Every cell has the same DNA. What makes a liver cell different is which genes it switches on. Welcome to gene expression.
Every sub-topic below feeds at least one of these questions.
How is gene expression changed in a cell?
How can patterns of gene expression be conserved through inheritance?
An extra 11 sub-topics for HL — same syllabus, deeper mechanism.
Gene expression as the mechanism by which information in genes has effects on the phenotype
Regulation of transcription by proteins that bind to specific base sequences in DNA
Control of the degradation of mRNA as a means of regulating translation
Epigenesis as the development of patterns of differentiation in the cells of a multicellular organism
Differences between the genome, transcriptome and proteome of individual cells
Methylation of the promoter and histones in nucleosomes as examples of epigenetic tags
Epigenetic inheritance through heritable changes to gene expression
Examples of environmental effects on gene expression in cells and organisms
Consequences of removal of most but not all epigenetic tags from the ovum and sperm
Monozygotic twin studies
External factors impacting the pattern of gene expression
Gene expression turns information in a gene into a functional product — usually a protein — that affects the organism's phenotype.
The steps: gene → mRNA (transcription) → protein (translation) → cellular function → contribution to phenotype. Genotype (the alleles carried) + environment determine which proteins are made and in what quantities, and that shapes the observable phenotype.
Three regulatory features control transcription: promoters, enhancers, and transcription factors.
The binding site for RNA polymerase. Required for transcription to begin.
Regulatory sequences that can be far from the gene. Loop around to interact with the promoter, boosting (or sometimes inhibiting) transcription.
Bind promoters and enhancers. Activate or repress transcription, often in response to cellular signals. The "switches" that make context-dependent gene expression possible.
After transcription, mRNA persists for a limited time before being broken down by nuclease enzymes. Controlling mRNA lifetime controls how much protein gets made.
In human cells, mRNA half-life ranges from minutes (for transient signals) to days (for housekeeping proteins). Each round of translation off a single mRNA produces another protein molecule; once the mRNA is degraded, translation stops. Adjusting mRNA stability is therefore a powerful way to tune protein levels.
Every cell in your body has the same DNA. The reason a liver cell differs from a neuron is that they express different subsets of that DNA — different gene expression patterns.
Epigenesis is the developmental process by which a single zygote produces a body with hundreds of differentiated cell types. Epigenetic tags — chemical markers added to DNA or histones — direct each cell type's gene expression pattern. The DNA sequence is identical; the patterns of which genes are active or silenced differ.
A cell's identity is captured at three levels — what it could be (genome), what it's currently transcribing (transcriptome), and what proteins it currently has (proteome).
The complete genetic content of a cell. Same in every cell of the body (with minor exceptions like mature B-cells).
All the RNA molecules currently being transcribed. Differs dramatically between cell types — liver vs neuron transcriptomes overlap only partially.
The complete set of proteins present in the cell right now. The final cellular phenotype is the proteome — what the cell can actually do.
Two main types of epigenetic methylation control gene expression: methylation of promoter DNA, and methylation of histone tails.
Epigenetic tags are usually erased during germ cell formation — but not always. Some tags persist across generations, transmitting environmental effects from parent to offspring.
When eggs and sperm form, most epigenetic tags are removed (reprogramming) to give the new individual a "clean slate" of gene regulation. But not all tags are erased — some heritable epigenetic patterns persist. These can transmit certain environmental effects (nutrition, stress, toxin exposure) from one generation to the next without any change in DNA sequence.
Diet, stress, toxin exposure, light, temperature — many environmental factors leave epigenetic marks that alter gene expression patterns.
Almost all epigenetic tags are removed from gametes — but a few are retained, producing genomic imprinting.
Reprogramming during gametogenesis erases most epigenetic marks, giving the zygote a "fresh start". A small number of genes retain their parental imprints — they are expressed only from one parent's allele. Mistakes in imprinting cause disorders like Prader-Willi syndrome and Angelman syndrome (both involve chromosome 15 imprinting errors).
Identical twins share the same genome but accumulate different epigenetic patterns through life. Comparing their phenotypes reveals epigenetic vs genetic contributions to traits.
Twin studies show that even identical twins increasingly diverge with age — in DNA methylation patterns, in disease susceptibility, even in physical traits like hair colour at older ages. The genome is fixed; the epigenome accumulates the effects of decades of distinct environments. This is some of the strongest evidence that environment-driven epigenetic change matters for phenotype.
If you can't define one of these in a sentence, that's where to revise next.
“What mechanisms are there for inhibition in biological systems?”
“In what ways does the environment stimulate diversification?”