Biology HL Paper 1A (May 2025, TZ2) · Question 10
Mixed-topicMatching part: 10
Also tests: 3.5 Translation and the Genetic Code
Practice this question →3.4 Transcription and Gene Activation
Build RNA from a DNA template and explain how promoters, transcription factors, enhancers and nucleosomes control initiation.
Estimated time: 47 minutes
IB syllabus: D1.2 · SL and HL
RNA Polymerase Makes a Complementary Transcript
A gene is a heritable DNA sequence that contributes to a functional product, which may be a polypeptide or a functional RNA. To express a protein-coding gene, a cell first transcribes its information into RNA. RNA polymerase locally separates the DNA strands, reads the template strand 3′→5′ and links ribonucleotides so the RNA grows 5′→3′. Only one strand of a particular gene is used as the template.
RNA nucleotides contain ribose, and uracil replaces thymine. During transcription, DNA adenine pairs with RNA uracil, while DNA thymine pairs with RNA adenine. Cytosine and guanine remain complementary. The RNA transcript detaches as polymerase advances and the DNA behind it re-forms a duplex. Transcription copies a selected region rather than the whole chromosome.
The coding DNA strand is not used as the template for that transcript. Its 5′→3′ base order matches the RNA product except that DNA contains T where RNA contains U. This relationship is useful for checking answers. If a template is written 3′-TAC GGA-5′, the mRNA is 5′-AUG CCU-3′; a code table then reads AUG and CCU in that displayed order.
HL extensionD1.2 AHL · D2.2
Promoters and Binding Proteins Select Genes
Transcription begins at a promoter, a non-coding DNA region associated with the start of a gene. In eukaryotes, general transcription factors assemble at promoter sequences and help recruit and position RNA polymerase. Additional sequence-specific transcription factors respond to cellular signals. The presence of a gene therefore does not guarantee its transcription: the correct regulatory proteins and accessible DNA are also required.
Enhancers are regulatory DNA sequences that may lie far from the promoter along the linear chromosome. Activator proteins bind enhancer sequences. DNA looping, supported by other proteins, brings the bound activators close to the promoter and transcription apparatus. Different combinations of factors can integrate several signals, allowing one gene to be strongly expressed in one cell type, weakly expressed in another and silent elsewhere.
Repressors reduce transcription by preventing assembly or function of the transcription complex, while activators increase it. These labels describe effects in a regulatory context, not permanent properties of every protein. Hormones, metabolites and developmental signals can alter a factor's abundance, location or activity. Gene expression thus links conditions inside and outside a cell to changes in protein production.
From transcript to translation laboratory
Follow a transcript in the shared genome model and inspect the directional relationship among template DNA, mRNA and codons.
Sequence · structure · expression
Genome and expression laboratory
Chromatin Controls Physical Access
Eukaryotic DNA is wrapped around histone proteins in nucleosomes and organized as chromatin. Packaging is essential because chromosomes are long, but it also affects access. A promoter tightly associated with compact chromatin is less available to transcription factors and polymerase. Nucleosomes are not fixed obstacles: they can slide, be repositioned or undergo chemical modifications that change how readily regulatory proteins reach DNA.
Prokaryotes have no nuclear envelope, so ribosomes can begin translating an mRNA while its downstream part is still being transcribed. In eukaryotes, transcription and RNA processing occur in the nucleus, whereas translation occurs on cytosolic ribosomes or ribosomes bound to rough endoplasmic reticulum. Compartmental separation creates extra opportunities for checking, processing, export and regulation.
Test Yourself
A mutation prevents an activator from binding an enhancer but leaves the protein-coding sequence unchanged. Which outcome is most direct?
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