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Q: What does H2 Biology: DNA Replication, Transcription & Translation cover? A: H2 Biology molecular biology notes covering DNA replication (semi-conservative), transcription, translation, gene expression regulation, and exam-style worked examples for 9744.
Molecular biology sits at the heart of Core Idea 3 (Genetics) in the H2 Biology syllabus. DNA replication, transcription, and translation form a tightly connected sequence - the central dogma - that examiners revisit in almost every sitting. Students who can name every enzyme, explain directionality, and link mutations to protein consequences tend to pick up full marks on structured questions.
Status: SEAB H2 Biology (9744) syllabus last checked 2026-03-23. Content aligns with Core Idea 3 - Genetics and the Molecular Basis of Inheritance. [1]
Quick revision box
What this topic tests: DNA structure, semi-conservative replication, transcription and post-transcriptional modification, translation, gene expression regulation, mutations, and their consequences for protein function.
Top mistakes to avoid: Writing that DNA polymerase reads 5' to 3' (it reads template 3' to 5' and synthesises 5' to 3'); omitting the role of primase; confusing the template strand with the coding strand during transcription.
20-minute sprint plan: 5 min enzyme table for replication; 10 min walk-through of transcription then translation with directionality; 5 min mutation type drill with worked example.
1 DNA Structure Review
Before studying the central dogma, ensure the following structural features are secure.
Double helix: Two polynucleotide strands wound around each other in a right-handed helix.
Antiparallel orientation: One strand runs 5' to 3' while the complementary strand runs 3' to 5'. This directionality is critical for understanding replication and transcription.
Complementary base pairing: Adenine pairs with thymine (two hydrogen bonds); guanine pairs with cytosine (three hydrogen bonds). This specificity underpins faithful replication.
Sugar-phosphate backbone: Phosphodiester bonds between the 3' carbon of one deoxyribose and the 5' carbon of the next sugar provide structural integrity. The bases project inward.
Major and minor grooves: Formed by the helical twist; proteins such as transcription factors interact with DNA primarily through the major groove.
2 DNA Replication
2.1 Overview
DNA replication is semi-conservative: each daughter molecule contains one original (parental) strand and one newly synthesised strand. Replication occurs during the S phase of interphase and ensures that every daughter cell receives a complete copy of the genome.
2.2 Evidence - the Meselson-Stahl experiment
Meselson and Stahl (1958) grew E. coli in a medium containing heavy nitrogen (15N), then transferred the bacteria to a 14N medium.
Generation 0: All DNA was heavy (15N–15N).
Generation 1: All DNA appeared at an intermediate density, consistent with one heavy strand and one light strand - ruling out conservative replication.
Generation 2: Half of the DNA was intermediate, half was light - ruling out dispersive replication and confirming semi-conservative replication.
2.3 Key enzymes and proteins
Enzyme / Protein
Role
Helicase
Unwinds the double helix by breaking hydrogen bonds between complementary base pairs
Single-strand binding (SSB) proteins
Stabilise the separated single strands and prevent them from re-annealing
Primase
Synthesises a short RNA primer complementary to the template strand, providing the free 3'-OH group that DNA polymerase III requires
DNA polymerase III
Adds deoxyribonucleotides to the 3' end of the growing strand (5' to 3' synthesis); also has 3' to 5' proofreading exonuclease activity
DNA polymerase I
Removes RNA primers and replaces them with DNA
DNA ligase
Seals the nicks (phosphodiester bond gaps) between Okazaki fragments on the lagging strand
2.4 Leading strand vs lagging strand
Because DNA polymerase III can only synthesise in the 5' to 3' direction:
Leading strand: Synthesised continuously in the same direction as the replication fork movement.
Lagging strand: Synthesised discontinuously as short Okazaki fragments (approximately 1,000–2,000 nucleotides in prokaryotes), each requiring its own RNA primer. Fragments are later joined by DNA ligase after primer removal.
2.5 Proofreading and error correction
DNA polymerase III possesses 3' to 5' exonuclease activity. If an incorrect nucleotide is incorporated, the enzyme reverses, excises the mismatch, and replaces it with the correct base. This proofreading reduces the error rate to approximately one mistake per 109 base pairs.
3 Transcription
Transcription is the synthesis of a messenger RNA (mRNA) molecule using one strand of DNA as a template. It occurs in the nucleus of eukaryotic cells.
3.1 Initiation
Transcription factors bind to the promoter region upstream of the gene.
RNA polymerase is recruited to the promoter and binds, forming the transcription initiation complex.
The DNA double helix unwinds locally, exposing the template strand (also called the antisense strand), which runs 3' to 5'.
3.2 Elongation
RNA polymerase reads the template strand in the 3' to 5' direction and synthesises the mRNA in the 5' to 3' direction.
Ribonucleoside triphosphates (ATP, GTP, CTP, UTP) are added by complementary base pairing with the template strand. Note that uracil replaces thymine in RNA.
Unlike DNA replication, no primer is required - RNA polymerase can initiate synthesis de novo.
The coding strand (sense strand) has the same sequence as the mRNA, except with thymine instead of uracil.
3.3 Termination
RNA polymerase reaches a terminator sequence on the DNA. The mRNA transcript is released, and the DNA re-anneals.
The primary transcript (pre-mRNA) undergoes three major modifications before leaving the nucleus:
5' cap: A modified guanine nucleotide is added to the 5' end. The cap protects the mRNA from degradation and assists ribosome recognition during translation.
3' poly-A tail: A string of adenine nucleotides (typically 100–250) is added to the 3' end. The poly-A tail increases mRNA stability and aids nuclear export.
Splicing: Non-coding sequences called introns are removed by the spliceosome. The remaining coding sequences, called exons, are joined together to form the mature mRNA. Alternative splicing allows one gene to encode multiple protein variants.
4 Translation
Translation is the synthesis of a polypeptide chain from an mRNA template. It occurs at ribosomes in the cytoplasm.
4.1 Ribosome structure
Small subunit: Binds the mRNA and ensures correct codon-anticodon pairing.
Large subunit: Catalyses peptide bond formation (peptidyl transferase activity).
Three binding sites on the ribosome:
A site (aminoacyl): Incoming aminoacyl-tRNA binds here.
P site (peptidyl): Holds the tRNA carrying the growing polypeptide chain.
E site (exit): Deacylated tRNA exits the ribosome from this site.
4.2 tRNA structure
Cloverleaf shape with an anticodon loop at one end and an amino acid attachment site (3' CCA end) at the other.
Each tRNA is charged with its specific amino acid by an aminoacyl-tRNA synthetase enzyme (one synthetase per amino acid).
The anticodon base-pairs with the complementary codon on the mRNA in an antiparallel fashion.
4.3 Initiation
The small ribosomal subunit binds to the 5' cap of the mRNA and scans until it reaches the start codon (AUG).
The initiator tRNA (carrying methionine) binds to the start codon at the P site via its anticodon (UAC).
The large ribosomal subunit then joins, forming the complete ribosome.
4.4 Elongation
An aminoacyl-tRNA enters the A site, with its anticodon complementary to the mRNA codon.
A peptide bond forms between the amino acid in the A site and the growing polypeptide in the P site (catalysed by the ribosome's peptidyl transferase).
The ribosome translocates one codon along the mRNA in the 5' to 3' direction. The tRNA in the P site moves to the E site and exits; the tRNA in the A site moves to the P site.
Steps 1–3 repeat, elongating the polypeptide chain.
4.5 Termination
When a stop codon (UAA, UAG, or UGA) enters the A site, no tRNA can bind.
A release factor binds to the stop codon in the A site.
The completed polypeptide is released, and the ribosomal subunits dissociate from the mRNA.
5 Gene Expression Regulation
Not all genes are expressed at all times. Regulation occurs at multiple levels and differs between prokaryotes and eukaryotes.
5.1 Prokaryotic regulation - operons
Lac operon (inducible):
In the absence of lactose, a repressor protein (encoded by the lacI gene) binds to the operator, blocking RNA polymerase from transcribing the structural genes (lacZ, lacY, lacA).
When lactose is present, allolactose (an isomer of lactose) binds to the repressor and changes its shape, causing it to detach from the operator. RNA polymerase can then transcribe the structural genes.
Trp operon (repressible):
When tryptophan levels are low, the repressor is inactive and the structural genes for tryptophan biosynthesis are transcribed.
When tryptophan accumulates, it acts as a co-repressor, binding to the repressor and activating it. The activated repressor binds the operator and blocks transcription.
5.2 Eukaryotic regulation
Eukaryotic gene expression is regulated at multiple levels:
Transcription factors: Proteins that bind to specific DNA sequences (e.g. enhancers or the promoter region) and recruit or block RNA polymerase. Activators increase transcription; repressors decrease it.
Enhancers and silencers: Regulatory DNA sequences that can be thousands of base pairs away from the gene they regulate. They function through DNA looping, bringing transcription factors into contact with the promoter.
Epigenetic modifications:
DNA methylation: Addition of methyl groups to cytosine bases (commonly at CpG sites). Methylation of a promoter region typically silences gene expression.
Histone modification: Acetylation of histone tails loosens chromatin (euchromatin), promoting transcription. Deacetylation or methylation of histones can compact chromatin (heterochromatin), repressing transcription.
6 Mutations
A mutation is a permanent change in the nucleotide sequence of DNA. Mutations can arise spontaneously during replication or be induced by mutagens (e.g. UV radiation, chemical agents).
6.1 Point mutations (single nucleotide changes)
Type
What happens
Effect on protein
Silent
Changed codon still codes for the same amino acid (degeneracy of the genetic code)
No change in protein
Missense
Changed codon codes for a different amino acid
May alter protein folding and function (e.g. sickle cell anaemia: GAG to GUG in the haemoglobin gene)
Nonsense
Changed codon becomes a premature stop codon
Truncated, usually non-functional protein
6.2 Frameshift mutations
Insertion or deletion of one or more nucleotides (not in multiples of three) shifts the reading frame. Every codon downstream of the mutation is altered, typically producing a non-functional protein. Frameshifts are generally more damaging than point mutations because they affect a larger portion of the polypeptide.
6.3 Consequences for protein function
A mutation in the active site of an enzyme may abolish catalytic activity.
A mutation affecting protein folding (e.g. altering a disulfide bond or hydrophobic core) can reduce stability.
Some mutations are neutral if they occur in non-coding regions or do not alter the protein's functional domains.
7 Comparison Table: Replication vs Transcription vs Translation
Feature
DNA Replication
Transcription
Translation
Template
Both DNA strands
One DNA strand (template / antisense strand)
mRNA
Product
Two identical DNA molecules
Pre-mRNA (then mature mRNA after processing)
Polypeptide
Key enzyme
DNA polymerase III
RNA polymerase
Ribosome (with peptidyl transferase activity)
Direction of synthesis
5' to 3'
5' to 3'
N-terminus to C-terminus (mRNA read 5' to 3')
Primer required?
Yes (RNA primer by primase)
No
No (initiator tRNA binds start codon)
Nucleotides used
dATP, dTTP, dGTP, dCTP
ATP, UTP, GTP, CTP
Amino acids (carried by tRNA)
Location (eukaryotes)
Nucleus
Nucleus
Cytoplasm (ribosomes)
Base pairing rule
A-T, G-C
A-U (template to mRNA), G-C
Codon-anticodon (mRNA to tRNA)
8 How Molecular Biology Appears in Exams
Paper 2 (structured): Expect diagrams of a replication fork or a ribosome at a mRNA, with blanks to label enzymes, sites, or directions. Data from the Meselson-Stahl experiment may be presented as a density gradient graph for interpretation.
Paper 3 (free-response): Essay prompts such as "Describe the process of translation" or "Compare and contrast DNA replication and transcription" are common. Full marks require precise enzyme names, directionality, and logical sequencing of steps.
Cross-topic links: Mutations connect to inheritance patterns and genetic disease (Core Idea 3). Gene regulation connects to cell differentiation (Core Idea 1) and evolution (Core Idea 4). Exam questions often span these boundaries.
Exam tip: When describing replication or transcription, always state the direction of synthesis (5' to 3') and the direction the template is read (3' to 5'). Omitting directionality is one of the most common reasons students lose marks.
Quick Retrieval Check
Name three enzymes involved in DNA replication and state the role of each.
Explain why the lagging strand is synthesised discontinuously.
Describe two post-transcriptional modifications and explain the function of each.
Outline the events that occur at the A site, P site, and E site during translation elongation.
Distinguish between a missense mutation and a nonsense mutation, giving one example of each.
Where can I find the full H2 Biology Notes series? Start at the H2 Biology Notes hub, then follow Core Ideas 1-4 and the Extension Topics.
Is this topic tested in Paper 4 (practical)? Molecular biology itself is rarely tested as a standalone practical. However, techniques such as gel electrophoresis and PCR - both of which rely on DNA replication and base-pairing principles - may appear as data-handling or planning questions in Paper 4.
Do I need to memorise every enzyme in the replication fork? Yes. SEAB expects you to name helicase, primase, DNA polymerase III, DNA polymerase I, DNA ligase, and SSB proteins, and to state each enzyme's specific role. Simply writing "DNA polymerase" without specifying III or I is insufficient for full marks. [1]
What is the difference between the template strand and the coding strand? The template strand (antisense strand) is the strand read by RNA polymerase during transcription (read 3' to 5'). The coding strand (sense strand) has the same base sequence as the mRNA (with T instead of U) and is not directly read during transcription.
How do prokaryotic and eukaryotic gene regulation differ? Prokaryotes primarily use operons (clusters of genes under one promoter, regulated by repressors and inducers). Eukaryotes regulate gene expression at multiple levels including chromatin remodelling, transcription factor binding to enhancers/silencers, mRNA splicing, and epigenetic modifications such as DNA methylation and histone acetylation. [1]
