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Educerie · IB Diploma · Biology
Theme A Unity and diversity · A1.2 Nucleic acids
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| State that DNA is the genetic material of all living organisms, and explain the exception of RNA viruses | SL, HL | Paper 1A item |
| Name the three components of a nucleotide and draw one with a circle, pentagon and rectangle | SL, HL | "Draw a labelled diagram of a nucleotide" (2–3 marks) |
| Explain the sugar–phosphate backbone and name the five bases | SL, HL | "Outline the structure of the backbone of a nucleic acid" (2 marks) |
| Explain RNA as a polymer made by condensation, and draw an RNA strand | SL, HL | "Draw a section of RNA showing three nucleotides" (3 marks) |
| Draw DNA as two antiparallel strands linked by hydrogen bonds between A–T and C–G | SL, HL | "Draw a section of DNA showing four nucleotides" (4–5 marks) |
| Distinguish DNA from RNA, and sketch ribose and deoxyribose | SL, HL | "Distinguish between the structures of DNA and RNA" (3 marks) |
| Explain how complementary base pairing allows replication and expression | SL, HL | "Explain the role of complementary base pairing" (3–4 marks) |
| Explain why DNA can store an effectively limitless amount of information, economically | SL, HL | Short answer, often with a 4ⁿ calculation |
| Explain why a shared genetic code is evidence for universal common ancestry | SL, HL | "Suggest why the universality of the genetic code…" (2 marks) |
| Explain the 5′ to 3′ directionality of strands and why it matters | HL only | "Explain the significance of the antiparallel strands" (3 marks) |
| Explain how purine-to-pyrimidine pairing keeps the helix a constant width | HL only | Paper 1A, or a 2-mark short answer |
| Describe the structure of a nucleosome | HL only | "Outline the structure of a nucleosome" (3 marks) |
| Explain how the Hershey–Chase results show DNA is the genetic material | HL only | Paper 1B data, or "explain" (4 marks) |
| Use Chargaff's data to show how the tetranucleotide hypothesis was falsified | HL only | Paper 1B data, or a nature-of-science question (3–4 marks) |
Before you start
You need three words from chemistry and one from A1.1. A polymer is a large molecule made of many repeating units, the monomers. A condensation reaction joins two molecules and releases a molecule of water. A covalent bond is a shared pair of electrons, and it is strong. From A1.1 you need the hydrogen bond: a weak attraction between a slightly positive hydrogen and a slightly negative atom, here nitrogen or oxygen.
1The idea in one paragraph
A nucleic acid is a chain of nucleotides, each one a phosphate, a sugar and a base. The phosphates and sugars link into a strong covalent backbone, and the bases stick out to the side. The order of the bases is the message: four letters, A, C, G and T in DNA, or A, C, G and U in RNA, spelling out instructions in any sequence and at any length. DNA is two strands running in opposite directions, held together by hydrogen bonds between bases that fit only one partner: A with T, C with G. That rule of complementary base pairing is what lets each strand act as a template, so the message can be copied for a new cell and read out to make proteins. Every organism uses the same molecule and the same code, and that is some of the best evidence that all life shares a single ancestor.
2DNA: the genetic material of all living things
Every living organism, from a bacterium to a blue whale, stores its genetic information in DNA (deoxyribonucleic acid). The DNA passes to offspring when cells divide, and its instructions build and run the organism.
There is one exception worth knowing, and the guide names it. Some viruses use RNA (ribonucleic acid) as their genetic material instead; influenza and the coronaviruses are examples. That does not break the rule, because viruses are not considered to be living organisms: they have no metabolism of their own and can reproduce only inside a host cell. (HL students meet viruses in full in A2.3.) So the statement stays exact: DNA is the genetic material of all living organisms.
3Nucleotides, the backbone and the bases
The monomer of every nucleic acid is the nucleotide, and it has three parts. The guide asks you to draw them with three simple shapes, shown in Figure 1.
- A phosphate group, drawn as a circle. It carries a negative charge, which is why nucleic acids are acids.
- A pentose sugar, drawn as a pentagon. Pentose means a sugar with five carbon atoms. In DNA the sugar is deoxyribose; in RNA it is ribose.
- A nitrogenous base, drawn as a rectangle. It is called nitrogenous because its rings contain nitrogen atoms.
The sugar sits in the middle. The phosphate is attached to one side of the sugar and the base to another, all by covalent bonds. Get the order right in a drawing: phosphate to sugar to base, never phosphate to base.
The sugar–phosphate backbone. When nucleotides join into a chain, the phosphate of one nucleotide bonds covalently to the sugar of the next. Repeat that and you get sugar–phosphate–sugar–phosphate, a continuous chain of covalently bonded atoms running the whole length of the strand. That chain is the sugar–phosphate backbone. Because every link in it is covalent, it is strong: the backbone does not come apart when DNA is copied, even though the two strands are separated. The bases are not part of the backbone. They hang off each sugar, one per nucleotide, facing to the side.
The bases. There are five, and you need their names.
| Base | Letter | Found in |
|---|---|---|
| Adenine | A | DNA and RNA |
| Cytosine | C | DNA and RNA |
| Guanine | G | DNA and RNA |
| Thymine | T | DNA only |
| Uracil | U | RNA only |
The backbone is the same in every nucleotide of every strand; only the base changes from one nucleotide to the next. So the backbone carries no information. The information is the sequence of bases, read along the strand like letters along a line of text. That is what the guide means by bases "that form the basis of a code".
4RNA: a polymer built by condensation
RNA is the simpler of the two nucleic acids, because it is usually a single strand. It is a polymer of RNA nucleotides joined by condensation reactions, as Figure 2 shows.
Each time a nucleotide is added, a covalent bond forms between the phosphate of the new nucleotide and the sugar of the last one in the chain, and one molecule of water is released. Joining monomers with the loss of water is exactly what condensation means. Build a strand of any length and you release one water molecule for every link:
The guide expects you to draw and recognise both a single nucleotide and an RNA polymer. For the polymer, stack three or four nucleotides so that each phosphate joins the sugar of the nucleotide above it, and put a base on every sugar, all on the same side.
5DNA: two antiparallel strands, linked by base pairs
DNA is two strands of nucleotides, wound around each other into a double helix. Figure 3 flattens the helix out into a ladder, which is the form you are expected to draw. The guide says outright that you need not draw the twist.
Three features, and each is worth a mark in a drawing.
Two backbones on the outside. Each strand has its own sugar–phosphate backbone, and the two backbones form the sides of the ladder.
The strands are antiparallel. They run in opposite directions: in Figure 3 the sugars on the right are turned upside down. Show it by drawing the second strand upside down relative to the first. (HL section 10 explains what "direction" means.)
Bases pair across the middle, held by hydrogen bonds. Each base on one strand faces a base on the other, and the two are held together by hydrogen bonds. Only two pairings fit: adenine with thymine and cytosine with guanine. These are complementary base pairs. Each hydrogen bond is weak, but there are millions of them along a DNA molecule, so together they hold the strands firmly, while still letting them be unzipped when the DNA must be copied or read. (You may meet the fact that A–T pairs share two hydrogen bonds and C–G pairs share three. The guide does not require you to know those numbers.)
The rungs of the DNA ladder are complementary base pairs, A with T and C with G, held by hydrogen bonds. The sides are two antiparallel sugar–phosphate backbones, held by covalent bonds.
Keep the two kinds of bond apart. Covalent bonds hold each strand together along its length. Hydrogen bonds hold the two strands to each other.
6How DNA and RNA differ
The guide lists three differences. Learn them as a table, and in the exam write each one as a comparison.
| DNA | RNA | |
|---|---|---|
| Number of strands | two, forming a double helix | one |
| Pentose sugar | deoxyribose | ribose |
| Bases | A, C, G and thymine | A, C, G and uracil |
The sugars differ at a single carbon atom, and you should be able to sketch the difference. Figure 4 shows it: the carbons of the sugar are numbered 1′ to 5′ ("one prime" to "five prime"), and on carbon 2′ ribose carries an OH group where deoxyribose carries only an H. Deoxyribose is literally "ribose without an oxygen".
You should also be familiar with examples of nucleic acids. DNA makes up the chromosomes of every cell, together with proteins. RNA comes in several forms with different jobs: messenger RNA (mRNA) carries a copy of a gene to the ribosome; transfer RNA (tRNA) brings amino acids to the ribosome; ribosomal RNA (rRNA) makes up much of the ribosome itself. And some viruses have RNA as their genome, as section 2 said.
7Complementary base pairing: copying the code and reading it
Because A pairs only with T (or U) and C pairs only with G, the sequence of one strand fixes the sequence of its partner exactly. If one strand reads ATGCCTAG, the other must read TACGGATC. Nothing else will fit. The pairing is based on hydrogen bonding: only the matching pairs can line up their hydrogen-bonding groups, so only they form stable bonds. That single fact does two jobs, and Figure 5 shows both.
Replication: copying the information. Before a cell divides, the two strands of its DNA separate. Each strand then serves as a template: free nucleotides pair with its exposed bases by the complementary rules, and are joined into a new strand. The result is two DNA molecules, each identical to the original and each made of one old strand and one new one. Because the pairing rule allows only one partner for each base, the copy is accurate. (The detail of replication is in D1.1.)
Expression: using the information. To use a gene, the cell copies its base sequence into RNA, again by complementary pairing, with one change: RNA has uracil, so an A in the DNA template is matched by U in the RNA. This is transcription, and it produces mRNA. At the ribosome the mRNA is read in groups of three bases called codons, and each tRNA recognises its codon by complementary pairing through its own three bases, the anticodon. So complementary pairing is used twice to turn a DNA sequence into a protein. (The detail is in D1.2.)
When a question says "explain the role of complementary base pairing", name the pairs, say they form by hydrogen bonding, and then show that it gives one and only one possible partner sequence, in replication and in transcription or translation.
8An unlimited store of information
A nucleic acid can be any length, and at every position it can carry any of four bases. The number of possible sequences therefore grows very fast:
A single gene is often more than a thousand bases long, and 4 raised to the power 1,000 is a number with more than 600 digits. That is what the guide means by limitless capacity: there is no sequence DNA cannot hold, and no limit on how long a message can be.
The storage is also remarkably economical. Each base carries two bits of information, since four choices fit in two binary digits. A human cell holds about 3 billion base pairs from each parent, around 2 metres of DNA, inside a nucleus a few thousandths of a millimetre across. No human technology stores data as densely.
9One code for all life
The genetic code is the set of rules by which a sequence of codons is translated into a sequence of amino acids. With minor variations, it is the same in every organism ever studied: a given codon codes for the same amino acid in a bacterium, a mushroom, an oak tree and you. (A few small exceptions exist, for example in mitochondria, but the core of the code is shared.)
Why is that evidence for universal common ancestry? Because nothing in chemistry forces a particular codon to mean a particular amino acid. The assignments look arbitrary, the way the words of a language are arbitrary. If life had arisen many times independently, we would expect many different codes, just as separate peoples invented different languages. One shared code is best explained by one origin: all living things inherited it from a common ancestor, and any lineage that changed it much would have scrambled every one of its proteins at once, so the code has been conserved ever since. The guide says you need not memorise specific codons; the argument is what is examined. (A2.1 takes this further, to the last universal common ancestor.)
10HLDirectionality: the 5′ and 3′ ends
SL students can skip to section 15.
The five carbon atoms of the pentose sugar are numbered 1′ to 5′. The base is attached to carbon 1′. The phosphate is attached to carbon 5′. And when a strand is built, the phosphate of the next nucleotide bonds to carbon 3′. Figure 6 shows the numbering and what it does to a strand.
Every link in the backbone therefore runs the same way: from the 5′ carbon of one nucleotide, through a phosphate, to the 3′ carbon of the next. That gives each strand a direction. One end has a free phosphate on a 5′ carbon, the 5′ end; the other has a free OH on a 3′ carbon, the 3′ end. A strand is written and read from 5′ to 3′ by convention. Antiparallel now has an exact meaning: the two strands of DNA run 5′→3′ in opposite directions, so the 5′ end of one strand lies alongside the 3′ end of the other.
This matters because the enzymes that build and read nucleic acids work in one direction only.
- Replication. DNA polymerase can add nucleotides only to the 3′ end of a growing strand, so new DNA is always built 5′→3′. Since the two template strands run opposite ways, the two new strands must be built in opposite directions, and one of them can only be made in short pieces. (That is D1.1, HL.)
- Transcription. RNA polymerase also builds RNA 5′→3′, reading the template strand from its 3′ end towards its 5′ end.
- Translation. The ribosome reads mRNA from its 5′ end towards its 3′ end, codon by codon, so the reading frame and the order of amino acids are fixed.
11HLPurine-to-pyrimidine pairing keeps the helix even
The five bases come in two shapes. Purines, adenine and guanine, are larger, with a double ring. Pyrimidines, cytosine, thymine and uracil, are smaller, with a single ring. Every complementary pair in DNA is one purine and one pyrimidine: A (purine) with T (pyrimidine), and G (purine) with C (pyrimidine). Figure 7 shows why that matters.
An A–T pair and a C–G pair have the same length. So every rung of the ladder spans the same distance, and the two backbones stay the same distance apart all the way along the molecule, whatever the sequence. Two purines together would be too long to fit between the backbones; two pyrimidines would be too short to reach. Because the helix has the same three-dimensional shape whatever the sequence, it is stable, and the enzymes that work on DNA can bind to any part of it in the same way: DNA can hold any message without changing shape.
12HLThe nucleosome
A human cell has about 2 metres of DNA and a nucleus a few micrometres wide. The DNA is packed, and the first level of packing is the nucleosome, shown in Figure 8.
A nucleosome has three parts, and the guide limits you to these:
- A core of eight histone proteins, two copies each of four different histones, packed into a disc.
- A length of DNA wrapped around the core, just under twice.
- One additional histone (called H1) attached to the linker DNA, the stretch that runs from one nucleosome to the next. It sits where the DNA enters and leaves the core and holds the wrapped DNA in place.
Along a chromosome, nucleosomes repeat every couple of hundred base pairs, which makes chromatin look like beads on a string under the electron microscope.
Why does DNA wrap so readily? Histones are rich in positively charged amino acids, and DNA's phosphates are negative, so the two attract. The guide asks you to see this with molecular visualisation software: a free viewer such as JSmol can display a nucleosome from the Protein Data Bank (search "nucleosome core particle"). Rotate it and find the DNA wrapped round the outside, the eight histones of the core, and the histone "tails" poking out between the turns; colour by charge if you can, and the positive histone surface faces the negative DNA backbone. Packing also affects gene expression (D2.2): DNA wound tightly on nucleosomes is harder to transcribe.
13HLHershey–Chase: the evidence that DNA is the genetic material
In the early 1950s it was still open whether genes were made of DNA or of protein. Chromosomes contain both, and many biologists thought protein, with twenty amino acids, a likelier carrier of information than DNA with four bases. In 1952 Alfred Hershey and Martha Chase used a bacteriophage (a virus that infects bacteria) called T2, made of just a protein coat with DNA inside. When it infects a bacterium, something from the phage enters the cell and instructs it to make new phages. Which of the two gets in? Figure 9 follows the method.
The trick was in two chemical differences. Protein contains sulfur (in some amino acids) but DNA contains none. DNA contains phosphorus (in its phosphates) but protein contains almost none. So:
- One batch of phages was grown with radioactive sulfur, ³⁵S, which labelled their protein. A second batch was grown with radioactive phosphorus, ³²P, which labelled their DNA.
- Each batch was allowed to infect bacteria (E. coli).
- The mixtures were agitated in a blender to shake the phage coats off the outside of the bacteria, then spun in a centrifuge. The heavy bacteria formed a pellet at the bottom of the tube; the light, empty phage coats stayed in the liquid above.
- Where had the radioactivity gone?
Results. With ³⁵S-labelled phages, the radioactivity was mostly in the liquid, with the empty coats. With ³²P-labelled phages, the radioactivity was mostly in the pellet, inside the bacteria, and some of it turned up in the new phages the bacteria later released.
Conclusion. The protein stayed outside the cell. The DNA went in. Since whatever enters the cell must carry the instructions for making new phages, DNA is the genetic material.
To evaluate it, note that the separation was not perfect: a little ³⁵S stayed with the pellet because some coats were not shaken off. The difference was strong, not absolute.
Nature of science: technology opens new experiments. The experiment was only possible because radioisotopes had become available as research tools. Radioactive sulfur and phosphorus behave chemically like the ordinary atoms, so cells build them into molecules normally, yet they can be detected wherever they end up. Without a way to label protein and DNA separately, the question "which one enters the cell?" could not have been asked.
14HLChargaff's data and the tetranucleotide hypothesis
Before DNA's structure was known, a popular idea, the tetranucleotide hypothesis, held that DNA was a simple repeating sequence of the four bases, the same four over and over. If that were true, DNA would contain exactly 25% of each base in every species, and it could carry no information at all: a message that says ACGT ACGT ACGT forever says nothing.
In the late 1940s the biochemist Erwin Chargaff measured the proportions of the four bases in DNA from many different organisms. Figure 10 shows the pattern he found, using invented figures for illustration.
Two results stand out.
- Base proportions differ between species. In one species adenine might be 30% of the bases; in another, 25%. They are not all 25%, so the tetranucleotide hypothesis must be wrong.
- Within any one species, A ≈ T and G ≈ C. The amount of adenine matches thymine, and guanine matches cytosine, whatever the species. That is exactly what complementary base pairing predicts, and Watson and Crick used it when they built their model of DNA in 1953.
The second result lets you calculate. If thymine is 18% of the bases in a sample of double-stranded DNA:
Nature of science: induction and falsification. The guide uses Chargaff to make a point about how science reaches conclusions. Suppose you test a hypothesis on ten species and it holds every time. You still cannot be sure it holds for the eleventh: no number of agreeing observations can prove a general claim true. That is the problem of induction. But one reliable observation that contradicts a hypothesis is enough to show it is false. That is the certainty of falsification. The tetranucleotide hypothesis predicted equal amounts of every base in every species. Chargaff found species with clearly unequal amounts, so the hypothesis was falsified. The logic is asymmetric: confirmation is always provisional, but a well-established contradiction is decisive. That is why good scientific hypotheses are ones that could, in principle, be shown to be false.
15Where marks are lost
Joining the phosphate to the base. In a nucleotide the phosphate and the base are both attached to the sugar. A drawing with the phosphate on the base loses the structure mark.
Putting the bases in the backbone. The backbone is sugar and phosphate only, linked by covalent bonds. The bases project from it.
Drawing the strands parallel. DNA strands are antiparallel. Draw the second strand upside down relative to the first, or label the ends 5′ and 3′ (HL).
Pairing A with G, or C with T. A pairs with T (or U in RNA) and C pairs with G. Every other pairing scores zero.
"Hydrogen bonds hold the nucleotides together." Covalent bonds join nucleotides along a strand; hydrogen bonds join bases across the two strands. Name the right bond for the right place.
Writing "uracil replaces thymine in DNA". Uracil is in RNA; thymine is in DNA. In a distinguish answer, write the comparison both ways: "DNA contains thymine, whereas RNA contains uracil".
Describing DNA and RNA separately. "Distinguish" needs paired contrasts: two strands versus one, deoxyribose versus ribose, thymine versus uracil.
HL: saying Hershey and Chase found protein in the pellet. The ³⁵S (protein) stayed in the liquid with the coats; the ³²P (DNA) entered the bacteria and ended up in the pellet.
16Draw it right
The guide expects you to draw a nucleotide, an RNA strand and a section of DNA. Markers look for the following.
- Shapes: phosphate as a circle, pentose sugar as a pentagon, base as a rectangle. Each labelled at least once.
- Attachments: phosphate and base both joined to the sugar, on different corners. The base goes on the corner to the right of the ring's top (carbon 1′); the phosphate on the corner to the left (towards carbon 5′).
- Backbone: in a strand, a line from each sugar to the phosphate of the next nucleotide, making a continuous sugar–phosphate chain. All bases on one side.
- RNA: one strand only, and U, not T, if you label bases.
- DNA: two strands side by side, the second one upside down relative to the first so they are antiparallel. HL: label the 5′ and 3′ ends of each strand.
- Base pairs: A opposite T, C opposite G, joined by dashed lines labelled "hydrogen bonds". The helix itself does not have to be drawn.
- Labels: backbone, phosphate, deoxyribose (or ribose), named bases, hydrogen bond, covalent bond. Draw with a ruler for the straight bonds, and make it big enough to label clearly.
17Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. Draw a labelled diagram to show a section of a DNA molecule containing four nucleotides, two on each strand. 4 marks
Q2. Distinguish between the structure of DNA and the structure of RNA. 3 marks
Q3. Explain how complementary base pairing allows genetic information to be both replicated and expressed. 4 marks
Q4. A student analysed the base composition of nucleic acid from four samples. The data are invented for this question.
| Sample | Adenine / % | Thymine / % | Guanine / % | Cytosine / % |
|---|---|---|---|---|
| P | ? | ? | ? | 21.0 |
| Q | 22.1 | 22.3 | 27.9 | 27.7 |
| R | 24.0 | 32.8 | 24.1 | 19.1 |
| S | 31.0 | 30.6 | 19.1 | 19.3 |
(a) Sample P is double-stranded DNA. Calculate the percentage of adenine in sample P. 2 marks
(b) One sample came from a virus whose genetic material is single-stranded DNA. Identify the sample, giving a reason. 2 marks
(c) Suggest why adenine and thymine are not exactly equal in sample Q, although it is double-stranded. 1 mark
Q5 (HL). Explain how the results of the Hershey–Chase experiment support the conclusion that DNA is the genetic material. 4 marks
Q6 (HL). Explain how Chargaff's data falsified the tetranucleotide hypothesis, and why a falsification is more certain than a confirmation. 4 marks
18In one breath
DNA is the genetic material of every living thing; some viruses use RNA, but viruses are not alive. A nucleotide is phosphate, pentose sugar and base: circle, pentagon, rectangle. Sugar joined covalently to phosphate, over and over, makes a strong backbone; the bases project from it, and their sequence is the code. RNA is one strand built by condensation, one water per link. DNA is two antiparallel strands held by hydrogen bonds between complementary bases, A–T and C–G; it has deoxyribose and thymine where RNA has ribose and uracil. One partner per base makes each strand a template, for replication and for expression. Four bases at any length give 4ⁿ sequences, a limitless, compact store, and one code shared by all life points to one ancestor. HL: strands run 5′→3′ and enzymes work only that way; purine–pyrimidine pairs are equal in length, so the helix is uniform; a nucleosome is DNA round eight histones, held by a ninth on the linker; Hershey and Chase used ³⁵S and ³²P to show DNA, not protein, enters the cell; Chargaff's data falsified the tetranucleotide hypothesis, and one clear contradiction outweighs any number of confirmations.
Answers
Q1. Two strands side by side, each of two nucleotides. Each nucleotide: phosphate (circle) joined to deoxyribose (pentagon) joined to a base (rectangle). Within each strand, the sugar of one nucleotide joined to the phosphate of the next, forming the backbone. The second strand drawn upside down (antiparallel). Bases paired across the middle, A with T and C with G, joined by dashed lines labelled hydrogen bonds. 1 for nucleotides with correct shapes and attachments, labelled, 1 for a sugar–phosphate backbone in each strand, 1 for antiparallel strands, 1 for correct complementary pairs joined by hydrogen bonds. Pairing A with C or G with T loses the fourth mark; a single strand scores 1 at most.
Q2. DNA is double-stranded, whereas RNA is single-stranded. DNA contains the sugar deoxyribose, whereas RNA contains ribose. DNA contains the base thymine, whereas RNA contains uracil in its place. 1 for each correct comparison. Two separate descriptions with no contrast drawn are capped at 2; "DNA has ATCG and RNA has AUCG" earns the base mark only if T and U are identified as the difference.
Q3. Bases pair only in complementary pairs, A with T (or U) and C with G, because only these pairs can form hydrogen bonds with each other. So the sequence of one strand fixes the sequence of the other. In replication, the two strands of DNA separate and each acts as a template: free nucleotides pair with the exposed bases, so each new strand is complementary to its template and the two new molecules are identical to the original. In expression, a gene's sequence is transcribed into mRNA by complementary pairing with the template strand (U pairing with A), and in translation tRNA anticodons pair with mRNA codons, so the base sequence determines the amino acid sequence. 1 for the pairing rules with hydrogen bonding, 1 for one sequence determining the other, 1 for templates in replication giving identical copies, 1 for transcription or translation by complementary pairing. An answer about replication only is capped at 3.
Q4. (a) C = 21.0%, so G = 21.0%. A + T = 100 − 42.0 = 58.0%, so A = 58.0 ÷ 2 = 29.0%. (b) Sample R: adenine (24.0%) does not equal thymine (32.8%), and guanine (24.1%) does not equal cytosine (19.1%), so the bases are not paired, which means the DNA is single-stranded. (c) Measurement error or uncertainty in the method used to measure base proportions; the difference of 0.2% is too small to be significant. (a) 1 for G = C = 21.0% and A + T = 58.0%, 1 for A = 29.0%. (b) 1 for R, 1 for the reason using A ≠ T or G ≠ C in the data. (c) 1 for a valid reason. In (b), "because it is a virus" scores 0 for the reason.
Q5 (HL). Phages grown with ³⁵S had radioactive protein, since sulfur is found in protein but not DNA; phages grown with ³²P had radioactive DNA, since phosphorus is in DNA but not protein. After infection, blending and centrifuging, the ³⁵S was found mostly in the liquid with the empty phage coats, so the protein did not enter the bacteria. The ³²P was found mostly in the pellet, inside the bacteria, and appeared in new phages. So the DNA entered the bacteria, and since the material that enters directs the production of new phages, DNA is the genetic material. 1 for the labelling logic, S in protein and P in DNA, 1 for ³⁵S in the supernatant, 1 for ³²P in the pellet or bacteria, 1 for the conclusion linked to what enters the cell. Reversing the two results scores 0 for both result marks.
Q6 (HL). The tetranucleotide hypothesis proposed that DNA was a repeating sequence of the four bases, which predicts equal proportions, 25% of each, in all species. Chargaff measured base proportions in DNA from different species and found that they varied between species (for example, A was clearly more than 25% in some and less in others), which contradicts that prediction, so the hypothesis was falsified. However many observations agree with a hypothesis, they cannot prove it true, because a future observation might contradict it: the problem of induction. But one reliable contradicting observation shows that a general claim is false, so falsification is more certain. 1 for the hypothesis's prediction of equal proportions, 1 for Chargaff's finding of variation between species, 1 for the problem of induction, 1 for the asymmetry of falsification. Citing A = T and G = C alone does not falsify the hypothesis, since the hypothesis also predicts A = T; it scores 0 for the second mark.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section A1.2 Nucleic acids. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
Check your understanding
The main ideas of this note. Tick each one you could do now, in an exam, without looking back up. Anything you cannot tick yet is the part to read again.