Factors affecting gene expressionEdexcel A-Level Biology B: Revision notes
Section 1
Transcription factors
Transcription factors are proteins that bind to specific base sequences on DNA, usually near the promoter of a gene. Each has a shape that is complementary to its binding sequence.
- Activators help RNA polymerase to bind to the promoter, so transcription starts and more mRNA is made.
- Repressors bind to DNA and block RNA polymerase, so transcription is reduced or stopped.
Cells differ in which transcription factors they contain, so different genes are expressed in different cells. A hormone such as oestrogen can also act by binding to a receptor to form a complex that enters the nucleus and acts as a transcription factor.
Transcription factors are proteins that bind to DNA. They are not RNA polymerase and they are not made of RNA.
Section 2
Post-transcription modification: RNA splicing
In eukaryotes the gene is transcribed into a primary transcript (pre-mRNA) that contains introns (non-coding) and exons (coding). Before leaving the nucleus the introns are removed and the exons are joined: RNA splicing.
The exons can be joined in different combinations in different cells or at different times. This is alternative splicing. It produces different mature mRNA molecules from one gene, so different proteins with different amino acid sequences and functions. A single gene can therefore code for more than one protein.
Example: a primary transcript with exons 1 to 6 gives mRNA with exons 1 to 6 in one tissue, but exons 1, 2, 4, 5 and 6 in another, producing a shorter protein.
Section 3
Epigenetic modification
Epigenetic modifications change gene expression without changing the base sequence of the DNA. Three are on the specification:
- DNA methylation: methyl groups are added to cytosine bases, often in the promoter. This prevents transcription factors and RNA polymerase binding, so the gene is silenced.
- Histone modification: DNA is wound round histones. Adding acetyl groups loosens the chromatin so genes can be transcribed; removing acetyl groups (deacetylation) makes it tighter and silences genes.
- Non-coding RNA: RNA molecules that are not translated can bind to DNA or to mRNA, or recruit enzymes, to silence a gene or to cause mRNA to be broken down.
Acetylation of histones opens the chromatin and switches genes on. Methylation of DNA usually switches genes off.
Section 4
Epigenetics and cell differentiation
Every body cell in an organism has the same genome, yet the cells have different structures and functions. The difference is in which genes are expressed.
As a cell differentiates, epigenetic modifications and transcription factors switch some genes off permanently and others on. These patterns are copied when the cell divides by mitosis, so daughter cells stay specialised. This is why epigenetic modification is important in ensuring cell differentiation.
Abnormal epigenetic change can contribute to disease, for example silencing of a tumour suppressor gene by methylation of its promoter.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Factors affecting gene expression
- A researcher studies a gene in liver cells that is transcribed only when a protein, factor F, enters the nucleus and binds to a DNA sequence next to the gene. In muscle cells factor F is absent and the gene is not transcribed, although muscle cells contain the same gene.Explain how factor F can increase the rate of transcription of the gene in liver cells.2 marks
- The gene for a muscle protein is transcribed in two different tissues. In both tissues the primary transcript has six exons. In tissue 1 the mature mRNA contains exons 1 to 6. In tissue 2 the mature mRNA contains exons 1, 2, 4, 5 and 6, and the protein it codes for is shorter and has a different function.Explain why the two mRNA molecules lead to proteins with different functions.2 marks
- Cells from a tumour show that the promoter region of a tumour suppressor gene has many methyl groups attached to its cytosine bases, and the histones around the gene have had their acetyl groups removed. The tumour suppressor gene is not expressed in these cells.Explain how these modifications prevent expression of the tumour suppressor gene.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).