What you need to know
- 2.9(i)Mononucleotide components, DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. and RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. polynucleotides, and condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. forming phosphodiester bondThe bond that links nucleotides: a phosphate group joins the sugar of one nucleotide to the sugar of the next..
- 2.9(ii)complementary base pairingSpecific pairing of bases by hydrogen bonds: adenine with thymine (or uracil in RNA), and cytosine with guanine. and hydrogen bonding between the strands of the DNA double helixThe shape of DNA: two antiparallel polynucleotide strands joined by base pairs and twisted into a spiral..
This lesson covers nucleic-acid structure. DNA replication, the genetic code and protein synthesis follow in 2B4–2B6. Strand-direction labels help you interpret the structure; detailed synthesis mechanisms belong to the later lessons.
By the end, you should be able to…
- Identify the three components of a DNA or RNA nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base..
- Explain how nucleotides form a sugar–phosphate backbone.
- Compare DNA and RNA and identify complementary DNA sequences.
- Use base-pairing rules in calculations, stating when the rules apply.
Build your understanding
Three parts make one nucleotide
A mononucleotide contains a pentose sugar, a phosphate group and a nitrogen-containing base. The base and phosphate are attached to the sugar. A base alone is not a nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base.: include all three components when describing or labelling one.
Three parts make one nucleotide (continued)
DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. stands for deoxyribonucleic acid. Its sugar is deoxyriboseThe pentose sugar in DNA nucleotides. and its bases are adenineA purine base. It pairs with thymine in DNA and with uracil in RNA. (A), thymineA pyrimidine base found only in DNA. It pairs with adenine. (T), cytosineA pyrimidine base. It pairs with guanine. (C) and guanineA purine base. It pairs with cytosine. (G). RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. stands for ribonucleic acid. Its sugar is riboseThe pentose sugar in RNA nucleotides and in ATP. and its bases are A, uracilA pyrimidine base found in RNA instead of thymine. It pairs with adenine. (U), C and G. Ribose has an –OH group at its 2′ carbon where deoxyribose has H.
Joining nucleotides into a strand
DNA and RNA are polynucleotides: many nucleotide residues joined into a chain. In the condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. description required by specification 2.9(i), joining nucleotides releases water and forms phosphodiester bondThe bond that links nucleotides: a phosphate group joins the sugar of one nucleotide to the sugar of the next.. These are strong covalent links between the sugar of one nucleotide and the phosphate associated with the next.
The repeating sugars and phosphates form the backbone; the bases project from the sugars. The order of bases can vary while the backbone repeats. A phosphodiester link connects the 3′ position of one sugar through phosphate to the 5′ position of the next, giving the strand a direction. The prime symbol distinguishes sugar-carbon numbers from positions in a base.
Two complementary strands form DNA
DNA normally consists of two polynucleotideA chain of nucleotides joined by phosphodiester bonds. strands twisted into a double helixThe shape of DNA: two antiparallel polynucleotide strands joined by base pairs and twisted into a spiral.. The sugar–phosphate backbones lie on the outside, with paired bases on the inside. The strands are antiparallel: one runs 5′ → 3′ while the other runs 3′ → 5′.
Two complementary strands form DNA (continued)
A pairs with T through two hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen.; C pairs with G through three hydrogen bonds. The shapes and hydrogen-bonding arrangements of these bases allow complementary pairing. Each pair combines a larger, two-ring base (A or G) with a smaller, one-ring base (T or C), helping maintain a consistent width.
Hydrogen bonds connect complementary bases on opposite strands. Phosphodiester bonds join neighbouring nucleotides within each strand. Separating the two strands by disrupting hydrogen bonding is different from cutting a sugar–phosphate backbone.
For example, opposite 5′–A C G T T A–3′ lies 3′–T G C A A T–5′. If asked to write that complementary strand in the 5′ → 3′ direction, reverse its displayed order: 5′–T A A C G T–3′. Check both complementary base pairingSpecific pairing of bases by hydrogen bonds: adenine with thymine (or uracil in RNA), and cytosine with guanine. and the direction requested.
RNA uses a different sugar and one different base
RNA is usually a single polynucleotide strand, containing ribose and uracil in place of deoxyribose and thymine. It still has a sugar–phosphate backbone joined by phosphodiester bonds. Single-stranded does not mean that every RNA molecule is a straight line: complementary regions within a strand can pair and make it fold.
Compare like with like: sugar with sugar, bases with bases, and strand organisation with strand organisation. The roles of messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. and transfer RNA in protein synthesis are developed in 2B6.
Use base pairing to reason with numbers
Across an entire double-stranded DNA molecule with standard complementary pairing, the number of A bases equals the number of T bases and the number of C bases equals the number of G bases. Together the four percentages total 100%. If A is 28%, T is 28%; C and G share the remaining 44%, so each is 22%.
Those equalities do not have to hold within one strand alone, or in a single-stranded RNA sample. Always identify what has been measured. Also distinguish base pairs from nucleotides: a double-stranded DNA fragment with 100 base pairs contains 200 nucleotides.
NOT: hydrogen bonds are between bases. Phosphodiester bonds are in the backbone.
- Two hydrogen bonds in A–T; three in C–G
- Strands are antiparallel: one 5′ to 3′, the other 3′ to 5′
- Complementary pairing lets each strand specify the other
- RNA uses ribose and uracil, and pairs A with U
Two traps this diagram blocks
- Do not say the two strands are joined by phosphodiester bonds. Those bonds run along each strand.
- Do not give DNA uracil or RNA thymine in this specification comparison.
From the diagram alone
Question
Describe how two DNA strands are held together, and state the pairing rules. [3]
Show a model answer
Hydrogen bonds form between complementary bases. Adenine pairs with thymine by two hydrogen bonds, and cytosine pairs with guanine by three. The strands are antiparallel. Phosphodiester bonds are within each backbone, not between the strands.
| Feature | DNA | RNA |
|---|---|---|
| Pentose sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Usual strand arrangement | Two antiparallel strands in a double helix | One strand, which may fold |
| Bonds along the backbone | Covalent phosphodiester bonds | Covalent phosphodiester bonds |
Make your reasoning visible
Name the bond and its location
Write “hydrogen bondA weak attraction between a partially positive hydrogen atom and a partially negative atom such as oxygen or nitrogen. between complementary bases on opposite strands” or “phosphodiester bondThe bond that links nucleotides: a phosphate group joins the sugar of one nucleotide to the sugar of the next. in the sugar–phosphate backbone”. “Bonds hold DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. together” does not distinguish their roles.
Make comparisons in paired statements
State that DNA contains deoxyriboseThe pentose sugar in DNA nucleotides. whereas RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. contains riboseThe pentose sugar in RNA nucleotides and in ATP., and DNA uses thymineA pyrimidine base found only in DNA. It pairs with adenine. whereas RNA uses uracilA pyrimidine base found in RNA instead of thymine. It pairs with adenine.. Both contain adenineA purine base. It pairs with thymine in DNA and with uracil in RNA., cytosineA pyrimidine base. It pairs with guanine., guanineA purine base. It pairs with cytosine. and phosphate.
Check what a percentage describes
A = T and C = G apply to totals across both complementary DNA strands. Do not impose those equalities on a single strand. Convert base pairs to two nucleotideThe monomer of nucleic acids: a pentose sugar, a phosphate group and a nitrogenous base. per pair when needed.
Keep the sequence aligned
Pair A with T and C with G one position at a time. If direction labels are supplied, show the opposite directions; reverse the complementary sequence only when the requested writing direction requires it.
Avoid this misconception
The claim: Hydrogen bonds join consecutive nucleotides along a DNA strand.
The correction: Consecutive nucleotides are linked by covalent phosphodiester bonds in the backbone. Hydrogen bonds form between complementary bases on the two strands.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
A nucleotide contains ribose, phosphate and guanine. Does the guanine identify it as DNA or RNA? Which component does distinguish it?
Show suggested answer
Guanine occurs in both DNA and RNA, so it does not distinguish them. Ribose identifies this as an RNA nucleotide; a DNA nucleotide contains deoxyribose.
Practice question
Retrieval and explanation · suggested answer
Describe how mononucleotides form a polynucleotide, naming the reaction, bond and resulting repeating structure.
Show suggested answer
In the specification’s condensation model, nucleotides join with release of water. Covalent phosphodiester bonds link sugar and phosphate groups of neighbouring nucleotide residues, producing a repeating sugar–phosphate backbone.
Practice question
Retrieval and explanation · suggested answer
One DNA strand is 5′–A G T C C A–3′. Write its complementary strand directly underneath, including direction labels.
Show suggested answer
3′–T C A G G T–5′. Each A pairs with T and each C with G; the strands run in opposite directions. Written instead in the 5′ → 3′ direction, the complementary strand is 5′–T G G A C T–3′.
Practice question
Retrieval and explanation · suggested answer
A double-stranded DNA fragment contains 200 base pairs. Adenine is 30% of all its bases. Calculate the number of adenine nucleotides and the percentage of guanine.
Show suggested answer
200 base pairs contain 400 nucleotides. Adenine count = 0.30 × 400 = 120. Thymine is also 30%, leaving 40% for C and G together. Guanine is 20% (80 nucleotides).
Practice question
Retrieval and explanation · suggested answer
Heating separates a DNA double helix into two intact polynucleotide strands. Which interactions have been disrupted, and which backbone bonds remain intact?
Show suggested answer
Hydrogen bonding between complementary bases has been disrupted. The covalent phosphodiester bonds along each sugar–phosphate backbone remain intact, so each strand remains a polynucleotide.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
In one strand of a double-stranded DNA fragment, A = 40%, T = 10%, C = 20% and G = 30%. Predict the base percentages in the complementary strand and in the complete molecule. Assume both strands have equal length and every base is paired.
Show a hint
Each A in the first strand requires T in the second. For the complete molecule, average the percentages from the two equal-length strands.
Explore the reasoning
The complementary strand has A = 10%, T = 40%, C = 30% and G = 20%. Across both strands, each base is 25%: for example A = (40 + 10)/2. One strand can have unequal A and T even though the complete double-stranded molecule has equal totals.
Recall this lesson
Lesson recall questions
Question 1 of 5: List the three components of a mononucleotide.
Nucleotides, DNA and RNA
Practice question
Retrieval and explanation · suggested answer
List the three components of a mononucleotide.
Lesson: 2B 3: The Structure of DNA and RNA
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Shakespeare written in DNA
2B · The Structure of DNA and RNAIn 2013, an EMBL-EBI research team reported storing digital files, including Shakespeare’s sonnets, in synthetic DNA and reconstructing the files after reading the DNA sequence. Researchers used a…Specification 2.9(i)Read & use the scienceClose article
Why this matters
In 2013, an EMBL-EBI research team reported storing digital files, including Shakespeare’s sonnets, in synthetic DNA and reconstructing the files after reading the DNA sequence. Researchers used a designed encoding system to translate digital information into a sequence of bases. This was a research demonstration, not evidence that DNA had replaced ordinary storage devices. Reading a sequence correctly is only part of the task: the coding scheme must preserve order and allow the original file to be reconstructed. Knowing the proportions of the four bases would lose most of that information, just as counting letters cannot recover a play. The cultural connection therefore tests a precise biological idea: a repeated backbone can carry information through a variable, ordered sequence.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.9(i) know the basic structure of mononucleotides (deoxyribose or ribose linked to a phosphate and a base, including thymine, uracil, adenine, cytosine or guanine) and the structures of DNA and RNA (polynucleotides composed of mononucleotides linked by condensation reactions to form phosphodiester bonds)
Use the science
Would knowing only the percentages of A, T, C and G let you reconstruct a stored sonnet? Explain using DNA structure.
Explore a possible response
No. Many different base sequences have the same overall percentages. The ordered sequence, together with the encoding rules, is needed to recover the information; composition alone loses that order.
Explore the source
Jurassic Park needs more than an amber mosquito
2B · The Structure of DNA and RNAJurassic Park imagines recovering dinosaur DNA from ancient insects and filling missing sequence with DNA from another animal. The Natural History Museum explains why that premise faces severe…Specification 2.9(i) · 2.9(ii)Read & use the scienceClose article
Why this matters
Jurassic Park imagines recovering dinosaur DNA from ancient insects and filling missing sequence with DNA from another animal. The Natural History Museum explains why that premise faces severe problems of preservation. A molecule’s visible surroundings can survive while its DNA has broken down. Even if short fragments remain, a complete genome is an ordered set of sequences, not merely the correct proportions of four bases. Substituting unrelated sequence does not recover information that has been lost. The film is therefore a useful way to distinguish a recognisable fossil, a chemical remnant and a usable genetic template.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.9(i) know the basic structure of mononucleotides (deoxyribose or ribose linked to a phosphate and a base, including thymine, uracil, adenine, cytosine or guanine) and the structures of DNA and RNA (polynucleotides composed of mononucleotides linked by condensation reactions to form phosphodiester bonds)
- 2.9(ii) know how complementary base pairing and the hydrogen bonding between two complementary strands are involved in the formation of the DNA double helix
Use the science
Why can complementary base pairing help copy an intact fragment but not reveal the original sequence of an entirely missing region?
Explore a possible response
A surviving template constrains complementary bases. If neither strand of a region survives, the pairing rule supplies no information about the original order there.
Explore the source
- Can we bring back dinosaurs?Natural History Museum