A1.2 Nucleic acids
Practice exam-style IB Biology questions for Nucleic acids, aligned with the syllabus and grouped by topic.
Question 1
What is the genetic material of all living organisms?
A.
Protein
B.
RNA
C.
DNA
D.
Lipid
Question 2
What are the three components of a nucleotide?
A.
Hexose sugar, fatty acid and amino group
B.
Glycerol, phosphate group and fatty acid
C.
Amino acid, peptide bond and nitrogenous base
D.
Pentose sugar, phosphate group and nitrogenous base
Question 3
What forms the strong backbone of a DNA or RNA strand?
A.
Alternating bases joined by hydrogen bonds
B.
Phosphate groups joined by weak hydrophobic interactions
C.
Pentose sugars joined directly to nitrogenous bases
D.
Alternating sugars and phosphates joined by covalent bonds
Question 4
What base is present in RNA but absent from DNA?
A.
Guanine
B.
Cytosine
C.
Uracil
D.
Thymine
Question 5
How many histone proteins form the core of a nucleosome?
A.
Sixty-four
B.
Sixteen
C.
Eight
D.
Four
Question 6
State the type of molecule that stores hereditary information in living organisms.
[1]
Some viruses have RNA genomes. State why this does not contradict the statement in (a).
[1]
Question 7
Distinguish between DNA and RNA using three differences. [3]
Question 8
A short DNA strand has the base sequence 5′-A G T C-3′. What is the complementary base sequence?
A.
T C A G
B.
G A C T
C.
U C A G
D.
A G T C
Question 9
What correctly distinguishes DNA from RNA?
A.
DNA contains deoxyribose and RNA contains ribose
B.
DNA contains uracil and RNA contains thymine
C.
DNA is always single-stranded and RNA is always double-stranded
D.
DNA has no phosphate groups and RNA has phosphate groups
Question 10
What observation supports universal common ancestry?
A.
Every codon codes for a different amino acid
B.
All organisms have identical DNA base sequences
C.
All organisms contain the same number of chromosomes
D.
The genetic code is conserved across almost all organisms
Question 11
What group is found at the 5′ terminal of a nucleic acid strand?
A.
A peptide bond attached to the phosphate group
B.
A free phosphate attached to carbon 5 of the sugar
C.
A nitrogenous base attached to carbon 2 of the sugar
D.
A free hydroxyl group attached to carbon 3 of the sugar
Question 12
Which bases are purines?
A.
Adenine and guanine
B.
Guanine and cytosine
C.
Adenine and thymine
D.
Cytosine and thymine
Question 13
What conclusion did Chargaff’s data falsify?
A.
DNA can contain adenine and guanine
B.
DNA contains phosphate groups in its backbone
C.
Some viruses contain RNA as genetic material
D.
DNA is a repeating tetranucleotide with equal amounts of all four bases
Question 14
The diagram shows a simplified nucleotide.
Identify structures X, Y and Z.
[1]
Question 15
Outline how the sugar–phosphate backbone contributes to the function of a nucleic acid strand. [2]
Question 16
Explain how complementary base pairing allows DNA replication to produce accurate copies. [3]
Question 17
Describe the formation of an RNA polymer from nucleotide monomers. [3]
Question 18
Explain why A–T and C–G base pairs help maintain a stable DNA helix. [3]
Question 19
Describe the structure of a nucleosome. [3]
Question 20
The table shows features of three nucleic acid samples extracted from cells.
| Sample | Sugar detected | Bases detected | Usual structure |
|---|---|---|---|
| P | Ribose | A, C, G, U | Usually single-stranded |
| Q | Deoxyribose | A, C, G, T | Usually double-stranded |
| R | Ribose | A, C, G, U | Usually single-stranded |
Identify the sample most likely to be DNA.
[1]
Give two pieces of evidence from the table for your answer to (a).
[1]
Suggest why one sample is described as usually single-stranded rather than always single-stranded.
[1]
Question 21
The graph shows the number of possible DNA base sequences for molecules of increasing length.
State the relationship between sequence length and the number of possible sequences.
[1]
Using the graph, estimate the number of possible sequences for the longest molecule shown.
[1]
Explain why DNA can store large amounts of information with great economy.
[1]
Question 22
How many different DNA base sequences are possible for a molecule that is 6 bases long?
A.
64
B.
24
C.
4096
D.
46656
Question 23
In what direction is a new DNA or RNA strand synthesized during replication or transcription?
A.
3′ to 5′, by addition to the 5′ end
B.
5′ to 3′, by addition to the 3′ end
C.
5′ to 5′, by joining two phosphate groups
D.
3′ to 3′, by joining two sugar hydroxyl groups
Question 24
Why does purine-to-pyrimidine pairing help stabilize the DNA helix?
A.
It makes all base sequences chemically identical
B.
It keeps the distance between the sugar–phosphate backbones constant
C.
It changes hydrogen bonds into covalent bonds
D.
It removes phosphate groups from the backbone
Question 25
In the Hershey–Chase experiment, which label would be expected mainly in the bacterial pellet if DNA entered the bacteria?
A.
³²P, because protein contains phosphorus
B.
³²P, because DNA contains phosphorus
C.
³⁵S, because DNA contains sulfur
D.
³⁵S, because protein enters the bacteria
Question 26
Which pattern in Chargaff’s data for double-stranded DNA supported complementary base pairing?
A.
Purines are absent from DNA in some species
B.
A is approximately equal to G and T is approximately equal to C
C.
A, T, G and C are always present in equal proportions
D.
A is approximately equal to T and G is approximately equal to C
Question 27
A DNA sequence contains 10 base positions.
Calculate the number of possible base sequences of this length.
[1]
Explain why this illustrates the capacity of DNA to store information.
[1]
Question 28
Explain how conservation of the genetic code supports universal common ancestry. [3]
Question 29
A diagram of a short nucleic acid strand shows a free phosphate at one end and a free carbon 3 position on the terminal sugar at the other end.
Label the 5′ end.
[1]
Label the 3′ end.
[1]
State the direction in which nucleotides are added during strand synthesis.
[1]
Question 30
Explain the significance of nucleic acid directionality in transcription. [3]
Question 31
Molecular visualization software shows DNA wrapped around a protein core in a nucleosome.
State one feature that would identify the DNA in the model.
[1]
Explain one interaction that helps DNA associate with histones.
[1]
Question 32
Outline the use of radioisotopes in the Hershey–Chase experiment. [4]
Question 33
Explain how the results of the Hershey–Chase experiment supported the conclusion that DNA is the genetic material. [3]
Question 34
The graph shows the percentage of newly synthesized DNA that matches the expected complementary sequence after different numbers of replication cycles in two experimental treatments.
State which treatment shows higher copying accuracy.
[1]
Describe the trend in copying accuracy over replication cycles for the lower-accuracy treatment.
[1]
Explain how complementary base pairing contributes to the high accuracy shown by the other treatment.
[1]
Question 35
The diagram and graph summarize an experiment in which activated RNA nucleotides were incubated under polymer-forming conditions.
| Time / min | Mean RNA length / nt | H2O formed per bond |
|---|---|---|
| 0 | 1.0 | 1 |
| 10 | 5.2 | 1 |
| 20 | 12.8 | 1 |
| 30 | 21.6 | 1 |
| 40 | 29.4 | 1 |
| 50 | 34.7 | 1 |
| 60 | 37.1 | 1 |
| 80 | 38.5 | 1 |
| 100 | 39.0 | 1 |
| 120 | 39.2 | 1 |
Identify the reaction type used to join the RNA nucleotides.
[1]
Describe the change in mean RNA polymer length over time.
[1]
Explain why polymer formation allows properties not shown by individual nucleotides.
[1]
Question 36
The table compares codon meanings in several organisms from different groups.
| mRNA codon | Bacterium | Plant | Animal | Fungus | Animal mitochondrion |
|---|---|---|---|---|---|
| UUU | Phenylalanine | Phenylalanine | Phenylalanine | Phenylalanine | Phenylalanine |
| UUA | Leucine | Leucine | Leucine | Leucine | Leucine |
| AUG | Methionine | Methionine | Methionine | Methionine | Methionine |
| GCU | Alanine | Alanine | Alanine | Alanine | Alanine |
| CCG | Proline | Proline | Proline | Proline | Proline |
| UGG | Tryptophan | Tryptophan | Tryptophan | Tryptophan | Tryptophan |
| UGA | Stop | Stop | Stop | Stop | Tryptophan |
State the general pattern shown by the table.
[1]
Calculate the number of possible codons made from four bases in groups of three.
[1]
Explain how the pattern in the table supports universal common ancestry.
[1]
Suggest why minor exceptions would not necessarily reject universal common ancestry.
[1]
Question 37
The figure shows three short nucleic acid strands with labelled 5′ and 3′ ends. A polymerase can add nucleotides only to one end of each strand.
Identify the strand to which a nucleotide can be added at the labelled site.
[1]
State the chemical feature of the end used for nucleotide addition.
[1]
Explain why incorrect orientation would prevent synthesis.
[1]
Question 38
A molecular visualization of a nucleosome is shown.
Identify structure X.
[1]
Identify structure Y.
[1]
Explain how the arrangement shown helps package eukaryotic DNA.
[1]
Question 39
Explain how Chargaff’s data addressed the tetranucleotide hypothesis. [4]
Question 40
The table shows predicted distances between the two sugar–phosphate backbones for different hypothetical base pairs.
| Base pair | Base classes | Predicted distance / nm |
|---|---|---|
| A–T | purine–pyrimidine | 2.0 |
| C–G | pyrimidine–purine | 2.0 |
| A–G | purine–purine | 2.4 |
| C–T | pyrimidine–pyrimidine | 1.6 |
Identify the two pair types that would keep the helix width constant.
[1]
Describe the effect of purine–purine and pyrimidine–pyrimidine pairs on predicted backbone distance.
[1]
Explain why stable DNA can contain any base sequence without large changes in helix shape.
[1]
Question 41
The graph shows the distribution of radioactivity after bacteria were infected with bacteriophages labelled either with ³²P or with ³⁵S, agitated and centrifuged.
State which isotope is found mainly in the bacterial pellet.
[1]
State which isotope is found mainly in the supernatant.
[1]
Explain why these results support the conclusion that DNA is the genetic material.
[1]
Suggest why radioisotopes were essential for this experiment.
[1]
Question 42
The table shows the percentage composition of bases in DNA from four species and in one RNA virus.
| Sample | Molecule | A / % | T or U / % | G / % | C / % |
|---|---|---|---|---|---|
| Human | DNA | 30.4 | 30.1 | 19.6 | 19.9 |
| Escherichia coli | DNA | 24.6 | 24.3 | 25.4 | 25.7 |
| Wheat | DNA | 27.3 | 27.1 | 22.8 | 22.8 |
| Salmon | DNA | 29.7 | 29.1 | 20.7 | 20.5 |
| Tobacco mosaic virus | RNA | 29.8 | 24.4 | 25.4 | 20.4 |
Identify one species in which A is approximately equal to T.
[1]
Describe one way the data falsify the tetranucleotide hypothesis.
[1]
Explain why A≈T and G≈C are expected in double-stranded DNA.
[1]
Suggest why the RNA virus may not show the same pattern.
[1]
Question 43
Describe the structure of one DNA nucleotide.
[1]
Explain how the structure of DNA allows hereditary information to be stored and accurately copied.
[1]
Question 44
State two similarities between DNA and RNA.
[1]
Compare and contrast DNA and RNA in structure and biological role.
[1]
Question 45
Outline the meaning of gene expression.
[1]
Explain the role of complementary base pairing in replication and gene expression.
[1]
Question 46
Calculate the number of possible DNA sequences that are 8 bases long.
[1]
Discuss how DNA combines stability with a vast capacity for information storage.
[1]
Question 47
Describe how the 5′ and 3′ ends of a nucleic acid strand differ.
[1]
Explain the significance of nucleic acid directionality for replication, transcription and translation.
[1]
Question 48
Distinguish between purines and pyrimidines, giving examples.
[1]
Discuss how purine-to-pyrimidine bonding contributes to the stability and information capacity of DNA.
[1]
Question 49
Outline the design of the Hershey–Chase experiment.
[1]
Evaluate how the results provided evidence that DNA, rather than protein, is the genetic material.
[1]
Question 50
Describe two patterns in Chargaff’s base-composition data for DNA.
[1]
Discuss how Chargaff’s data contributed to understanding DNA as genetic material and illustrate falsification in science.
[1]
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