Explain differentiation and environmental effects without changing every DNA sequence.
Method and worked examples
Most nucleated somatic cells share broadly the same genome but express different combinations of genes. Transcription factors influence transcription; other mechanisms regulate RNA and protein production. Differentiation therefore need not remove unused genes or change the genetic code. Environmental signals can also alter expression.
DNA methylation and histone modifications are epigenetic mechanisms that can affect gene access without changing base order. Effects depend on location and context. Altered regulation of cell-cycle genes can contribute to cancer. In a supplied expression table, compare the stated RNA amounts directly: an increase in RNA is an expression change, not automatically a mutation or a proportional increase in functional protein.
Example 1
A neuron and liver cell have the same gene but different amounts of its mRNA. Explain.
Different regulatory factors can cause different transcription rates or RNA stability. The result does not require a difference in the gene’s DNA sequence.
Example 2
A gene’s mRNA rises from 20 to 50 arbitrary units. Find the increase.
The increase is 50−20=30 units, or 150% of the initial value. This is an expression measurement, not a DNA-sequence change.
Watch out:Most specialised nucleated cells retain the genome; selective expression produces many of their differences.
Gene regulation and epigenetics | Year 12 Biology | Ascent Arcade