3.8 The control of gene expression (A-level only)

Cells are able to control their metabolic activities by regulating the transcription and translation of their genome. Although the cells within an organism carry the same coded genetic information, they translate only part of it. In multicellular organisms, this control of translation enables cells to have specialised functions, forming tissues and organs.

There are many factors that control the expression of genes and, thus, the phenotype of organisms. Some are external, environmental factors, others are internal factors. The expression of genes is not as simple as once thought, with epigenetic regulation of transcription being increasingly recognised as important.

Humans are learning how to control the expression of genes by altering the epigenome, and how to alter genomes and proteomes of organisms. This has many medical and technological applications.

Consideration of cellular control mechanisms underpins the content of this section. Students who have studied it should develop an understanding of the ways in which organisms and cells control their activities. This should lead to an appreciation of common ailments resulting from a breakdown of these control mechanisms and the use of DNA technology in the diagnosis and treatment of human diseases.

3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins (A-level only)

Content

Opportunities for skills development

Gene mutations might arise during DNA replication. They include addition, deletion, substitution, inversion, duplication and translocation of bases.

Gene mutations occur spontaneously. The mutation rate is increased by mutagenic agents. Mutations can result in a different amino acid sequence in the encoded polypeptide.

  • Some gene mutations change only one triplet code. Due to the degenerate nature of the genetic code, not all such mutations result in a change to the encoded amino acid.

  • Some gene mutations change the nature of all base triplets downstream from the mutation, ie result in a frame shift.

Students should be able to relate the nature of a gene mutation to its effect on the encoded polypeptide.

 

3.8.2 Gene expression is controlled by a number of features (A-level only)

3.8.2.1 Most of a cell’s DNA is not translated (A-level only)

Content

Opportunities for skills development

Totipotent cells can divide and produce any type of body cell.

During development, totipotent cells translate only part of their DNA, resulting in cell specialisation.

Totipotent cells occur only for a limited time in early mammalian embryos.

Pluripotent cells are found in embryos; multipotent and unipotent cells are found in mature mammals and can divide to form a limited number of different cell types.

  • Pluripotent stem cells can divide in unlimited numbers and can be used in treating human disorders.
  • Unipotent cells, exemplified by the formation of cardiomyocytes.
  • Induced pluripotent stem cells (iPS cells) can be produced from adult somatic cells using appropriate protein transcription factors.

Students should be able to evaluate the use of stem cells in treating human disorders.

AT i

Students could produce tissue cultures of explants of cauliflower (Brassica oleracea).

3.8.2.2 Regulation of transcription and translation (A-level only)

Content

Opportunities for skills development

In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus. The role of the steroid hormone, oestrogen, in initiating transcription.

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 the DNA or
  • decreased acetylation of associated histones.

The relevance of epigenetics on the development and treatment of disease, especially cancer.

In eukaryotes and some prokaryotes, translation of the mRNA produced from target genes can be inhibited by RNA interference (RNAi).

Students should be able to:

  • interpret data provided from investigations into gene expression
  • evaluate appropriate data for the relative influences of genetic and environmental factors on phenotype.
 

3.8.2.3 Gene expression and cancer (A-level only)

Content

Opportunities for skills development

The main characteristics of benign and malignant tumours.

The role of the following in the development of tumours:

  • tumour suppressor genes and oncogenes
  • abnormal methylation of tumour suppressor genes and oncogenes
  • increased oestrogen concentrations in the development of some breast cancers.

Students should be able to:

  • evaluate evidence showing correlations between genetic and environmental factors and various forms of cancer
  • interpret information relating to the way in which an understanding of the roles of oncogenes and tumour suppressor genes could be used in the prevention, treatment and cure of cancer.
 

3.8.3 Using genome projects (A-level only)

Content

Opportunities for skills development

Sequencing projects have read the genomes of a wide range of organisms, including humans.

Determining the genome of simpler organisms allows the sequences of the proteins that derive from the genetic code (the proteome) of the organism to be determined. This may have many applications, including the identification of potential antigens for use in vaccine production.

In more complex organisms, the presence of non-coding DNA and of regulatory genes means that knowledge of the genome cannot easily be translated into the proteome.

Sequencing methods are continuously updated and have become automated.

 

3.8.4 Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the design of new industrial and medical processes (A-level only)

3.8.4.1 Recombinant DNA technology (A-level only)

Content

Opportunities for skills development

Recombinant DNA technology involves the transfer of fragments of DNA from one organism, or species, to another. Since the genetic code is universal, as are transcription and translation mechanisms, the transferred DNA can be translated within cells of the recipient (transgenic) organism.

Fragments of DNA can be produced by several methods, including:

  • conversion of mRNA to complementary DNA (cDNA), using reverse transcriptase
  • using restriction enzymes to cut a fragment containing the desired gene from DNA
  • creating the gene in a ‘gene machine’.

Fragments of DNA can be amplified by in vitro and in vivo techniques.

The principles of the polymerase chain reaction (PCR) as an in vitro method to amplify DNA fragments.

The culture of transformed host cells as an in vivo method to amplify DNA fragments.

  • The addition of promoter and terminator regions to the fragments of DNA.
  • The use of restriction endonucleases and ligases to insert fragments of DNA into vectors. Transformation of host cells using these vectors.
  • The use of marker genes to detect genetically modified (GM) cells or organisms. (Students will not be required to recall specific marker genes in a written paper.)

Students should be able to:

  • interpret information relating to the use of recombinant DNA technology
  • evaluate the ethical, financial and social issues associated with the use and ownership of recombinant DNA technology in agriculture, in industry and in medicine
  • balance the humanitarian aspects of recombinant DNA technology with the opposition from environmentalists and anti-globalisation activists
  • relate recombinant DNA technology to gene therapy.

AT g

Students could investigate the specificity of restriction enzymes using extracted DNA and electrophoresis.

3.8.4.2 Differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions (A-level only)

Content

Opportunities for skills development

The use of labelled DNA probes and DNA hybridisation to locate specific alleles of genes.

The use of labelled DNA probes that can be used to screen patients for heritable conditions, drug responses or health risks.

The use of this information in genetic counselling and personalised medicine.

Students should be able to evaluate information relating to screening individuals for genetically determined conditions and drug responses.

 

3.8.4.3 Genetic fingerprinting (A-level only)

Content

Opportunities for skills development

An organism’s genome contains many variable number tandem repeats (VNTRs). The probability of two individuals having the same VNTRs is very low.

The technique of genetic fingerprinting in analysing DNA fragments that have been cloned by PCR, and its use in determining genetic relationships and in determining the genetic variability within a population.

The use of genetic fingerprinting in the fields of forensic science, medical diagnosis, animal and plant breeding.

Students should be able to:

  • explain the biological principles that underpin genetic fingerprinting techniques
  • interpret data showing the results of gel electrophoresis to separate DNA fragments
  • explain why scientists might use genetic fingerprinting in the fields of forensic science, medical diagnosis, animal and plant breeding.

AT g

Students could use gel electrophoresis to produce ‘fingerprints’ of food dyes.