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3.8 The control of gene expression

6 dot points across 6 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How can gene expression be switched off without changing the DNA base sequence, and can these changes be inherited?

Epigenetic control of gene expression in eukaryotes. Epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA. These changes are caused by changes in the environment that inhibit transcription by increased methylation of DNA or decreased acetylation of associated histones. The increased methylation of DNA and decreased acetylation of histones can inhibit transcription. Epigenetic changes can be inherited and have a role in the development of disease.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on epigenetics. Explains how increased DNA methylation and decreased histone acetylation inhibit transcription without changing the base sequence, how these heritable changes respond to the environment, and their role in disease such as cancer.
10 min answer →

How do changes in the expression of just a few genes turn a normal cell into a tumour cell?

The role of the increased and decreased expression of genes in the development of tumours. The roles of tumour suppressor genes and oncogenes in the development of tumours. The role of abnormal methylation of tumour suppressor genes and oncogenes in the development of tumours. The increased exposure to oestrogen can increase the chances of developing some breast cancers. Benign and malignant tumours can be distinguished by their characteristics.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on cancer. Explains how oncogenes and tumour suppressor genes control the cell cycle, how mutation and abnormal methylation lead to tumours, the difference between benign and malignant tumours, and the role of oestrogen in some breast cancers.
11 min answer →

How can we find one specific gene or person within a sea of DNA, and what is it used for?

The use of labelled DNA probes that can be used to locate specific genes by complementary base pairing (DNA hybridisation). The use of these techniques in medical diagnosis. The principles of DNA sequencing and the development of high-throughput sequencing. Genetic fingerprinting and its use in determining genetic relationships and the genetic variability within a population, based on variable number tandem repeats (VNTRs), separated by size using gel electrophoresis.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on gene probes, sequencing and genetic fingerprinting. Explains labelled DNA probes and hybridisation, their use in medical diagnosis, the principles of DNA sequencing, and how VNTRs and gel electrophoresis produce a genetic fingerprint for forensics and relationship testing.
12 min answer →

How do mutations alter the polypeptide a gene codes for, and how can one genome give rise to so many specialised cell types?

Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include addition, deletion, substitution, inversion, duplication and translocation of bases. The degenerate nature of the genetic code means that some substitutions do not change the amino acid coded for. Some gene mutations change only one triplet code; the position of a deletion or addition mutation within a gene is important. Mutagenic agents increase the rate of mutation. Stem cells are unspecialised cells capable of dividing and differentiating, and are described as totipotent, pluripotent, multipotent or unipotent.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on gene mutations and cell specialisation. Covers substitution, deletion, addition, inversion, duplication and translocation, the role of the degenerate code, mutagenic agents, and totipotent, pluripotent, multipotent and unipotent stem cells with their uses.
11 min answer →

How do scientists move a gene from one organism into another and get it to work?

Recombinant DNA technology involves transferring fragments of DNA from one organism, or species, to another. Because the genetic code is universal, the transferred DNA can be translated in the recipient. Fragments of DNA can be produced by conversion of mRNA to complementary DNA using reverse transcriptase, by using restriction endonucleases to cut a fragment containing the desired gene, and by creating the gene in a gene machine. DNA fragments can be amplified using in vivo techniques involving vectors and the use of the polymerase chain reaction (PCR) in vitro. The use of recombinant DNA technology to produce transformed organisms that benefit humans, and the use of gene therapy.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on recombinant DNA technology. Covers isolating a gene with reverse transcriptase, restriction endonucleases and the gene machine, amplification by in vivo cloning with vectors and in vitro PCR, transformation and marker genes, and the principles of gene therapy.
13 min answer →

How is a cell able to switch genes on and off so that the right polypeptides are made at the right time?

The control of transcription by specific transcription factors which move from the cytoplasm to the nucleus. In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcription factors bind to DNA. The effect of oestrogen on gene transcription. The control of translation of mRNA by RNA interference using small interfering RNA (siRNA), which can lead to the breakdown of mRNA or block its translation.
An exam-focused answer to the AQA A-Level Biology 3.8 dot point on regulating gene expression. Explains how specific transcription factors control transcription, how oestrogen acts as a transcription factor complex, and how siRNA in RNA interference breaks down or blocks mRNA to control translation.
11 min answer →