How Is Gene Expression Controlled?
How is gene expression controlled? Learn how transcription factors, DNA methylation and histone acetylation affect genes in AQA A Level Biology, with exam tips.
Gene expression is controlled by regulating whether a gene is transcribed and whether its messenger RNA (mRNA) is translated into a protein. Two important controls are transcription factors, which act on particular genes, and epigenetic changes, which affect how accessible genes are.
Transcription factors
A transcription factor is a protein that binds to DNA and stimulates or inhibits transcription. Transcription is the process of making an RNA copy of a gene. A transcription factor can move from the cytoplasm into the nucleus, bind near a target gene and change whether that gene is transcribed.
For example, oestrogen can lead to a transcription factor stimulating transcription of target genes. This means that cells can respond to a signal by changing which proteins they produce. Cells do not express all their genes all the time.
Epigenetic changes
Epigenetics means heritable changes in gene function without a change to the DNA base sequence. Increased methylation of DNA can inhibit transcription. Decreased acetylation of histones can also inhibit transcription: histones are proteins associated with DNA, and reducing their acetylation makes the DNA less accessible to the proteins needed for transcription.
These controls can affect the same outcome in different ways. A transcription factor acts at a particular target gene, while an epigenetic change can alter how readily the transcription machinery can access DNA. Gene expression can also be controlled after transcription: RNA interference (RNAi) inhibits translation of mRNA.
Exam mistake to avoid: Do not describe epigenetics as a mutation. A mutation changes the DNA base sequence; an epigenetic change affects gene function without changing that sequence. Also, making mRNA is transcription, while using mRNA to make a polypeptide is translation.
