IB DP Biology D2.2 Gene expression Study Notes - New Syllabus -2025
IB DP Biology D2.2 Gene expression Study Notes – New syllabus 2025
IB DP Biology D2.2 Gene expression Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- How is gene expression changed in a cell?
- How can patterns of gene expression be conserved through inheritance?
Additional higher level: 3 hours
D2.2.1—Gene expression as the mechanism by which information in genes has effects on the phenotype
- Genotype & Phenotype:
- Genotype: Genetic information of an organism.
- Phenotype: Totality of structures and functions of an organism.
- Gene expression: Process of converting genotype into phenotype.
Stages of Gene Expression
Transcription:
- Genes within the genotype are selected for RNA production.
- DNA base sequence of a gene determines the RNA sequence via complementary base pairing.
- Messenger RNA (mRNA) is produced by transcribing protein-coding genes.
- mRNA is transported to ribosomes in the cytoplasm.
Translation:
- mRNA is used to produce polypeptides.
- The base sequence of mRNA determines the amino acid sequence of the polypeptide.
- Translation occurs on ribosomes.
- Proteins may consist of one or more polypeptides linked together, sometimes with additional non-polypeptide components.
Protein Function:
- Proteins contribute to the organism’s structure.
- Some proteins have functions not visible in outward appearance but still influence phenotype.
- Many proteins act as enzymes, affecting phenotype by catalyzing reactions.
- Example: Lactose digestion is due to protein lactase.
- Gene Expression is Not Binary:
- It is not simply “on” or “off”; instead, it can be measured through the quantities of proteins or their effects.
D2.2.2—Regulation of transcription by proteins that bind to specific base sequences in DNA
Transcription Regulation:
- Gene expression requires selective transcription of RNA.
- RNA polymerase binds to a promoter (a sequence near the gene).
- Eukaryotic promoters often include the TATA box (TATAAAA sequence).
- A general transcription factor binds to the promoter, signaling RNA polymerase to start transcription.
Gene Selection for Transcription:
- Every gene has a promoter, but only some genes are transcribed at different life stages.
- Selectivity of transcription depends on DNA-binding proteins that attach reversibly to specific DNA sequences.
- DNA has two grooves:
- Major groove: Wider, where most protein binding occurs.
- Minor groove: Narrower, with less protein binding.
Transcription Factors & Regulation
Transcription factors: DNA-binding proteins that regulate gene transcription.
- Bind to sites upstream of the gene.
- Binding sequences vary widely, allowing regulation of individual genes or gene groups.
- Unlike these unique binding sites, promoter sequences (where RNA polymerase binds) are shared among genes.
Regulatory Role of Transcription Factors:
- Can act as activators or repressors to turn genes “on” or “off”.
- Bind to regulatory sequences called enhancers and silencers:
- Enhancers: Increase transcription rate.
- Silencers: Decrease transcription rate.