Regulation of transcription and translationAQA A-Level Biology: Revision notes
Section 1
Transcription factors
In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcription factors move from the cytoplasm into the nucleus. Transcription factors are proteins that bind to DNA near a gene. Only genes with matching binding sites respond, so each cell type switches on its own set of genes.
Section 2
Oestrogen and transcription
Oestrogen is a steroid hormone. Being lipid soluble, it diffuses through the phospholipid bilayer into the cell. In the cytoplasm it binds to a specific receptor on a transcription factor, forming an oestrogen-receptor complex. The complex enters the nucleus, binds to DNA and stimulates transcription of target genes.
Only target cells, which have the receptor, respond.
Say that the receptor is specific to oestrogen to explain why only target cells respond.
Section 3
Epigenetic control
Epigenetics involves heritable changes in gene function without changes to the base sequence of DNA. They are caused by environmental changes that inhibit transcription by:
- increased methylation of the DNA
- decreased acetylation of the associated histones, so histones bind DNA more tightly and transcription factors cannot reach the gene
Epigenetic changes are not mutations. The base sequence stays the same.
Section 4
Epigenetics and disease
Epigenetic changes affect the development and treatment of disease, especially cancer. For example, increased methylation can switch off tumour suppressor genes. Because epigenetic changes are reversible, drugs that inhibit methylation or increase acetylation can be used in treatment. Methylation patterns may help diagnosis.
Section 5
RNA interference (RNAi)
In eukaryotes and some prokaryotes, translation of mRNA from target genes can be inhibited by RNAi. Double-stranded RNA is cut into short fragments by an enzyme. One strand binds to the target mRNA by complementary base pairing, and the mRNA is broken down, so the protein is not made.
Section 6
Interpreting data: genes and environment
Identical twins share all alleles, so differences between them suggest environmental or epigenetic influence. Higher concordance in identical than non-identical twins suggests a genetic influence. Evaluate by noting sample size, shared environments and that concordance below 100% in identical twins shows environment matters.
Quote the figures and compare them in any data question.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Regulation of transcription and translation
- Oestrogen is a steroid hormone released by the ovaries. In a target cell of the uterus lining, oestrogen enters the cytoplasm and binds to a specific receptor protein. The oestrogen-receptor complex then moves into the nucleus. Cells that lack this receptor protein, such as most muscle cells, do not respond to oestrogen.Explain why oestrogen changes gene expression in cells of the uterus lining but not in most muscle cells.2 marks
- Researchers followed a pair of identical twins from childhood to the age of 50. Both twins have the same DNA base sequence. At the age of 3 the pattern of DNA methylation in their cells was almost identical. At 50 one twin, who smoked and had a very different diet, had much more methylation of the promoter region of one gene, and cells from this twin contained far less mRNA from that gene.Explain how the two twins can show different gene expression even though their DNA base sequences are identical.2 marks
- Researchers are developing a treatment for a disorder in which liver cells translate too much of a harmful protein. They design short double-stranded RNA molecules, with a base sequence matching part of the mRNA for the protein, and add them to cultured human liver cells. The protein production fell by 85%, but the amount of DNA and the rate of transcription of the gene did not change.Describe how RNA interference (RNAi) can inhibit translation of the mRNA for the harmful protein.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).