How is DNA copied accurately, and how does its base sequence code for proteins?
2.1.3 Nucleotides and nucleic acids: the semi-conservative replication of DNA and the roles of DNA helicase, DNA polymerase and the complementary base pairing rule; the nature of the genetic code as a triplet code that is degenerate and non-overlapping; the roles of mRNA and tRNA in protein synthesis.
A focused answer to the OCR H420 2.1.3 dot point on DNA replication and the genetic code. Covers semi-conservative replication, the roles of DNA helicase and DNA polymerase, the Meselson-Stahl evidence, and the triplet, degenerate, non-overlapping code with transcription and translation.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
OCR wants you to describe semi-conservative DNA replication with the roles of helicase and polymerase, explain the experimental evidence, and define the genetic code as a triplet, degenerate, non-overlapping code, then outline how mRNA and tRNA translate it into protein.
The answer
Semi-conservative replication
DNA copies itself before cell division so that each daughter cell receives a complete copy. The mechanism is semi-conservative: each new molecule keeps one original strand and gains one newly built strand.
- DNA helicase unwinds the double helix and breaks the hydrogen bonds between complementary base pairs, separating the two strands.
- Each original strand acts as a template.
- Free DNA nucleotides line up opposite their complementary bases (adenine with thymine, cytosine with guanine).
- DNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides, building a new strand on each template.
- The two molecules produced are identical to the original, each with one conserved and one new strand.
The evidence (Meselson and Stahl)
Meselson and Stahl grew bacteria in a medium containing heavy nitrogen () so all DNA was heavy, then switched them to light nitrogen (). After one round of replication, all the DNA had an intermediate density (one heavy strand, one light strand), ruling out conservative replication. After two rounds, half was intermediate and half was light, exactly as the semi-conservative model predicts. This is a classic data-interpretation context in OCR papers.
The genetic code
A gene is a sequence of DNA bases that codes for the amino acid sequence of a polypeptide. The code has three defining properties:
- Triplet: each amino acid is coded for by three bases (a codon). With four bases, possible triplets code for 20 amino acids plus stop signals.
- Degenerate: most amino acids have more than one codon, so some base changes (especially at the third position) do not alter the amino acid, reducing the impact of mutations.
- Non-overlapping: each base belongs to only one triplet; the code is read in fixed blocks of three.
The code is also universal (the same triplets code for the same amino acids in almost all organisms), which is why genetic engineering across species is possible.
Translating the code: mRNA and tRNA
- Transcription (in the nucleus): RNA polymerase makes a single-stranded mRNA copy of the template strand of a gene, using complementary base pairing (uracil replaces thymine). The mRNA leaves through a nuclear pore.
- Translation (at a ribosome): the ribosome reads the mRNA codon by codon. Each tRNA carries a specific amino acid and has an anticodon complementary to an mRNA codon. tRNAs bring amino acids in the order set by the codons; the ribosome joins them by peptide bonds, building the polypeptide until a stop codon is reached.
Examples in context
Example 1. The polymerase chain reaction. PCR copies DNA in vitro by repeatedly separating strands and using a polymerase, mirroring natural replication, which you meet in manipulating genomes in Module 6.
Example 2. Sickle-cell anaemia. A single base substitution changes one codon and one amino acid in haemoglobin; the fact that the code is degenerate explains why many other base changes would have no effect, linking replication errors to mutation.
Try this
Q1. State the role of DNA helicase in replication. [1 mark]
- Cue. It unwinds the double helix and breaks the hydrogen bonds between complementary bases to separate the strands.
Q2. Explain why a base substitution in a degenerate code may not change the protein. [2 marks]
- Cue. Several triplets code for the same amino acid, so a change (especially at the third base) may still code for the same amino acid, leaving the primary structure unchanged.
Q3. Name the bond formed by DNA polymerase between adjacent nucleotides. [1 mark]
- Cue. A phosphodiester bond.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20195 marksDescribe the process of semi-conservative replication of DNA, including the roles of the enzymes involved.Show worked answer →
Five marks for a clear sequence with named enzymes and base pairing.
- DNA helicase unwinds the double helix and breaks the hydrogen bonds between the complementary base pairs, separating the two strands.
- Each original strand acts as a template.
- Free DNA nucleotides align opposite their complementary bases (adenine with thymine, cytosine with guanine).
- DNA polymerase joins adjacent nucleotides by catalysing the formation of phosphodiester bonds, building a new strand on each template.
- Each new molecule has one original (conserved) strand and one new strand, which is why replication is described as semi-conservative.
Markers reward helicase unwinding and breaking hydrogen bonds, the complementary base pairing, DNA polymerase forming phosphodiester bonds, and the semi-conservative outcome.
OCR H420/02 20223 marksThe genetic code is described as a triplet code that is degenerate and non-overlapping. Explain what each of these three terms means.Show worked answer →
One mark per term, each precisely defined.
- Triplet code
- each amino acid is coded for by a sequence of three bases (a codon).
- Degenerate
- most amino acids are coded for by more than one triplet, so some changes to the third base do not change the amino acid.
- Non-overlapping
- each base is part of only one triplet; the code is read in fixed blocks of three from a start point, so a base is not shared between adjacent codons.
Markers reward the precise meaning of each term, not just a restatement of the word.
Related dot points
- 2.1.2 Biological molecules: the structure and function of triglycerides and phospholipids; the structure of amino acids, the formation of peptide bonds and the four levels of protein structure; the structure of nucleotides, DNA and RNA; the biochemical tests for lipids (emulsion test) and proteins (biuret test).
A focused answer to the OCR H420 2.1.2 dot point on lipids, proteins and nucleic acids. Covers triglycerides and phospholipids, amino acids and the four levels of protein structure, nucleotide and DNA and RNA structure, and the emulsion and biuret tests.
- 6.1.1 Cellular control: the nature of gene mutations and their effects on proteins; the control of gene expression at the transcriptional level, including operons (the lac operon) and transcription factors; the role of homeobox (Hox) genes in body plan development; and the role of apoptosis (programmed cell death).
A focused answer to the OCR H420 6.1.1 dot point on cellular control. Covers gene mutations and their effects, the control of transcription by the lac operon and transcription factors, the role of homeobox (Hox) genes in body plan development, and apoptosis.
- 2.1.6 Cell division: the cell cycle and its regulation by checkpoints; the main stages of mitosis (prophase, metaphase, anaphase and telophase) and cytokinesis; the significance of mitosis in growth, repair and asexual reproduction; the calculation and use of the mitotic index.
A focused answer to the OCR H420 2.1.6 dot point on the cell cycle and mitosis. Covers interphase and checkpoints, the four stages of mitosis and cytokinesis, the significance of mitosis, the link to cancer, and the mitotic-index calculation.
- 2.1.4 Enzymes: the role of enzymes as biological catalysts in metabolic reactions; the mechanism of enzyme action including the lock-and-key and induced-fit models; the effects of temperature, pH, enzyme and substrate concentration on the rate of reaction; the action of competitive and non-competitive inhibitors; the roles of cofactors, coenzymes and prosthetic groups.
A focused answer to the OCR H420 2.1.4 dot point on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, activation energy, the effects of temperature, pH and concentration, competitive and non-competitive inhibition, and cofactors.
- 6.1.3 Manipulating genomes: the principles of DNA sequencing, the polymerase chain reaction (PCR) and gel electrophoresis; the use of restriction enzymes and ligase to produce recombinant DNA in genetic engineering; the principles of gene editing; and the use of DNA profiling.
A focused answer to the OCR H420 6.1.3 dot point on manipulating genomes. Covers DNA sequencing, the polymerase chain reaction and gel electrophoresis, restriction enzymes and ligase in genetic engineering, the principles of gene editing, and DNA profiling.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)