week 6

• Mutation


  A stable, heritable change in a sequence of DNA
• Mutations


  • May be beneficial, neutral or harmful
  • Changes in the genotype which may or may not affect the phenotype
• Mutation types


  • Base substitutions
  • Frameshift mutations
  • Silent mutation
  • Missense mutation
  • Nonsense mutation
• Base substitution


  The most common type of mutation, a single base pair is replaced by another
• Frameshift mutation


  One or more base pairs are inserted or deleted in the DNA, results in a change in the reading of codons
• Silent mutation


  Base change results in no change of the amino acid sequence of the translated protein, has no effect on phenotype
• Missense mutation


  Base change results in the change of an amino acid in the translated protein
• Nonsense mutation


  Base change generates a stop codon in place of that coding for an amino acid, results in production of a truncated protein, usually results in a non-functional protein
• Causes of mutations


  • Spontaneous mutations
  • Induced mutations
• Spontaneous mutations


  Occur in the absence of mutation causing agents, due to occasional mistakes in DNA replication
• Induced mutations


  Caused by mutagens, agents such as chemicals and radiation which induce mutations
• Chemical mutagens


  • Ethidium bromide inserts between bases causing frameshift mutations
• Ionizing radiation


  Causes the formation of ions that can react with nucleotides (causing base changes) and the deoxyribose-phosphate backbone (causes chromosomes to break)
• UV radiation


  Induces formation of covalent bonds between adjacent thymines to form thymine dimers which can not be replicated
• Effects of mutations


  • Cell or colony morphology
  • Viability
  • Biochemical pathways
  • Susceptibility to environmental conditions
• Positive selection


  Mutants grow differently for example become resistant to an antibiotic while the "wild type" are unable to grow
• Negative selection


  Selection detects mutants unable to grow by replica plating, wild type bacteria are able to grow without histidine but mutants can not
• Ames test


  Uses Salmonella histidine mutants, mutagens will cause reversion (or back mutation) of these mutants to allow them to grow in the absence of histidine
• A silent mutation changes the amino acid - False, it has no effect on amino acid sequence
• A missense mutation has no effect on the amino acid sequence - False, it changes an amino acid
• A frameshift mutation may code for a longer protein - True, it changes many amino acids and can lead to either a longer or shorter protein, the normal stop codon is altered and out of frame
• Plants and animals typically pass genes from parents to offspring by what is known as "vertical transfer"
• Horizontal gene transfer


  Bacteria transmit DNA to offspring by means of horizontal transfer, allowing DNA to move from cell to cell
• Mechanisms of horizontal gene transfer

  • Conjugation
  • Transformation
  • Transduction
  • Transposons
• Conjugation


  Direct physical interaction transfers genetic material from donor to recipient cell
• Transformation


  DNA released from a dead bacterium into the environment is taken up by another bacteria
• Transduction


  A virus transfers genetic information from one bacterium to another
• Transposons


  DNA sequences that are inserted into new locations, known as "jumping genes"
• Types of plasmids


  • Resistance plasmids (R factors)
  • Degradative plasmids
  • Col-plasmids
  • Virulence plasmids
  • Fertility plasmids (F factors)
• Resistance plasmids (R factors)


  Contain genes that confer resistance against antibiotics and other types of toxins
• Degradative plasmids


  Carry genes that enable the bacterium to digest and utilize an unusual substance
• Col-plasmids


  Contain genes that encode proteins that kill other bacteria
• Virulence plasmids


  Carry genes that turn a bacterium into a pathogenic strain
• Fertility plasmids (F factors)


  Allow bacteria to mate with each other
• Transformation


  Does not require direct contact between bacterial cells, living bacterial cell imports a strand of DNA that another bacterium released into the environment when it died, only competent cells with competence factors can do this
• Transduction


  Viruses that infect bacteria (bacteriophages) may transfer DNA from one bacterium to another, usually an error in a phage lytic cycle where newly assembled phages incorporate piece of host DNA instead
• Transposons


  Known as "jumping genes", sequences of DNA that move from one location in a genome to another location on the genome, contain the genes for transposase which allows the transposon to cut into DNA and insert itself back into a different position in the DNA
• Transposons can insert themselves into a different position on the same genome or move between different bacteria, may also contain other genes for eg antibiotic resistance genes
• The transposon has an inverted repeat sequence at each end
• Constitutive genes


  Genes that make proteins all the time at a constant rate, also known as "housekeeping genes"