Nucleotides and nucleic acids — OCR A-Level Biology
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Nucleotides and nucleic acids explained
Nucleotides and nucleic acids covers 8 specification points, including (a) the structure of a nucleotide as the… and (b) the synthesis and breakdown of….
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Open a point below for its explanation and revision guidance.
(a) the structure of a nucleotide as the monomer from which nucleic acids are made
A nucleotide is the repeating monomer that polymerises into nucleic acids. Teach the three-part structure: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base. The base is either a purine (adenine, guanine) or a pyrimidine (cytosine, thymine in DNA, uracil in RNA). The sugar and base join by a glycosidic bond to form a nucleoside; adding phosphate gives the nucleotide. In DNA the sugar is deoxyribose and bases are A, T, C, G; in RNA the sugar is ribose and uracil replaces thymine. Recognise that the phosphate and sugar form the backbone while bases project inward. A common MCQ asks which component differs between DNA and RNA nucleotides, or which bond links sugar to base.
(b) the synthesis and breakdown of polynucleotides by the formation and breakage of phosphodiester bonds
Polynucleotides form when nucleotides join in a condensation reaction, releasing water and creating a phosphodiester bond between the phosphate of one nucleotide and the sugar of the next. The backbone is a sugar–phosphate chain with bases projecting from it. Breakdown occurs by hydrolysis, where water is added to break the phosphodiester bond, releasing nucleotides. Teach the direction and chemistry: condensation builds, hydrolysis breaks, and both involve the same bond. In cells, polymerases catalyse synthesis and nucleases catalyse hydrolysis. A typical MCQ asks which reaction forms the bond, what is released during condensation, or which enzyme class breaks the bond.
(c) the structure of ADP and ATP as phosphorylated nucleotides
ADP and ATP are phosphorylated nucleotides: each is built on a nucleotide framework in which a nitrogenous base (adenine) is joined to a pentose sugar (ribose), and the sugar carries phosphate groups. In ADP (adenosine diphosphate) the ribose carries two phosphate groups; in ATP (adenosine triphosphate) it carries three, so ATP has one extra phosphate compared with ADP. The phosphate groups are attached in a chain, and adding the terminal phosphate to ADP (phosphorylation) stores energy, while removing it (dephosphorylation) releases energy. Recognise the components: adenine base, ribose sugar, and two or three phosphate groups respectively. A useful check is to count phosphates: diphosphate means two, triphosphate means three. Distinguish these from a plain nucleotide such as AMP, which has a single phosphate.
(d) (i) the structure of DNA (deoxyribonucleic acid)
DNA is a polymer of nucleotides, and each nucleotide has three parts: a phosphate group, a deoxyribose sugar and a nitrogenous base. The bases are adenine, thymine, cytosine and guanine. Nucleotides join through the sugar–phosphate backbone, with phosphodiester bonds linking the phosphate of one nucleotide to the sugar of the next. Two strands run in opposite directions (antiparallel) and wind into a double helix. The bases on opposite strands pair by complementary base pairing: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds). The sugar–phosphate backbone lies on the outside and the paired bases on the inside. Recognise that deoxyribose, not ribose, is the sugar, and that thymine replaces uracil found in RNA.
(ii) practical investigations into the purification of DNA by precipitation
DNA precipitation separates DNA from other cell components by exploiting its chemistry. In a practical investigation you would break open cells, remove protein and other material, then add cold ethanol so DNA comes out of solution as a visible white precipitate that can be spooled out. The method depends on DNA being soluble in water but insoluble in alcohol, and on salt helping DNA clump. Questions test why each step is done: why the sample is chilled, why ethanol is layered carefully, why salt is added, and how the precipitate is collected. You must link each manipulation to the property of DNA it exploits, and recognise that precipitation purifies DNA rather than simply extracting a crude mixture.
(e) semi-conservative DNA replication
Semi-conservative replication means each new DNA molecule keeps one original strand and gains one newly built strand. The two strands of the double helix separate, and each acts as a template. Free nucleotides line up by complementary base pairing (A with T, C with G), and enzymes join them into a new strand. The result is two identical DNA molecules, each half old and half new. Questions test the meaning of semi-conservative, the role of complementary base pairing, the template function of each strand, and why the products are genetically identical. You should be able to explain the process step by step and predict the outcome for daughter molecules.
(f) the nature of the genetic code
The genetic code is the set of rules by which the base sequence of DNA is translated into the amino acid sequence of a polypeptide. Teach learners that the code is a triplet code: three bases (a codon) specify one amino acid, and because there are four bases, 4³ = 64 possible codons arise. Of these, 61 code for amino acids while three are stop codons that terminate translation; a start codon also signals initiation. The code is degenerate, meaning most amino acids are encoded by more than one codon, yet it is unambiguous because each codon specifies only one amino acid. It is non-overlapping, so each base is read once, and it is universal, since the same codons carry the same meaning in almost all organisms. A concrete method: read a short mRNA sequence in groups of three from the start codon and assign each codon to its amino acid.
(g) transcription and translation of genes resulting in the synthesis of polypeptides.
Transcription makes a messenger RNA copy of a gene in the nucleus. DNA helicase unwinds the double helix, and RNA polymerase joins free RNA nucleotides to the template strand by complementary base pairing (adenine pairs with uracil in RNA). The mRNA strand is formed, then leaves the nucleus through a nuclear pore to a ribosome. Translation occurs at the ribosome: tRNA molecules carry specific amino acids and their anticodons pair with complementary mRNA codons. A start codon begins translation, and a stop codon ends it. Peptide bonds form between adjacent amino acids, building a polypeptide whose sequence is determined by the order of codons.
Your focus
- Identify the three components of a nucleotide.
- Distinguish DNA and RNA nucleotides by sugar and base.
- Explain why nucleotides are described as monomers of nucleic acids.
Show all 24 objectives
- Describe how condensation forms phosphodiester bonds between nucleotides.
- Explain how hydrolysis breaks polynucleotides into nucleotides.
- Identify the groups joined by a phosphodiester bond.
- Identify the components of ADP and ATP as adenine, ribose and phosphate groups.
- State the number of phosphate groups in ADP (two) and ATP (three).
- Explain that ATP is a phosphorylated nucleotide formed by adding a phosphate to ADP.
- Describe the components of a DNA nucleotide: phosphate, deoxyribose and a base.
- State the four DNA bases and the complementary base-pairing rules.
- Explain how nucleotides join to form the sugar–phosphate backbone of the double helix.
- Explain why cold ethanol causes DNA to precipitate from solution.
- Describe the role of salt in allowing DNA to aggregate and precipitate.
- Explain how collecting the precipitate purifies DNA by removing dissolved contaminants.
- Explain what is meant by semi-conservative replication.
- Describe how complementary base pairing and template strands produce new DNA.
- Predict that each daughter molecule contains one original and one new strand.
- Describe the genetic code as a triplet, non-overlapping code.
- Explain why the code is described as degenerate and unambiguous.
- State that the code is universal and that three codons are stop codons.
- Describe the stages of transcription, including complementary base pairing.
- Explain how tRNA anticodons pair with mRNA codons during translation.
- Relate the order of codons to the sequence of amino acids in the polypeptide.
Nucleotides and nucleic acids exam tips
Marking Points
- A nucleotide has three components: a pentose sugar, a phosphate group and a nitrogenous base.
- DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose.
- Purine bases are adenine and guanine; pyrimidine bases are cytosine, thymine (DNA) and uracil (RNA).
- The sugar and base are joined by a glycosidic bond, forming a nucleoside; the phosphate is attached to the sugar.
- Nucleotides are the monomers that polymerise to form polynucleotides (nucleic acids).
- Polynucleotides are synthesised by condensation reactions that form phosphodiester bonds between adjacent nucleotides.
- Each condensation releases a water molecule.
- The phosphodiester bond links the phosphate group of one nucleotide to the pentose sugar of the next.
- Hydrolysis breaks phosphodiester bonds by adding water, releasing individual nucleotides.
- Enzymes such as polymerases catalyse synthesis and nucleases catalyse hydrolysis.
- ADP and ATP are both phosphorylated nucleotides, built from adenine, ribose and phosphate groups.
- ADP (adenosine diphosphate) has two phosphate groups attached to the ribose sugar.
- ATP (adenosine triphosphate) has three phosphate groups attached to the ribose sugar.
- ATP differs from ADP by one additional phosphate group, added by phosphorylation.
- The base is adenine and the pentose sugar is ribose in both molecules.
- Adding a phosphate to ADP stores energy; removing the terminal phosphate from ATP releases energy.
- DNA is a polymer of nucleotides, each made of a phosphate group, a deoxyribose sugar and a nitrogenous base.
- The four bases are adenine, thymine, cytosine and guanine.
- Nucleotides are joined by phosphodiester bonds forming a sugar–phosphate backbone.
- DNA consists of two antiparallel strands forming a double helix.
- Complementary base pairing occurs: adenine pairs with thymine and cytosine pairs with guanine.
- Adenine–thymine pairs have two hydrogen bonds; cytosine–guanine pairs have three.
- DNA is soluble in water but insoluble in cold ethanol, so adding cold ethanol causes DNA to precipitate out of solution.
- Chilling the ethanol and sample reduces DNA solubility and slows enzyme activity, encouraging precipitation and protecting DNA.
- Adding salt (for example sodium chloride) neutralises the negative charges on the phosphate backbone so DNA molecules can aggregate and precipitate.
- The white precipitate can be spooled or collected, separating purified DNA from soluble contaminants left in the supernatant.
- Precipitation is a purification step: contaminants that stay dissolved are removed when the DNA precipitate is separated from the liquid.
- The double helix unwinds and the two strands separate, each acting as a template for a new strand.
- Free nucleotides align opposite the template by complementary base pairing: adenine with thymine and cytosine with guanine.
- Enzymes catalyse the joining of nucleotides into a new strand along each template.
- Each resulting DNA molecule contains one original (conserved) strand and one newly synthesised strand, which is what semi-conservative means.
- Because base pairing is complementary, the two daughter molecules are identical to the original, conserving the genetic information.
- States that the code is a triplet code in which three bases (a codon) code for one amino acid.
- Explains that 4³ = 64 codons exist, of which 61 code for amino acids and three are stop codons.
- Describes the code as degenerate because most amino acids are coded for by more than one codon.
- Describes the code as unambiguous because each codon specifies only one amino acid.
- States that the code is non-overlapping, with each base read only once.
- States that the code is universal, as the same codons carry the same meaning in almost all organisms.
- Transcription: DNA helicase unwinds the double helix and RNA polymerase joins free RNA nucleotides to the template strand by complementary base pairing.
- In RNA, adenine pairs with uracil, so the mRNA sequence is complementary to the DNA template.
- mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome.
- Translation: tRNA molecules carry specific amino acids, and their anticodons pair with complementary mRNA codons.
- A start codon initiates translation and a stop codon terminates it.
- Peptide bonds form between adjacent amino acids, producing a polypeptide whose sequence is set by the codon order.
Examiner Tips
- 💡Learn the three components and be ready to identify them on a diagram.
- 💡Memorise which sugar and which bases distinguish DNA from RNA nucleotides.
- 💡Practise naming the bond between sugar and base (glycosidic) and between nucleotides (phosphodiester).
- 💡Link each reaction to whether water is released or used.
- 💡State precisely which groups the phosphodiester bond joins.
- 💡Use the terms condensation and hydrolysis accurately in explanations.
- 💡Learn the component list (adenine, ribose, phosphate groups) and the phosphate count for each molecule.
- 💡Use the prefixes di- and tri- to recall two versus three phosphate groups.
- 💡Practise labelling a simple diagram of ATP showing base, sugar and the phosphate chain.
- 💡Link phosphorylation to energy storage and dephosphorylation to energy release when explaining the difference.
- 💡Learn the three components of a DNA nucleotide and the four bases.
- 💡Use complementary base pairing rules to work out the partner base on the opposite strand.
- 💡Remember the hydrogen-bond counts: two for A–T and three for C–G.
- 💡Describe the double helix as two antiparallel strands with a sugar–phosphate backbone outside and bases inside.
- 💡For each step, name the property of DNA it exploits rather than just describing the action.
- 💡Use the words soluble and insoluble precisely when explaining the role of ethanol and water.
- 💡Link the visible white precipitate to purification, explaining what remains dissolved and is therefore removed.
- 💡Define semi-conservative explicitly in terms of one old and one new strand per molecule.
- 💡State the complementary base-pairing rules precisely when describing template alignment.
- 💡Explain the outcome (two identical molecules) and link it to conservation of genetic information.
- 💡Learn the named features of the code (triplet, degenerate, unambiguous, non-overlapping, universal) and be ready to match each to its definition.
- 💡Use the calculation 4³ = 64 to justify the number of codons rather than recalling it blindly.
- 💡When a sequence is given, group the bases into threes from the start codon before assigning amino acids.
- 💡Sequence the stages clearly: unwinding, complementary base pairing, mRNA formation, movement to ribosome, codon–anticodon pairing, peptide bond formation.
- 💡Use the terms template strand, anticodon and codon precisely to gain credit.
- 💡Link the final polypeptide sequence to the order of mRNA codons.
Common Mistakes
- Confusing nucleotide with nucleoside: a nucleoside lacks the phosphate group; correct by stating a nucleotide has sugar, base and phosphate.
- Claiming uracil occurs in DNA: uracil is found in RNA, replacing thymine; DNA uses thymine.
- Stating the base joins the phosphate directly: the base bonds to the sugar, and the phosphate also attaches to the sugar.
- Saying hydrolysis forms the bond: condensation forms it and releases water; hydrolysis breaks it and uses water.
- Describing the bond as linking two bases: the phosphodiester bond links phosphate to sugar in the backbone.
- Forgetting the water: condensation releases water, hydrolysis requires water.
- Confusing the number of phosphates: ADP has two and ATP has three. Correction: count the 'di' (two) and 'tri' (three) prefixes.
- Thinking ATP contains deoxyribose. Correction: the sugar in ADP and ATP is ribose, not deoxyribose.
- Treating ATP as a nucleotide with a single phosphate. Correction: ATP is a phosphorylated nucleotide carrying three phosphate groups.
- Believing the base differs between ADP and ATP. Correction: both contain adenine; only the phosphate count differs.
- Naming ribose as the sugar in DNA. Correction: DNA contains deoxyribose.
- Including uracil among DNA bases. Correction: DNA contains thymine instead of uracil.
- Saying the strands are parallel. Correction: the two strands are antiparallel, running in opposite directions.
- Placing the bases on the outside of the helix. Correction: the sugar–phosphate backbone is on the outside and the bases are on the inside.
- Mismatching base pairs. Correction: adenine pairs with thymine and cytosine pairs with guanine.
- Thinking DNA dissolves in ethanol: correct this by stating DNA is insoluble in cold ethanol and precipitates, while it dissolves in water.
- Believing salt breaks the DNA: correct this by explaining salt neutralises phosphate charges to allow aggregation, not strand breakage.
- Assuming precipitation destroys DNA: correct this by noting the DNA remains intact and can be collected and redissolved for further use.
- Thinking both strands of a daughter molecule are new: correct this by stating one strand is original and one is new.
- Mixing up base pairs, for example pairing adenine with cytosine: correct this by stating A pairs with T and C pairs with G.
- Believing the original DNA is destroyed: correct this by explaining the original strands are conserved as templates in the products.
- Thinking one base codes for one amino acid; correct this by stating that three bases form a codon coding for one amino acid.
- Confusing degenerate with ambiguous; correct this by explaining that degeneracy means several codons share an amino acid, while each codon still specifies only one amino acid.
- Assuming all 64 codons code for amino acids; correct this by noting that three are stop codons.
- Believing the code overlaps so bases are shared between codons; correct this by stating that each base is read once in a non-overlapping triplet code.
- Saying thymine pairs with adenine during transcription: correct this by stating uracil pairs with adenine in RNA.
- Confusing the template and coding strands: the mRNA is complementary to the template strand, not identical to it.
- Describing translation as occurring in the nucleus: state that mRNA moves to a ribosome in the cytoplasm for translation.