Bacterial genetics

• What are the advantages of bacteria for genetics?

  easily cultured

  short generation time: 20-30 min

  Haploid
• Where is the DNA found in the E.coli?

  Nucleoid
• Describe the nucleoid of E.coli:

  • Chromosome is a single double stranded circular DNA molecule 4.6 million base pairs in length and contains around 4500 genes
  • Bacterial DNA is compact by coiling up in the nucleoid
  • Single circular chromosome
• What are the classes of bacteriophages?

  T7, lambda, T5, fd, T4
• what are the life cycles of bacteriophage?

  Lytic

  temperate
• what is the lytic stage of a bacteriophage?

  Multiply and the lyse the bacterial cell releasing progeny bacteriophage particles
• What is the temperate stage of bacteriophage?

  Integrate into the bacterial chromosome and remain dormant replicating along with the bacterial DNA
• Lytic life cycle of bacteriophages

  1. Phage tail fiber sticks to receptor sites on E.coli cell

  2. Sheath of tail contracts, thrusting hollow core through cell wall and membrane

  3. Phage injects DNA into cell

  4. Cell's DNA is hydrolyzed

  5. Cell's machinery produces phage proteins and nucleotides to make copies of phage genome

  6. Phage parts assemble

  7. Phage produces lysozyme to digest bacterial cell wall

  8. Cell swells and bursts, releasing 100-200 phage particles
• T4 phage

  Uses its tail fiber to stick to specific receptor sites on the outer surface of an E.coli cell
• The empty capsid of the phage is left as a ghost outside the cell
• What are the stages of the temperate phage lifecycle?

  Phage DNA circularizes

  Phage DNA integrates into the bacterial chromosome, becoming a prophage.

  The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells.

  Many cells division produce a colony of bacteria infected with prophage

  Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle
• How can genes be transferred between bacteria?

  transformation, transduction, conjugation
• what does genetic recombination produce?


  New bacterial strains, due to new combination of alleles.

  Mutation create new alleles
• what is transformation?

  The uptake of naked DNA
• what is transduction?

  Transfer of bacterial genes from one bacteria to another by a bacteriophage
• what are the 2 types of transduction?

  Generalised

  specialised
• what is generalise transduction?

  Occurs only with virulent phage
• what is a specialised phage?

  Occurs only with temperature phage
• What are the stages of generalized transduction?

  1. Phage infects bacterial cell.
  2. Host DNA is hydrolysed into pieces, and phage DNA and proteins are made
  3. Occasionally bacterial DNA fragment is package in a phage capsid
  4. Crossing over occurs, Transducing phage phages infect new host cells, where recombination (crossing over) can occur
  5. The recombinants have genotyep A+B- which difference for donor (A+B+) or (A-B-)
• What are the stages of specialized transduction?

  1. Bacterial cell has prophage integrated between genes A and B
  2. Occasionally, prophage DNA exist incorrectly, taking adjoining bacterial DNA with it
  3. Phage particles carry bacterial DNA along with Phage DNA
  4. Crossing over, transducing phages infect new host cells, where recombination can occur
  5. The recombinants have genotypes
• What is conjugation?

  The ability to from sex pili and to transfer DNA by conjugation is determined by a plasmid called an F factor
• What is the role of the F factor?

  The F factor replicates in synchrony with the bacterial chromosome.

  The F factor replicates in such a way that one end of the DNA molecule passes through the cytoplasmic bridge into the recipient cell where it cirularises

  The recipient cell is called an excinjugant

  The donor keeps a copy of the F factor
• Stages of conjuagtion

  1. conversion of F+ male into an Hfr male
  2. conjugation between an hfr and F- bacterium
  3. recombination between the transferred fragment and the F- chromosome
• How do individual bacteria adapt to their environment?

  Genes which code for proteins required all the time by the bacterial cell are constitutively expressed

  Other genes are only active (expressed) when they are required - regulated genes
• What controls gene expression in bacteria?

  Controlled at the level of initiation of transcription
• What controls trytophan biosynthesi?

  Regulated synthesis of repressible enzyme
• What type of regulation controls tryptophan biosynthesis?

  Negative regulation - binding of repressor/tryptophan to operator blocks transcription
• How is lactose metabolism controled?

  Regulated synthesis of inducible enzyme
• describe lactose metabolism:

  When lactose is absent:

  • lacI regulatory gene codes for a repressor protein that bind to operator making it inactive, so no translation occurs

  When lactose is present:

  • lacI regulatory gene codes for a repressor protein which binds to allolactose changing its shape leading it to be unable able to bind to lac operator leading to translation occuring
• What are they types of regulation of gene expression in bacteria?

  negative

  positive
• what is negative regualtion?


  Binding of a protein to an operator prevents transcription.
• Examples of negative regulation

  trp operon- expression is off when tryptophan binds to repressor which then binds to the operator

  lac operon- expression is off in the absence of lactose when the repressor binds to the operator
• what is positive regulation?


  Binding of a molecule to the operator turns on gene expression
• Regulation

  positive - activator protein

  negative- repressor protein
• Positive regulation of gene expression - cAMP receptor protein

  Lactose present and Glucose scarce (cAMP high)

  • abundant lac mRNA synthesis

  lactose present and Glucose abundant (cAMP low)

  • little lac mRNA synthesis