What you need to know
- 2.6–2.13Proteins, enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction., nucleic acids, replication and protein synthesis.
By the end, you should be able to…
- Retrieve key mechanisms before checking the answers.
- Apply ideas to unfamiliar numerical and experimental examples.
- Identify and correct missing steps in an explanation.
Build your understanding
Separate three information processes
Replication makes DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. from DNA; transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. makes RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. from a DNA template; translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids. makes a polypeptideA chain of many amino acids joined by peptide bonds. using messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome.. Assign enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction., bonds and locations to the correct process.
Check what your rate measures
A change in absorbance per second is not automatically a chemical amount per second. Use a tangent at the start and state when calibrationChecking or setting a measuring instrument against a known standard, such as finding what one eyepiece division is worth. would be needed.
Make your reasoning visible
Separate an observation from an explanation
First state what was measured, such as the position of a DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. density band. Then explain what it supports or rules out. An intermediate band after one generation alone cannot distinguish semi-conservative from dispersive replication.
Answer the command word
Describe the pattern, explain the mechanism or evaluate the evidence as requested. A correct fact may still miss the question.
Show workings and units
Write a calculation before its result, and distinguish fractions, percentages and percentage-point changes.
Use answers diagnostically
Attempt the question first, then identify the missing step rather than only checking whether your final number matches.
Avoid this misconception
The claim: A longer answer automatically earns more credit.
The correction: Relevance, accuracy and the requested reasoning matter.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Explain why an enzyme changes reaction rate without changing the overall reactant-to-product energy difference.
Show suggested answer
It provides a pathway with lower activation energy. The energies of reactants and products, and their difference, remain unchanged.
Practice question
Retrieval and explanation · suggested answer
An enzyme assay’s initial absorbance falls from 0.80 to 0.62 in 30 s along a straight initial segment. Calculate the magnitude of the rate.
Show suggested answer
(0.80 − 0.62)/30 = 0.0060 absorbance units s⁻¹. This is an optical rate proxy, not mol s⁻¹ without calibration.
Practice question
Retrieval and explanation · suggested answer
Which Meselson–Stahl observation distinguishes semi-conservative from simple dispersive replication?
Show suggested answer
After two generations in light nitrogen, separate intermediate and light DNA bands appear. Dispersive replication predicts one progressively lighter band.
Practice question
Retrieval and explanation · suggested answer
Transcribe template DNA 3′–TAC AAA ACT–5′ and translate using AUG = methionine, UUU = phenylalanine and UGA = stop.
Show suggested answer
mRNA is 5′–AUG UUU UGA–3′, producing methionine–phenylalanine before termination.
Practice question
Retrieval and explanation · suggested answer
Distinguish a phosphodiester bond from a peptide bond by naming what each joins.
Show suggested answer
Phosphodiester bonds join nucleotide residues along a sugar–phosphate backbone. Peptide bonds join amino-acid residues in a polypeptide.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
After three generations in light nitrogen, a synchronised culture derived from fully heavy DNA shows 25% hybrid molecules. Predict the hybrid fraction after one more generation.
Show a hint
Each original heavy strand persists in only one molecule while the total molecule count doubles.
Explore the reasoning
The hybrid fraction halves to 12.5%; 87.5% are fully light. This assumes complete replication and no additional heavy nitrogen incorporation.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the lock-and-key model with the induced-fit model of enzyme specificity.
Enzyme action and location
Practice question
Retrieval and explanation · suggested answer
Compare the lock-and-key model with the induced-fit model of enzyme specificity.
Lesson: 2B 2: How Enzymes Work
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
An experiment that separates explanations
2B · How DNA WorksMeselson and Stahl’s 1958 paper tested competing replication models by following nitrogen labels through generations. It remains an example of designing an observation that distinguishes explanations. The wider…Specification 2.10(ii)Read & use the scienceClose article
Why this matters
Meselson and Stahl’s 1958 paper tested competing replication models by following nitrogen labels through generations. It remains an example of designing an observation that distinguishes explanations. The wider research lesson is to look for a prediction on which rival models disagree. An observation can be consistent with an explanation without uniquely supporting it over every competitor. Sampling after another generation added information because the predicted distributions diverged. This is a principle students can carry into unfamiliar investigations: identify what each model predicts before deciding whether a result distinguishes them. The original paper is a landmark source; the replication mechanism itself remains part of the required specification.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.10(ii) understand how Meselson and Stahl’s classic experiment provided new data that supported the accepted theory of replication of DNA and refuted competing theories
Use the science
Why would sampling only before transfer and after one generation leave an important uncertainty?
Explore a possible response
The first intermediate band is compatible with both semi-conservative and dispersive models. Later sampling tests where their predictions diverge.
Explore the source
- Meselson and Stahl: original 1958 paperProceedings of the National Academy of Sciences (1958)
Molecular photocopying has a template problem
2B · How DNA WorksPCR lets laboratory scientists make many copies of a selected DNA region from a small starting sample. It uses repeated rounds of strand separation and DNA synthesis, with…Specification 2.10(i) · 2.9(ii)Read & use the scienceClose article
Why this matters
PCR lets laboratory scientists make many copies of a selected DNA region from a small starting sample. It uses repeated rounds of strand separation and DNA synthesis, with short primers helping select the target. This adapts the template principle and DNA polymerase activity you already know. It does not turn an unknown or missing sequence into the correct original, and copying a contaminant can create a misleading signal. The value of amplification therefore depends on sample handling and controls as well as chemistry. PCR’s detailed stages belong later in the course; here the extension is what template-directed copying can and cannot establish.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.10(i) understand the process of DNA replication, including the role of DNA polymerase
- 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 should a reaction containing all the reagents but no intended sample be included when searching for a particular DNA sequence?
Explore a possible response
It can reveal contamination or unintended amplification in reagents or handling. A positive signal is only interpretable against appropriate controls.
Explore the source
- Polymerase Chain Reaction (PCR) Fact SheetNational Human Genome Research Institute