to bases

double stranded

single stranded

Exam points

State the complementary base pairs: A–T in DNA, A–U in RNA, and C–G.
Explain how complementary base pairing enables accurate DNA replication, transcription and translation in gene expression.

[Maximum number: 1]

The image of a nucleosome was obtained using molecular visualization software.

Adenine forms hydrogen bonds with the nitrogenous base X . Identify X .

Exam points

Describe how DNA length and base sequence vary and how genome size and gene number differ widely between organisms.
Explain how diversity in DNA base sequences gives DNA a large information-storage capacity.

[Maximum number: 4]

At all levels of biological organization there is diversity.

Explain the diversity of possible base sequences in nucleic acids.

a. four bases in DNA / bases in DNA are adenine cytosine guanine and thymine

OR

four bases in RNA / bases in RNA are adenine cytosine guanine and uracil;

b. purines and pyrimidines

OR

uracil instead of thymine in RNA;

c. bases/nucleotides can be arranged in any sequence;

d. many/4 ${ }^{\mathrm{n}}$ possible permutations/combinations/sequences (of the four bases)

OR

64 possible triplets of bases/codons;

e. nucleic acids/RNA/DNA can have any number of nucleotides / unlimited in length;

f. mutations can change base sequence/increase diversity;

g. many different proteins/proteins so many sequences needed to code for them;

h. three/triplet of bases needed to code for each amino acid

OR

degeneracy of genetic code/more than one codon per amino acid;

Marking guidance:

Award mark point a and mark point b if the answer includes the four bases in both DNA and RNA.

Do not accept letters instead of base names in mark point a.

Mark points d and g are different: mark point d is for the idea that the four bases in any sequence gives many possible base sequences; mark point g is for the idea that the many different proteins necessitate many different base sequences in the genome.

Exam points

Explain that the 64 codons have nearly the same meanings across life.
Use conserved genes for transcription, translation and ribosomes as evidence supporting common ancestry.
Explain how synonymous mutations can preserve an amino acid sequence.

[Maximum number: 1]

What provides evidence for the universal nature of the genetic code?

Uracil replaces thymine in RNA

The amount of A is equal to the amount of T and the amount of G is equal to the amount of C

Nucleic acids contain the same bases in all species

mRNA codons are assigned to the same amino acids in different species

Exam points

Explain how 3′–5′ phosphodiester bonds create the 5′ and 3′ ends of a nucleic-acid strand.
Describe DNA strands as antiparallel, with one running 5′ to 3′ and the other 3′ to 5′.
State that RNA is synthesized and translated in the 5′ to 3′ direction.

[Maximum number: 1]

Which points to the $3^{\prime}$ end of a strand of DNA?

III

No public sample questions are available for this syllabus point yet.

Exam points

Describe DNA wrapping nearly twice around a histone octamer and the binding of histone H1 to linker DNA and the histone core.
Explain how nucleosomes package DNA and allow regulated access to genes.

[Maximum number: 1]

What is a nucleosome?

DNA wrapped around histone proteins in a eukaryotic cell

RNA bound to ribosomes in a eukaryotic cell

A region of the nucleus in a eukaryotic cell

Genetic material within a viral capsule

Exam points

Describe how Hershey–Chase used ³²P to label bacteriophage DNA and ³⁵S to label protein.
Explain how ³²P-labelled DNA entered E. coli and appeared in new viruses, supporting the conclusion that DNA rather than protein carries genetic information.

[Maximum number: 1]

Hershey and Chase used bacteriophages to investigate the chemical nature of genes. The phosphorus in the DNA and the sulphur in the protein of the bacteriophages were radioactively labelled.

The data obtained after the infection and then centrifugation of bacterial cells is shown in the table.

What did Hershey and Chase conclude from their experiment?

Viral DNA was found within the bacterial cells.

Viruses infect bacterial cells with proteins.

DNA was mainly outside the bacterial cells.

Neither protein nor DNA were chemicals making up genes in viruses.

No public sample questions are available for this syllabus point yet.

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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