Mutation and gene editing

• Gene mutations

  Structural changes to genes at the molecular level
• Types of gene mutations

  • Substitutions
  • Insertions
  • Deletions
• Despite its stability, DNA can sometimes undergo changes called gene mutations
• Substitutions


  One base swapped for another
• Insertions

  Extra bases added
• Deletions

  Bases removed
• Substitutions are most common and can arise from errors during DNA replication or chemical modifications
• Insertions and deletions, requiring breaks in the sugar-phosphate backbone, are more significant changes
• Single-nucleotide polymorphisms (SNPs)

  The result of base substitution mutations
• Because of the degeneracy of the genetic code, SNPs may or may not change a single amino acid in a polypeptide
• Consequences of base substitutions

  • No effect in non-coding regions
  • Same-sense mutations (no effect)
  • Nonsense mutations (introduce stop codon, often non-functional proteins)
  • Mis-sense mutations (alter single amino acid, may be harmless or disrupt protein function)
• Many diseases, like sickle cell disease, arise from mis-sense mutations
• Rarely, beneficial mutations can occur, increasing an organism's chance of survival
• SNPs are the most common type of genetic variation and can influence disease risk or other traits
• Consequences of insertions and deletions

  • Often disrupt protein function completely
  • Even minor insertions or deletions can cause frameshift mutations
  • Frameshifts alter the reading frame, leading to jumbled amino acids and non-functional proteins
• Causes of gene mutation

  • Mistakes during DNA replication or repair
  • Exposure to mutagens
• Mutagens


  External agents that increase mutation rates
• Types of mutagens

  • Radiation (X-rays, gamma rays, ultraviolet rays)
  • Chemicals
• Chemical mutagens

  • Heterocyclic amines (HCAs)
  • Benzene
  • Herbicides and pesticides
• The chance of mutation is naturally low, but mutagens significantly increase this risk
• Mutations can occur anywhere in the base sequences of a genome, although some bases have a higher probability of mutating than others
• No natural mechanism is known for making a deliberate change to a particular base with the purpose of changing a trait
• Most mutations are neutral or harmful because genes have been refined through long periods of evolution
• There's no known mechanism to test and pass on only beneficial mutations acquired during an organism's life
• Mutations in germline cells (sperm and egg) can be passed on, influencing future generations
• Randomness in mutation is a key driver of evolution, shaping traits over vast stretches of time
• Consequences of mutation in germ cells and somatic cells

  • Mutations in germ cells can be passed to offspring
  • Mutations in somatic cells can lead to uncontrolled division and cancer
• Gene mutation is the original source of all genetic variation
• Although most mutations are either harmful or neutral for an individual organism, in a species they are in the long term essential for evolution by natural selection
• Gene knockout


  Technique for investigating the function of a gene by changing it to make it inoperative
• A library of knockout organisms is available for some species used as models in research
• CRISPR-Cas9


  A gene editing tool inspired by a natural bacterial defense system
• How CRISPR-Cas9 works

  1. Guide RNA targets specific DNA sequence

  2. Cas9 enzyme cuts DNA at that location

  3. Cell tries to repair the break, allowing introduction of desired genetic change
• Successful uses of CRISPR-Cas9

  • Correcting cystic fibrosis mutation in human cells
  • Modifying mosquito genes to make them resistant to malaria parasite
• Conserved sequences in genes show little change over time, either within a species or across many
• Hypotheses for conserved or highly conserved sequences

  • Functional requirements for the gene products
  • Slower rates of mutation
• Some conserved sequences remain unchanged for millions of years, while others can evolve faster depending on their role
• CFTR is the gene which is affected in cystic fibrosis
• CRISPR sequences and the enzyme Cas9 corrects repairs the gene in cystic fibrosis
• conserved sequences are identical or similar across a species or group of species