3.8.2 gene expression

• Stem cell


  Undifferentiated cells, that can divide indefinitely and turn into other specific cell types
• Types of stem cell


  • Totipotent
  • Pluripotent
  • Multipotent
  • Unipotent
• Totipotent


  Can develop into any cell type including the placenta and embryo
• Pluripotent


  Can develop into any cell type excluding the placenta and embryo
• Multipotent


  Can only develop into a few different types of cell
• Unipotent


  A cell that can only develop into one type of cell
• Unipotent cell example


  • Cardiomyocytes (heart cells)
• Totipotent and pluripotent cells are found in embryos, multipotent and unipotent cells are only found in mature mammals
• Uses of stem cells


  • Medical therapies e.g. bone marrow transplants, treating blood disorders
  • Drug testing on artificially grown tissues
  • Research e.g. on formation of organs and embryos
• How induced pluripotent stem cells are produced


  1. From mature, fully specialised (somatic) cells

  2. The cell regains capacity to differentiate through the use of proteins, in particular transcription factors
• Transcription factor


  A protein that controls the transcription of genes so that only certain parts of the DNA are expressed, e.g. in order to allow a cell to specialise
• How transcription factors work

  1. Move from the cytoplasm into nucleus

  2. Bind to promoter region upstream of target gene

  3. Makes it easier or more difficult for RNA polymerase to bind to gene, increasing or decreasing rate of transcription
• Epigenetics


  A heritable change in gene function without change to the base sequence of DNA
• Increased methylation of DNA


  Prevents transcription factors from binding, therefore gene transcription is suppressed
• Decreased acetylation of histones


  Binding between DNA and histones becomes too tight, preventing transcription factors from accessing the DNA, therefore gene transcription is suppressed
• Effects of epigenetic changes on humans


  They can cause disease, either by over activating a gene's function (such as in cancer) or by suppressing it
• Applications of epigenetics


  • Treatments of various diseases
  • Development of ways to reverse epigenetic changes
• RNA interference


  RNA molecules act to inhibit gene expression, usually by destroying mRNA so that it cannot be translated. Occurs in eukaryotes and some prokaryotes
• Characteristics of benign tumours


  • Slow growth
  • Defined by a clear boundary due to cell adhesion molecules
  • Cells retain function and normal shape
  • Don't spread easily
  • Easy to treat
• Characteristics of malignant tumours


  • Rapid, uncontrollable growth
  • Ill-defined boundary (finger-like projections)
  • Cells do not retain function and often die
  • Spreads quickly and easily (metastasis)
  • Difficult to treat
• Tumour-suppressor genes


  Code for proteins that control cell division; in particular, stopping the cell cycle when damage is detected. They are also involved in programming apoptosis i.e. 'self destruction' of the cell
• Mutation or epigenetic changes in tumour-suppressor genes


  Cells will divide uncontrollably resulting in a tumour
• Proto-oncogenes


  Control cell division; in particular, code for proteins that stimulate cell division
• Mutation or epigenetic changes in proto-oncogenes


  Results in uncontrolled cell division and formation of a tumour
• Abnormal methylation of genes


  Can impair the function of tumour-suppressor genes or oncogenes and cause the cell to divide uncontrollably
• High oestrogen concentration


  Can lead to uncontrolled cell division in areas like the breasts as oestrogen is an activator of RNA polymerase
• What happens to totipotent cells during embryonic development?


  Certain parts of the DNA are selectively translated so that only some genes are 'switched on', in order to differentiate the cell into a specific type and form the tissues that make up the foetus
• How does increased methylation of DNA affect gene transcription?


  Involves addition of CH3 group to cytosine bases which are next to guanine. Prevents transcription factors from binding - gene transcription is suppressed
• How does decreased acetylation of DNA affect gene transcription? 


  Positively-charged histones bind to negatively charged DNA. Decreasing acetylation increases positive charge of histones. Binding becomes too tight and prevents transcription factors from accessing the DNA - gene transcription is suppressed