Control of Gene Expression

• What is a mutation?

  A change in the base sequence of DNA or the number of chromosomes, which is spontaneous and random, thus meaning it cannot be predicted when or where they occur
• Why can mutations not be predicted?

  They are random
• A change in the DNA base sequence leads to a change in the mRNA base sequence, which could lead to a change in amino acid, which could lead to a change in the amino acid sequence, which could lead to a change in the structure of a protein
• Why does a change in the DNA base sequence not always alter the protein?

  The genetic code is degenerate
• The consequences of a mutation are that the DNA is passed on to the next generation of cells, and there may be a change to the codon, which could result in a faulty protein, which may result in a disease
• If a change in the DNA base sequence changes the structure of an enzyme, it could no longer form enzyme-substrate complexes due to the active site potentially changing shape
• Most mutations have little or no effect on the phenotype of organisms
• What are advantageous mutations?

  Positive mutations that can give organisms advantages, which can lead to evolution
• What are the different types of mutation?

  Insertion, deletion, substitution, inversion, duplication and translocation
• What is a frame shift?

  When the reading frame is changed in the DNA sequence, which could then affect the amino acid sequence
• How can frame shift mutations have an effect on the base sequence of a gene?

  They change the number of bases in the DNA code, causing a shift to all the base triplets that follow, so that all codons after the mutation are changed and the DNA code will be transcribed differently
• What is an insertion mutation?

  When a nucleotide is randomly inserted into the DNA sequence
• If the change is not a factor of three, an insertion mutation causes a frame shift to occur
• The further away an insertion mutation occurs on a gene, the lower the potential impact on the proteins structure
• Why does an insertion mutation later in the gene have less impact on a protein structure?

  Fewer codons are affected by the frame shift
• What is a deletion mutation?

  When a nucleotide is removed from the DNA sequence
• A deletion mutation can cause a frame shift
• If frame shifts occur later in the gene, it impacts fewer codons, so it will have less of an impact on the proteins structure than a frame shift earlier in the gene
• What is a substitution mutation?

  When a base in the DNA sequence is randomly swapped for a different base
• Substitution mutations dont cause frame shifts
• What are the three types of substitution mutations?

  Silent, missense and nonsense
• What are silent mutations?

  Mutations that do not alter the amino acid sequence of the polypeptide
• Why could silent mutations happen?

  The new codon may code for the same amino acid as previously
• What is a missense mutation?

  Alters a single amino acid in the polypeptide chain
• What is a nonsense mutation?

  Mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function
• What is an inversion mutation?

  When a sequence of bases is reversed
• When do inversion mutations typically occur?

  During crossing over in meiosis
• Inversion mutations can result in a silent mutations but dont cause frame shifts
• What is a duplication mutation?

  When a whole gene or section of a gene is duplicated so that two copies of the gene appear on the same chromosome
• A duplication mutation can cause a frame shift but can also result in increasing repeating subunits and cause genetic diseases
• How can duplication mutations cause genetic diseases?

  Overtime, the copy may undergo further mutations which could enable it to develop new functions
• Duplication mutations are an important source of evolutionary change
• What is a translocation mutation?

  When a sequence of bases is moved from one location in the genome to another, this could be movement within the same chromosome or to a different chromosome
• In translocation mutations, the cut gene and the gene that has gained the section are both likely to be non-functional
• If a section of a proto-oncogene is translocated onto a gene controlling cell division, it could boost expression and lead to tumours
• Mutagenic agents can increase the rate of mutation
• What are some examples of mutagenic agents?

  High energy ionising radiations and chemicals such as tobacco smoke
• Polypeptides affect the phenotype of an organism through specific cellular mechanisms, so if a mutation occurs it can affect the phenotype
• Stem cells are types of cells that can retain the ability to divide repeatedly and differentiate into a range of cell types
• What is potency?

  The ability of stem cells to differentiate into more specialised cell types