IB Biology · Theme A Unity and diversity · Molecules
A1.2 Nucleic acids
DNA holds the heritable information of every living organism in its base sequence. Nucleotides link into strands, and two antiparallel strands pair by hydrogen bonding. Complementary pairing lets that information be copied, expressed and stored without limit.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
Learn
A1.2.1 DNA is the genetic material of every living organism
The genetic material is the molecule that stores heritable information.
In all living organisms that molecule is DNA; its base sequence carries the information.
Some viruses carry RNA instead, but viruses are not considered living.
So viruses are not exceptions to the rule.
Students often think HIV or influenza show that living things can use RNA. In fact viruses are not living organisms, so DNA remains universal in living things.
Students often think proteins carry the genetic information because they do the work. In fact the information is in DNA; proteins are made from it.
A1.2.2 A nucleotide has three parts, with the sugar in the middle
From 2028 this is supplied in the Biology data booklet — you need to recognise and interpret it, not reproduce it from memory.
A nucleotide has a pentose sugar, a phosphate group and a nitrogenous base.
The pentose sugar is central; the phosphate and the base each bond to it.
The pentose is ribose in RNA or deoxyribose in DNA.
In diagrams: circle for phosphate, pentagon for sugar, rectangle for base.
Students often think the nucleotide is just the base letter. In fact it is all three parts, covalently bonded.
Students often draw the phosphate bonded to the base. In fact both bond to the sugar, never to each other.
A1.2.3 Nucleotides link sugar to phosphate to make a strong backbone
The phosphate of one nucleotide bonds to the sugar of the next by condensation.
This is a covalent bond, so the chain is strong.
Alternating sugars and phosphates form the sugar–phosphate backbone of each strand.
The bases project from the backbone; they do not link nucleotides.
Students often think nucleotides in a strand are joined by hydrogen bonds. In fact they are joined by covalent sugar–phosphate bonds.
Students often think adjacent nucleotides join through their bases. In fact the phosphate bonds to the next sugar; bases stay free.
A1.2.4 Four bases in each nucleic acid form the code
2028 guide: scope reduced — Reported (unverified) that the 2028 guide no longer requires the purine/pyrimidine distinction at SL. The 2025 guidance for this statement asks only for the names of the bases, so no SL object here classifies bases as purines or pyrimidines; that distinction appears only in HL objects on A1.2.12 and A1.2.15. All items on this statement are therefore valid under both guides. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
DNA has adenine, cytosine, guanine and thymine.
RNA has adenine, cytosine, guanine and uracil in place of thymine.
The backbone is the same along the strand, so only the base sequence varies.
That sequence is the basis of the genetic code.
Students often put uracil in DNA or thymine in RNA. In fact thymine is DNA only; uracil is RNA only.
Students often say DNA has five bases. In fact it has four; uracil belongs to RNA.
A1.2.5 RNA is a polymer built by condensation of nucleotides
A condensation reaction joins two nucleotides and releases one molecule of water.
The bond forms between the phosphate of one and the sugar of the next.
RNA is a single strand of these linked ribonucleotides: a polynucleotide.
A strand of n nucleotides has n − 1 sugar–phosphate bonds.
Students often think nucleotides are joined by hydrolysis. In fact condensation joins them; hydrolysis splits the polymer.
Students often count one water per nucleotide. In fact one water is released per bond, so 20 nucleotides release 19.
A1.2.6 Two antiparallel strands, paired by hydrogen bonds, make the double helix
DNA is two strands wound into a double helix, backbones outside, bases inside.
The strands are antiparallel: they run in opposite directions.
A pairs with T and G pairs with C: complementary base pairing.
Each pair is held by hydrogen bonds, weak alone but many along the molecule.
Draw the strands antiparallel; the helical twist is not required in a drawing.
Students often think both strands carry the same sequence. In fact they are complementary: A opposite T, G opposite C.
Students often think covalent bonds hold the strands together. In fact hydrogen bonds do, which is why the strands can be separated.
A1.2.7 How DNA and RNA differ: strands, sugar, bases
DNA is usually double-stranded; RNA is usually single-stranded.
DNA contains deoxyribose; RNA contains ribose.
Deoxyribose has one fewer oxygen: –H where ribose has –OH on the ring.
DNA has thymine; RNA has uracil instead.
The cell's RNAs include mRNA, which carries a gene's sequence; tRNA, which carries amino acids; and rRNA, part of the ribosome.
Students often think RNA is just one strand of DNA. In fact the sugar and one base differ too.
Students often think deoxyribose has an extra oxygen. In fact it has one fewer; the name means "without oxygen".
A1.2.8 Complementary pairing lets DNA be copied and expressed
A template strand directs the assembly of a new strand against it.
Each base can hydrogen-bond stably only with its complementary base.
In replication both strands are templates, giving two identical DNA molecules.
In transcription one strand templates a complementary mRNA, which is then translated.
Students often think mRNA has the same sequence as the strand it was copied from. In fact it is complementary to the template, with U opposite A.
A1.2.9 Any length, any sequence: DNA's storage capacity has no limit
Any of four bases can sit at any position along a strand.
So a strand of n nucleotides has 4ⁿ possible sequences.
A DNA molecule can be any length, so the possibilities have no upper limit.
Each nucleotide stores one of four options in under a nanometre: great economy.
Students often calculate 4 × n or n⁴. In fact it is 4ⁿ, because each position is independent.
Students often think four symbols is too few to store much. In fact unlimited length means unlimited capacity.
A1.2.10 One genetic code for all life points to one common ancestor
The genetic code maps each base triplet to an amino acid or a stop.
The same triplets mean the same amino acids in essentially all organisms: the code is universal.
So a gene from one organism can be expressed correctly in another.
No chemistry forces a triplet onto an amino acid; the assignments are arbitrary.
Shared arbitrary assignments are best explained by inheritance from a common ancestor.
Students often think each species has its own code. In fact the code is conserved across all life forms.
Students often think a universal code means shared genes. In fact only the triplet-to-amino-acid rules are shared, not the sequences.
A1.2.11 Strands have direction: 5' to 3' HL
The ends are named after carbons of the pentose sugar.
5' end: phosphate on carbon 5. 3' end: free –OH on carbon 3.
Each backbone phosphate links carbon 3 of one sugar to carbon 5 of the next.
Nucleotides add only to the free 3' –OH, so strands grow 5' to 3'.
The template is read 3' to 5', and the ribosome reads mRNA 5' to 3'.
Students often think a strand can grow at either end. In fact only the 3' end accepts new nucleotides.
Students often call one strand "the 5' strand". In fact every strand has both a 5' end and a 3' end.
A1.2.12 Purine with pyrimidine keeps the helix a constant width HL
Purines have two rings: adenine and guanine.
Pyrimidines have one ring: cytosine, thymine and uracil.
Every DNA base pair is one purine bonded to one pyrimidine.
So A–T and G–C pairs are the same length, and the helix shape is constant.
Students often think G–C-rich regions are wider. In fact every pair is purine plus pyrimidine, so all pairs are equal in length.
Students often think purines pair with purines. In fact a purine always pairs with a pyrimidine.
A1.2.13 A nucleosome: DNA wrapped around eight histones HL
A nucleosome is DNA wrapped around a core of eight histone proteins.
An additional histone attached to linker DNA holds the DNA on the core.
Linker DNA is the stretch between one nucleosome core and the next.
Molecular visualisation software shows the DNA wound around the outside of the core.
Students often think histones wrap around the DNA. In fact the DNA wraps around the histone core.
Students often count nine histones in the core. In fact the core has eight; the ninth sits on the linker DNA.
A1.2.14 Hershey and Chase traced DNA, not protein, into bacteria HL
A bacteriophage is a virus of bacteria, made only of DNA and protein.
DNA was labelled with ³²P; protein was labelled with ³⁵S.
After infection, ³²P was inside the bacteria and ³⁵S outside.
So DNA enters and directs new phage production: DNA is the genetic material.
Radioisotopes becoming available as research tools made the experiment possible.
Students often swap the labels. In fact DNA has phosphorus but no sulfur; protein has sulfur but no phosphorus.
Students often think the idea came first and the technology followed. In fact the new technology opened the way for the experiment.
A1.2.15 Chargaff's ratios falsified the tetranucleotide hypothesis HL
In every organism, A = T and G = C, so purines equal pyrimidines.
The ratio of A–T to G–C differs between species.
The tetranucleotide hypothesis predicted a repeating sequence with equal amounts of all four bases.
Chargaff's data contradicted it, so the hypothesis was falsified.
No number of agreeing observations proves a hypothesis; one reliable contrary result can disprove it.
Students often think every organism has 25% of each base. In fact A = T and G = C, but the ratio varies between species.
Students often think Chargaff's data proved the double helix. In fact they only falsified the earlier hypothesis; structure came later.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement about the genetic material of organisms is correct?
Answer and reasoning
Every living organism, from bacteria to humans, uses DNA as its genetic material. — DNA is the genetic material of all living organisms. The only entities that use RNA as genetic material are certain viruses, and viruses are not considered to be living.
Some living organisms use RNA in place of DNA as their genetic material. — A student who counts RNA viruses as living organisms picks this. Viruses are not considered to be living, so no living organism uses RNA as its genetic material.
Genetic information in cells is stored in proteins rather than in DNA. — A student who confuses a gene with its protein product picks this. Genes are lengths of DNA; the protein is what the gene's base sequence specifies, not the store of information.
Living organisms vary in whether their genes are made of DNA or RNA. — A student who has generalized from RNA viruses picks this. Every living organism uses DNA; RNA viruses do not count because viruses are not living.
2 Which is a correct description of a single nucleotide?
Answer and reasoning
A nitrogenous base only; the sugar and phosphate belong to the backbone. — A student who uses 'base' and 'nucleotide' as synonyms picks this. The sugar and phosphate that make up the backbone are parts of each nucleotide, along with its base.
A phosphate group bonded to a nitrogenous base, which is bonded to a sugar. — A student who reads the circle–pentagon–rectangle diagram as a chain picks this. The phosphate and the base are each bonded to the sugar; they are not bonded to each other.
A pentose sugar bonded to a phosphate group and to a nitrogenous base. — A nucleotide has three components: a pentose sugar, a phosphate group and a nitrogenous base. The sugar is central, with the phosphate and the base each bonded to it.
A six-carbon sugar, glucose, bonded to a phosphate and to a base. — A student who takes 'sugar' to mean glucose picks this. The sugar of a nucleotide is a pentose, with five carbon atoms: ribose or deoxyribose.
3 Which list names the nitrogenous bases found in DNA?
Answer and reasoning
Adenine, cytosine, guanine and uracil: four bases. — A student who attaches uracil to the wrong nucleic acid picks this. Uracil is found in RNA, where it takes the place of thymine; DNA contains thymine.
Adenine, cytosine, guanine and thymine: four bases. — DNA contains four bases: adenine (A), cytosine (C), guanine (G) and thymine (T). The order of these bases along the strand forms the basis of the genetic code.
Adenine, cytosine, guanine, thymine and uracil. — A student who has pooled every base name into one list picks this. Each nucleic acid has exactly four bases; uracil is not found in DNA.
Alanine, cysteine, glycine and threonine: four bases. — A student who blurs DNA with protein picks this: alanine, cysteine, glycine and threonine are amino acids, the monomers of protein, whose names resemble the DNA bases adenine, cytosine, guanine and thymine. The bases of DNA are adenine, cytosine, guanine and thymine.
4 How is a molecule of RNA formed from nucleotides?
Answer and reasoning
Hydrolysis reactions join nucleotides through their sugars and phosphates, taking in a water molecule each time. — A student who has condensation and hydrolysis the wrong way round picks this. Hydrolysis breaks polymers by adding water; nucleotides are joined by condensation, which releases water.
Hydrogen bonds form between the phosphate group of one nucleotide and the sugar of the next in the chain. — A student who thinks hydrogen bonds are the bonds of nucleic acids picks this. The bonds linking nucleotides in a strand are covalent, formed by condensation.
Condensation reactions bond the phosphate of one nucleotide to the sugar of the next, releasing water. — RNA is a polymer formed by condensation of nucleotide monomers. Each condensation forms a covalent sugar–phosphate bond and releases one molecule of water.
Condensation reactions bond the base of one nucleotide to the base of the next, releasing water. — A student who pictures the bases as the links in the chain picks this. Condensation occurs between the phosphate of one nucleotide and the sugar of the next; the bases are not joined to each other.
5 Which row correctly gives differences between DNA and RNA?
Answer and reasoning
DNA: two strands, deoxyribose, thymine. RNA: one strand, deoxyribose, thymine. — A student who thinks RNA is simply one strand of DNA picks this. RNA also differs in its sugar (ribose) and in having uracil instead of thymine.
DNA: two strands, ribose, thymine. RNA: one strand, deoxyribose, uracil. — A student who has the sugars the wrong way round picks this. Deoxyribose (one oxygen fewer than ribose) is the sugar of DNA, as its name says; ribose is the sugar of RNA.
DNA: two strands, deoxyribose, thymine. RNA: one strand, ribose and uracil. — DNA is double-stranded, contains the pentose deoxyribose and has thymine; RNA is single-stranded, contains ribose and has uracil in place of thymine.
DNA: two strands, deoxyribose, thymine. RNA: two strands, ribose, uracil. — A student who draws every nucleic acid as a double helix picks this. RNA is single-stranded; the number of strands is one of the three differences between the molecules.
6 How does complementary base pairing allow the genetic information in a DNA molecule to be replicated?
Answer and reasoning
Each separated strand is a template on which free nucleotides pair by hydrogen bonding, so both new molecules keep the original sequence. — Because each base can hydrogen-bond stably only with its complement, the sequence of a template strand determines the sequence of the strand built against it. Both strands serve as templates, so two identical molecules result.
Each separated strand acts as a template on which free nucleotides pair by covalent bonding, so both new molecules carry the original sequence. — A student who assumes the pairing is covalent picks this. Complementarity is based on hydrogen bonding; the covalent bonds formed during replication are those of the new backbone.
Each strand is copied directly, base for base, so the new strand is identical to the strand it was built against, preserving the sequence. — A student who thinks copying means making an identical strand picks this. The new strand is complementary to its template; the original sequence is restored only across the pair of strands.
Only one strand acts as a template, because the other strand has the same base sequence and would give exactly the same product if it were copied too. — A student who thinks the two strands are identical picks this. The strands are complementary, and both act as templates so that each daughter molecule has one old and one new strand.
7 The human gene for insulin was inserted into a bacterium, which then produced insulin identical to the human protein. Which conclusion does this best support, and why?
Answer and reasoning
Humans and bacteria share the same genes, because the bacterium already carried all the information needed to make human insulin. — A student who takes a universal code to mean shared sequences picks this. Bacteria have no insulin gene; the gene had to be inserted, and only the code for reading it is shared.
The bacterium adopted the human genetic code in place of its own bacterial code when the human gene was inserted. — A student who thinks each species has its own code picks this. There is no separate human code to adopt; the same code is used by both organisms, which is why the gene worked.
Any organism could translate any gene, because the code we observe is the only one that is chemically possible. — A student who assumes the codon assignments are fixed by chemistry picks this. The assignments are arbitrary, so sharing them is evidence of common ancestry, not chemical necessity.
The genetic code is shared by humans and bacteria, because the bacterium translated human triplets into the same amino acids. — The bacterium read the human gene's base triplets exactly as human cells do, producing the same amino acid sequence. This conservation of the genetic code across life forms is evidence of universal common ancestry.
8 What do the labels 5' and 3' refer to in a strand of DNA or RNA? HL
Answer and reasoning
The carbon atoms of the pentose sugar that carry the phosphate group and the hydroxyl group. — The numbers name carbon atoms of the pentose: the phosphate is attached to the 5' carbon and a hydroxyl group to the 3' carbon. A strand has a 5' end and a 3' end depending on which group is free.
The positions of the first and last nucleotides in the strand, counted along it from the starting end. — A student who reads the numbers as positions along the sequence picks this. They are the numbers of carbon atoms in each nucleotide's sugar, not counts along the strand.
The names given to the two different strands that together make up the DNA double helix. — A student who has seen 5' at the left of one strand and 3' at the left of the other picks this. Every strand has both a 5' end and a 3' end; the labels name ends, not strands.
The two ends of a strand, either of which can accept new nucleotides as the strand is extended. — A student who thinks a strand can be extended from either end picks this. Nucleotides are added only to the 3' end, which is the reason the labels matter.
9 Which statement describes the structure of a nucleosome? HL
Answer and reasoning
Eight histone proteins are wrapped around a length of DNA, held in place by an additional histone on linker DNA. — A student who pictures the proteins as a coating on the DNA picks this. It is the DNA that is wound around the outside of the histone core, not the histones around the DNA.
DNA is wrapped around a core of nine histone proteins, one of which is attached to the linker DNA between the cores. — A student who adds the linker histone to the core picks this. The core is eight histones; the additional histone on the linker DNA holds the DNA in place but is not part of the core.
DNA is covalently bonded to a core of eight histone proteins, with the linker DNA left free between one core and the next. — A student who reasons from a stable association to covalent bonding picks this. DNA is held to the histones by non-covalent attraction between its negative phosphates and the positive proteins, which allows it to be released for copying.
DNA is wrapped around a core of eight histone proteins, held in place by an additional histone attached to linker DNA. — A nucleosome is a length of DNA wound around a core of eight histone proteins. An additional histone protein, attached to the linker DNA where the DNA enters and leaves the core, holds the nucleosome together.
10 Hershey and Chase labelled one batch of bacteriophages with radioactive phosphorus (³²P) and another with radioactive sulfur (³⁵S). Why were these two elements chosen? HL
Answer and reasoning
Protein contains phosphorus but not sulfur, and DNA contains sulfur but not phosphorus, so each label marked only one component. — A student who has the labels attached to the wrong molecules picks this. DNA's phosphate groups contain phosphorus; the sulfur is in the amino acids of the protein coat.
DNA contains phosphorus but no sulfur, and protein sulfur but no phosphorus, so each label marked one component. — Phosphorus occurs in the phosphate groups of DNA and not in protein; sulfur occurs in some amino acids of protein and not in DNA. Each radioisotope therefore traced one component of the phage exclusively.
The labels showed that the phage particles contained both DNA and protein, which was enough to identify the genetic material. — A student who thinks finding DNA present is the same as finding it responsible picks this. The point of labelling was to trace which component entered the bacterium and directed new phage production.
The labels identified which one of the phage's many proteins carries its genetic information into the bacterial cell. — A student who thinks genes are made of protein picks this. The experiment compared DNA with protein; it showed that DNA, not any protein, enters the cell and carries the genetic information.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
17 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 The influenza virus contains RNA and no DNA. Why does this not contradict the statement that DNA is the genetic material of all living organisms?
Answer and reasoning
The statement is a generalization, and influenza is one of its known exceptions. — A student who treats the virus as a living organism picks this. The statement has no living exceptions; viruses fall outside it because they are not living.
Viruses are not considered to be living, so the statement does not apply to them. — The statement is about living organisms. Viruses are not cells and are not considered to be living, so a virus with RNA as its genetic material is not an exception to the rule.
The virus's RNA is not genetic material; its protein coat carries the genes. — A student who thinks proteins carry genetic information picks this. The RNA of an influenza virus is its genetic material; the protein coat is not.
RNA is just a single strand of DNA, so the virus's genetic material still counts as DNA. — A student who thinks RNA and DNA differ only in strand number picks this. RNA differs from DNA in its sugar (ribose) and in having uracil, so it is a distinct nucleic acid, not a form of DNA.
2 What links one nucleotide to the next within a single strand of DNA?
Answer and reasoning
A hydrogen bond between the phosphate of one nucleotide and the sugar of the next. — A student who thinks all the bonds in DNA are hydrogen bonds picks this. Within a strand the bonds are covalent; hydrogen bonds occur only between bases on opposite strands.
A covalent bond between the base of one nucleotide and the base of the next one. — A student who pictures the bases as beads on a string picks this. The bases project from the backbone and are bonded only to their own sugar, not to each other.
A bond formed by adding water between the sugar of one nucleotide and the next. — A student who confuses hydrolysis with condensation picks this. The sugar–phosphate bond forms by condensation, which releases water; adding water breaks the bond.
A covalent bond from the phosphate of one nucleotide to the sugar of the next. — Sugar–phosphate bonding makes a continuous chain of covalently bonded atoms along the strand, forming the strong backbone of the molecule.
3 A solution of DNA is heated to 95 °C. The two strands separate, but each strand remains a continuous, intact chain of nucleotides. Which explanation is consistent with these observations?
Answer and reasoning
The backbone of each strand is covalently bonded, so it survives heating that breaks the weaker hydrogen bonds between the strands. — Sugar–phosphate bonding makes a continuous chain of covalently bonded atoms in each strand, forming a strong backbone. The strands are linked only by hydrogen bonds, which are weak enough to be broken by heat.
The hydrogen bonds of the backbone are stronger than the covalent bonds between strands, so only the strands come apart. — A student who has the two bond types the wrong way round picks this. The backbone is covalent and the link between strands is hydrogen bonding; the observation is explained only that way round.
Heating breaks the covalent bonds that join the two strands, while the hydrogen bonds within each strand stay intact. — A student who assumes the strands are joined by covalent bonds picks this. If that were so, the strands would not separate before the backbones broke, which is the opposite of what is observed.
The bases along each strand are covalently bonded to one another, which holds the strand together when the pairs separate. — A student who thinks the bases link nucleotides within a strand picks this. Each base is bonded only to its own sugar; it is the sugar–phosphate backbone that keeps the strand intact.
4 A short RNA molecule is made by joining 20 nucleotides into a single strand. What happens to water during its formation?
Answer and reasoning
20 molecules of water are released, one for each nucleotide. — A student who counts nucleotides instead of bonds picks this. There is one bond fewer than there are nucleotides in a linear chain, so 19 water molecules are released.
19 molecules of water are taken in, one to form each bond. — A student who thinks the joining reaction uses water picks this. Bonds between nucleotides form by condensation, which releases water; water is taken in only when bonds are hydrolysed.
No water is involved, because the bonds are hydrogen bonds. — A student who thinks nucleotides are linked by hydrogen bonds picks this. The links are covalent sugar–phosphate bonds, each formed by a condensation that releases water.
19 molecules of water are released, one per bond formed. — Joining 20 nucleotides into one strand requires 19 sugar–phosphate bonds, and each bond is formed by a condensation reaction that releases one molecule of water: 19 in total.
5 Which statement describes the structure of a DNA molecule?
Answer and reasoning
Two antiparallel strands linked by hydrogen bonds between complementary bases, A with T and G with C. — DNA is a double helix of two antiparallel strands of nucleotides, linked by hydrogen bonding between complementary base pairs: adenine with thymine and guanine with cytosine.
Two strands with identical base sequences linked by hydrogen bonds between matching bases. — A student who thinks the second strand duplicates the first picks this. The strands are complementary, not identical: A on one strand lies opposite T on the other, and G opposite C.
Two antiparallel strands linked by covalent bonds between complementary base pairs, A–T and G–C. — A student who reasons from the stability of DNA to strong bonding picks this. The strands are linked by hydrogen bonds; the covalent bonds are within each strand's backbone.
Two parallel strands linked by hydrogen bonds between complementary bases, A with T and G with C. — A student who has never given a strand a direction picks this. The two strands run in opposite directions: they are antiparallel, and diagrams of DNA should show this.
6 Part of one strand of a DNA molecule has the base sequence ATGGCA. What is the base sequence of the complementary strand at the same position?
Answer and reasoning
ATGGCA — A student who thinks the two strands carry the same sequence picks this. The second strand is complementary to the first, so every base is replaced by its partner.
TACCGT — Each base on one strand is paired with its complement on the other: A with T, T with A, G with C and C with G. So ATGGCA is paired with TACCGT.
UACCGU — A student who puts uracil into DNA picks this. Uracil occurs only in RNA; in the complementary DNA strand adenine pairs with thymine.
GCAATG — A student who pairs A with G and C with T picks this. The complementary pairs in DNA are A with T and G with C.
7 A student sketches the ring structures of ribose and deoxyribose. Which difference should the sketch show?
Answer and reasoning
One carbon of the deoxyribose ring has an extra –OH that ribose lacks. — A student who has not parsed 'deoxy' as 'lacking an oxygen' picks this. Deoxyribose has one fewer oxygen than ribose, not one more.
The deoxyribose ring has six carbon atoms where the ribose ring has five. — A student who thinks of sugars as six-carbon molecules picks this. Both are pentoses with five carbon atoms; they differ only in one oxygen atom.
The rings are identical, and the sugars differ only in which base is attached. — A student who thinks the only difference between DNA and RNA is the number of strands picks this. The sugars themselves differ: deoxyribose lacks one oxygen atom that ribose has.
One carbon of the deoxyribose ring carries –H where ribose carries –OH. — Deoxyribose is ribose with one hydroxyl group replaced by hydrogen, so it has one fewer oxygen atom. The sketch shows –H at that carbon in deoxyribose and –OH in ribose.
8 The template strand of a gene has the base sequence TACGGA at one position. What base sequence will the mRNA transcribed from it have at that position?
Answer and reasoning
UACGGA — A student who thinks the mRNA copies the template's sequence picks this. The mRNA is complementary to the template strand, not identical to it.
AUGCCU — RNA nucleotides are added by complementary base pairing with the template: A pairs with U, T with A, C with G and G with C. So TACGGA gives AUGCCU.
ATGCCT — A student who puts thymine into RNA picks this. RNA contains uracil in place of thymine, so the base paired with template A is U.
CGUAAG — A student who pairs A with G and C with T picks this. The complementary pairs are A with U (or T) and G with C.
9 How many different base sequences are possible for a single DNA strand that is 10 nucleotides long?
Answer and reasoning
4 × 10, which is exactly 40 — A student who combines the two numbers by multiplying them picks this. The possibilities multiply at each position, giving 4¹⁰, not 4 × 10.
10⁴, which is exactly 10 000 — A student who puts the numbers into the wrong places in the power picks this. It is the number of bases (4) raised to the number of positions (10): 4¹⁰.
4¹⁰, which is about 1 000 000 — Each of the 10 positions can be any of four bases, independently of the others, so the number of possible sequences is 4 × 4 × … (10 times) = 4¹⁰ = 1 048 576.
2¹⁰, which is exactly 1024 — A student who counts two kinds of base pair rather than four bases picks this. Each position on a strand can hold any of four bases, so the base is 4, not 2.
10 During replication, in which direction is a new DNA strand synthesized, and why? HL
Answer and reasoning
3' to 5', because a nucleotide can be added only to the free phosphate on the 5' carbon of the growing strand. — A student who has the direction reversed picks this. The incoming nucleotide's phosphate joins the free 3' hydroxyl of the chain, so growth is at the 3' end and synthesis is 5' to 3'.
5' to 3', because nucleotides can be added only to the free hydroxyl on the 3' carbon of the strand. — The phosphate of each incoming nucleotide bonds to the free 3' hydroxyl of the last sugar in the chain, so the strand can only grow at its 3' end: synthesis is 5' to 3'. This determines how the antiparallel template is read.
5' to 3', because the template strand is read in the same direction as that in which the new strand grows. — A student who forgets that the new strand is antiparallel to its template picks this. Synthesis is 5' to 3', which means the template is read 3' to 5', not in the same direction.
In either direction, because nucleotides can be joined to whichever end of the growing strand is free. — A student who thinks a chain can grow at both ends picks this. Only the 3' end carries the free hydroxyl that can react with an incoming nucleotide's phosphate.
11 Why does the DNA double helix have the same three-dimensional structure whatever its base sequence? HL
Answer and reasoning
Purines pair with purines and pyrimidines with pyrimidines, so both bases in every pair are the same size. — A student who expects 'matching' bases to be alike picks this. A purine always pairs with a pyrimidine; two purines would be too wide for the helix and two pyrimidines too narrow.
G–C pairs are longer than A–T pairs, but the helix adjusts its width locally so its shape is preserved. — A student who thinks the two kinds of pair differ in size picks this. They do not: each is one purine plus one pyrimidine and the pairs have equal length, so no adjustment is needed.
Every base pair is one two-ring purine with one one-ring pyrimidine, so A–T and C–G pairs are of equal length. — Adenine and guanine are purines (two rings); thymine and cytosine are pyrimidines (one ring). Because each pair combines one of each, all pairs are the same length and the helix has a uniform structure, which contributes to its stability.
Every base pair is one two-ring pyrimidine with one one-ring purine, so A–T and C–G pairs have equal length. — A student who has the classes reversed picks this. Purines (A, G) have two rings and pyrimidines (C, T) one; the conclusion about equal length is right but the classification is wrong.
12 A student uses molecular visualization software to examine a nucleosome, colouring the DNA one colour and the proteins another. What would the student observe about the association between the DNA and the proteins? HL
Answer and reasoning
The DNA lies on the outside, wound around a compact core of eight protein molecules, with its phosphate backbone against the protein surface. — In the visualization the DNA double helix winds around the outside of the histone core, which is a compact assembly of eight histone proteins; the phosphate backbone of the DNA lies against the positively charged surface of the proteins.
The proteins lie on the outside, forming a coat around a compact loop of DNA, with the protein surfaces facing outward, away from the DNA backbone. — A student who expects histones to wrap the DNA picks this. The software shows the reverse: the DNA is the outer coil and the proteins form the core it is wound around.
The DNA passes straight through the centre of the protein core, with the eight proteins arranged symmetrically around it, like a sleeve around a rod. — A student who imagines the DNA threaded through the proteins picks this. The DNA does not pass through the core; it is wound around the outside of it.
The DNA and the proteins share covalent bonds at many points, with the backbone of the DNA fused to the surface of the protein core. — A student who assumes the association must be covalent picks this. Visualization shows the DNA lying against the protein surface without covalent links; the attraction is between charged groups.
13 In the Hershey–Chase experiment, bacteria were infected with labelled phages, the mixture was agitated in a blender to remove phage coats from the bacterial surface, and then centrifuged so that the bacteria formed a pellet. Most of the ³²P was in the pellet and most of the ³⁵S was in the liquid above it, and the new phages produced by the bacteria contained ³²P. What do these results show? HL
Answer and reasoning
Phage protein entered the bacteria and directed the production of new phages, while the DNA stayed outside. — A student who has swapped the labels picks this. ³²P marks DNA, and it was the ³²P that entered the bacteria and appeared in the new phages; the ³⁵S-labelled protein stayed outside.
The phages are composed of both DNA and protein, and both components are needed for the phage to infect a bacterium. — A student who reads the result as a statement about composition picks this. The experiment separated the two components and showed that only the DNA entered and was passed on to new phages.
Phage DNA entered the bacteria and directed the production of new phages, while the phage protein coats stayed outside them. — ³²P labels DNA and was found in the bacterial pellet and in the progeny phages; ³⁵S labels protein and stayed in the liquid with the detached coats. So DNA, not protein, entered the cell and carried the information for making new phages.
Protein is the genetic material, because the labelled protein stayed with the intact phages rather than being used. — A student who expects genes to be protein picks this. The protein that stayed outside could not have directed production of new phages inside the bacteria; the DNA that entered did.
14 Which statement best describes the relationship between technology and the Hershey–Chase experiment? HL
Answer and reasoning
Hershey and Chase designed the experiment first, and radioisotope techniques were then developed specifically to carry it out. — A student who assumes ideas always precede tools picks this. Radioisotopes were already available as research tools; their availability is what opened up the possibility of the experiment.
The experiment could have been done at any earlier time; radioisotopes only made the measurements more precise. — A student who sees technology as a refinement rather than an enabler picks this. Without a way to label DNA and protein separately there was no experiment to perform, precisely or otherwise.
Radioisotopes revealed which molecules the phage contained, and that was sufficient to establish the genetic material. — A student who thinks composition settles the question picks this. Phages were already known to contain DNA and protein; the radioisotopes were needed to follow where each component went.
Radioisotopes becoming available as research tools made it possible to trace DNA and protein separately during infection. — When radioisotopes were made available to scientists as research tools, the experiment became possible: ³²P and ³⁵S allowed the two components of the phage to be followed separately. Technological developments can open up new possibilities for experiments.
15 Chargaff measured the percentage of each base in DNA from different organisms. For human DNA he found adenine 30.9%, thymine 29.4%, guanine 19.9% and cytosine 19.8%. For the bacterium E. coli he found adenine 24.7%, thymine 23.6%, guanine 26.0% and cytosine 25.7%. Which conclusion do these data support? HL
Answer and reasoning
In each organism A ≈ T and G ≈ C, but base composition differs between organisms, so DNA is not a simple repeating sequence of the bases. — In both organisms adenine matches thymine and guanine matches cytosine (purines equal pyrimidines), yet human DNA is about 40% G + C and E. coli about 52%. A repeating tetranucleotide would give 25% of each base in every organism, so the data falsify that hypothesis.
All four bases are present in roughly similar amounts in both organisms, which is consistent with DNA being a simple repeating tetranucleotide. — A student who expects 25% of each base picks this. Human DNA has about 31% adenine but only 20% guanine, and the proportions differ from those in E. coli, which a repeating tetranucleotide cannot produce.
The equal amounts of A and T and of G and C prove that DNA is a double helix in which each A is hydrogen-bonded to a T and each G is bonded to a C. — A student who reads the later explanation back into the data picks this. Base composition alone does not reveal structure; the double helix was proposed later and accounted for these equalities.
The different base compositions show that humans and bacteria must use different genetic codes to specify their proteins. — A student who conflates the genetic code with base composition picks this. The code (which triplets specify which amino acids) is the same in both organisms; only the proportions of the bases differ.
16 The tetranucleotide hypothesis proposed that DNA was a repeating sequence of the four bases. Which statement correctly describes the role of Chargaff's data in relation to this hypothesis? HL
Answer and reasoning
The hypothesis had been proved by earlier supporting measurements, so Chargaff's data could only refine it rather than reject it. — A student who thinks accumulated confirmations prove a hypothesis picks this. Confirmations never prove a general hypothesis; contradicting data falsify it, and that is what Chargaff's data did.
Confirming cases cannot prove a hypothesis, but Chargaff's contrary measurements were enough to show it false. — This is how the certainty of falsification addresses the problem of induction. No amount of supporting data proves a general claim, but data that contradict it, such as Chargaff's unequal base proportions, show it to be false.
Chargaff's data replaced the hypothesis with proof that DNA is a double helix with complementary base pairs throughout. — A student who credits Chargaff with the structure picks this. His data measured base composition and falsified the tetranucleotide hypothesis; the double helix model came later and explained his findings.
Chargaff's data confirmed the hypothesis, since every organism he examined had all four bases present in its DNA. — A student who takes 'all four bases present' as equivalent to 'a repeating sequence of the four bases' picks this. The hypothesis predicts equal amounts; Chargaff found unequal, organism-specific amounts.
17 Which statement best explains why the capacity of DNA for storing information is described as limitless?
Answer and reasoning
Capacity is not limitless: with only four bases, DNA can store far less than a code with more symbols could. — A student who judges capacity by the number of symbols picks this. Capacity depends on length as well; with any length allowed, four symbols give an unlimited number of sequences.
Each position can be one of only two base pairs, but the molecule is long enough to make up for the small alphabet. — A student who counts base pairs as two kinds picks this. Each position on a strand can hold any of four bases (A–T, T–A, G–C or C–G), so the alphabet is four, not two.
A DNA molecule can be of any length and have any base sequence, so the number of possible sequences has no upper bound. — Any length of DNA molecule and any base sequence is possible. The number of sequences, 4ⁿ, grows without limit as n increases, and each nucleotide stores its information in a very small space, so the capacity is enormous and economical.
Each nucleotide added creates four more possible sequences, so a long molecule stores a great deal. — A student who thinks possibilities add rather than multiply picks this. Each added nucleotide multiplies the number of possible sequences by four; that is why 4ⁿ becomes so large so quickly.
That was your twenty minutes. Real practice on A1.2 is past-paper questions marked against the mark scheme.
What the exam asks of A1.2
Paper 1A asks you to identify nucleotide parts, name bases, or pick the correct pairing. Paper 1B may give base-composition data and ask you to apply A = T, G = C, or to count bonds and water molecules. Paper 2 uses *draw* for a nucleotide or an antiparallel pair, *outline* for DNA–RNA differences, and *explain* for why complementary pairing allows replication. At HL, expect *explain* on directionality and *discuss* on Hershey–Chase or Chargaff: give the evidence, the conclusion, and the reasoning that links them.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
DP Science Cafe