The control of gene expression

• Cells


  Able to control their metabolic activities by regulating the transcription and translation of their genome
• Cells within an organism carry the same coded genetic information, but they translate only part of it
• Control of translation in multicellular organisms


  Enables cells to have specialised functions, forming tissues and organs
• Factors that control the expression of genes and the phenotype of organisms


  • External, environmental factors
  • Internal factors
• 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
• Cellular control mechanisms


  Underpin 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
• Gene mutations


  Might arise during DNA replication, including addition, deletion, substitution, inversion, duplication and translocation of bases
• Gene mutations occur spontaneously, and 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, and 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, resulting in a frame shift
• Totipotent cells can divide and produce any type of body cell, but during development they translate only part of their DNA, resulting in cell specialisation
• Totipotent cells occur only for a limited time in early mammalian embryos
• Types of stem cells

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


  Heritable changes in gene function, without changes to the base sequence of DNA, 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)
• The main characteristics of benign and malignant tumours
• Factors in the development of tumours

  • Tumour suppressor genes
  • Oncogenes
  • Abnormal methylation of tumour suppressor genes and oncogenes
  • Increased oestrogen concentrations in the development of some breast cancers
• 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
• 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
• 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
• Methods to produce fragments of DNA

  • 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'
• Amplification of DNA fragments

  • In vitro using the polymerase chain reaction (PCR)
  • In vivo by culturing transformed host cells
• 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, and the transformation of host cells using these vectors
• The use of marker genes to detect genetically modified (GM) cells or organisms
• 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, and the use of this information in genetic counselling and personalised medicine