Unit 8: The control of Gene Expression

    Cards (193)

    • Gene mutations
      Change in base sequence of DNA
    • Gene mutations
      • Can arise spontaneously during DNA replication, during interphase of the cell cycle
    • Mutagenic agents
      • Ionising radiation (gamma and X-rays)
      • Carcinogens e.g. mustard gas
      • Some viruses
    • Mutagenic agents

      Increase the rate of gene mutation (above the rate of naturally occurring mutations)
    • Types of gene mutations and their effects on amino acid sequences
      • Substitution
      • Addition
      • Deletion
      • Inversion
      • Duplication
      • Translocation
    • Substitution
      1 base replaced with another
    • Addition
      1 or more bases added to base sequence
    • Deletion
      1 or more bases lost from base sequence
    • Inversion
      A sequence of bases is separated from DNA and inserted at the same position, backwards
    • Duplication
      A sequence of bases is inserted twice, or multiple times
    • Translocation
      Sequence of bases taken out and inserted at a different position on the same, or a different chromosome
    • Some gene mutations (substitution) change only 1 codon
    • New codon might still code for same amino acid because genetic code is degenerate (meaning the same amino acid can be coded for by more than one triplet)
    • Some gene mutations occur in the introns (non-coding sequences within genes) and therefore won't affect amino acid sequences
    • Frameshift
      A frameshift occurs when gene mutations such as insertion or deletion change the number of nucleotides by any number not divisible by 3
    • This shifts the way the genetic code is read, so all the DNA triplets / mRNA codons downstream from the mutation change
    • The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant
    • Stop codons
      There are 3 stop codons in the genetic code (UAA, UGA, UAG) that don't code for amino acids, so they terminate translation
    • A mutation (substitution or frameshift) may create a premature stop codon and result in the production of a shorter and often non-functional polypeptide
    • How mutations can lead to the production of a non-functional protein / enzyme
      1. Change in base / triplet sequence of DNA / gene
      2. Changes sequence of codons on mRNA
      3. Changes sequence of amino acids in primary structure of polypeptide
      4. Changes position of hydrogen / ionic / disulphide bonds in protein tertiary structure
      5. Changes tertiary structure / shape of protein and in the case of enzymes, the active site will change shape
      6. In the case of enzymes, the substrate will be unable to bind to active site and form an enzyme-substrate complex
    • Stem cells
      Unspecialised cells capable of self-renewal and specialisation/differentiation
    • Stem cell specialisation
      1. Stimulus e.g. chemical
      2. Causes selective activation of genes – some genes activated while others inactivated
      3. mRNA only transcribed from active genes → translated on ribosomes = proteins
      4. These proteins modify cell permanently and determine cell structure / function
    • Types of stem cells
      • Totipotent cells
      • Pluripotent cells
      • Multipotent cells
      • Unipotent cells
    • Totipotent cells

      Occur for a limited time in early mammalian embryos, can divide and differentiate into every cell type in body (including the cells that support the embryo, such as the placenta)
    • Pluripotent cells
      Found in embryos, can divide and differentiate into most cell types (every cell type in body but not the cells of the placenta)
    • Multipotent cells

      Found in mature mammals, can divide and differentiate into a limited number of cell types
    • Unipotent cells
      Found in mature mammals, can divide and differentiate into just one cell type
    • Stem cells in medicine
      • Regrow damaged tissues
      • Drug testing
      • Developmental biology research
    • Induced pluripotent stem cells
      Produced from adult somatic cells (non-pluripotent cells or fibroblasts) by putting in specific protein transcription factors associated with pluripotency, causing the cell to express genes associated with pluripotency (reprogrammed)
    • Induced pluripotent stem cells can be used in medical treatment instead of embryonic cells to overcome immune rejection and some ethical issues
    • Arguments for and against the use of stem cells in treating human disorders
      • For: Use of embryonic stem cells (tiny ball of cells, incapable of feeling pain, not equivalent to a human, would otherwise be destroyed)
      • Against: Use of embryonic stem cells (embryo is a potential human, should be given rights, scientific issues with induced pluripotent stem cells)
    • Transcription factors
      Proteins that move from cytoplasm to nucleus, bind to DNA at a specific base sequence on a promotor region, and stimulate ('activator') or inhibit ('repressor') transcription of target gene(s)
    • Role of oestrogen in initiating transcription
      1. Oestrogen diffuses into cytoplasm and binds to receptor of inactive transcription factor, forming hormone-receptor complex
      2. Inactive transcription factor changes shape, resulting in active transcription factor
      3. Active transcription factor diffuses into nucleus and binds to specific DNA base sequence on a promotor region
      4. Stimulates transcription of genes by helping RNA polymerase to bind
    • Epigenetics
      Heritable changes in gene function (expression) without changes to the base sequence of DNA, caused by changes in the environment
    • Epigenetic changes
      • Methylation of DNA
      • Decreased acetylation of associated histones
    • Epigenetic changes that increase the expression of an oncogene, or that silence a tumour suppressor gene, can lead to tumour development
    • Tests can be used to see if a patient has abnormal levels of methyl and acetyl – early indicator of cancer (called a biomarker)
    • Epigenetic changes could be manipulated to treat cancer i.e. drugs to prevent histone acetylation / DNA methylation that may have caused these genes to be switched on/off, resulting in cancer
    • RNA interference (RNAi)

      RNA molecules inhibit translation of mRNA produced by transcription (gene is 'switched on' but encoded protein not produced = 'silenced' gene)
    • Types of RNAi
      • Micro-RNA (miRNA)
      • Small interfering RNA (siRNA)
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