LECTURE 4 Bacterial genetics

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LECTURE 4Bacterial genetics

• Lecture aims
• To have a brief look at how procaryotic and
  eucaryotic DNAs differ
• To become aware of how bacteria grow
• To understand how bacteria can exchange genetic
  material
• Refs Black Chp 6 p137-140Chp8 p196-210

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Basics of DNA, RNA Protein synthesis

• Like eucaryotes, procaryotes have dsDNA
• This is transcribed into ss mRNA
• The mRNA is translated into protein by ribosomes

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Eucaryotic v procarotic DNA

• Eucaryotic DNA
• 2 stage mRNA synthesis
• DNA has introns exons
• Introns need to be removed from RNA and exons
  respliced together before protein is synthesized

DNA
hmRNA
mRNA
mRNA
Protein
ribosome
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Procaryotic DNA

• Bacterial DNA does not contain introns
• The bacteria do not have the mechanism to remove
  introns and to resplice exons
• Bacteria have one large genome (chromosome)
• Many bacteria have extra DNA (plasmids and
  bacteriophages)
• These can be extrasomal or integrate into the
  chromosome

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Bacterial reproduction

• Bacterial growth refers to increase in numbers
  not size
• This occurs by Binary Fission
• Put simply bacteria double their numbers every
  generation time

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Counting bacteria
Add 1 mL of original broth to 9mL of diluent

• If we start with a broth containing about 1011
  bacteria per/mL
• How do we count them? By dilution!
• ie. dilute something 1million -the original must
  have a million more

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Counting bacteria contin...

• Lets look at a real sample count (well do this
  in the lab)
• The following are E.coli counts obtained from
  100µL of a particular dilution
• BUT we always express the answer as per mL
• ie 10 times greater

100µL of 10-7
gt1000
100µL of 10-8
170
17
100µL of 10-9
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Counting nos contin...

• So the count is expressed as colonies/mL
• The actual count x original dilution x final
  dilution (lets look at the last plate of 17)
  colonies as an example
• 17 (actual count) x 109 (original) x 10
  (µL-gtmL)
• 1.7 x 1011 colonies/mL
• (Note you get the same answer if you use the
  10-8 dilution- try it!)

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How can bacteria change genetically?

• Mutation of chromosome
• Transformation
• Transduction
• Conjugation
• The 1st involves a change in the bacterias
  chromosome
• The last 3 means extra DNA comes into the
  bacteria from outside

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Transformation

• Free DNA may become available from dying or
  disintegrating bacteria
• This can enter a living bacterial cell and join
  into its chromosome
• ie become a part of the chromosome by a process
  called recombination

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Transduction1. Generalized transduction

• Transduction means a piece of bacterial DNA is
  transported to another bacteria by a bacterial
  virus
• The bacterial virus is also called a
  bacteriophage)
• This transfer can happen in a couple of ways

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2. Specialized transduction

• The bacteriophage transports foreign DNA into a
  new bacteria as before
• In this case the virus was integrated ( a part
  of) into the old bacterial chromosome
• And when it leaves it takes a bit of the old
  bacteria with it to a new host bacterium

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Conjugation

• Recall earlier that bacteria can contain extra
  DNA as plasmids
• These plasmids can integrate into the bacterial
  chromosome as well

Integrated plasmid
Extrasomal plasmid
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Conjugation contin...

• Comjugation is a process mediated (caused by) a
  plasmid in a bacteria
• The plasmid DNA codes for the formation of a
  conjugation tube to form between a F and an F-
  bacteria
• The plasmid replicates, causes a tube to form to
  a F-

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Conjugation contin...

• A more complicated form of conjugation is (like
  with the phage) the plasmid integrates into the
  donor chromosome and when conjugation occurs it
  transfers a replicated piece of the donor
  chromosome as well

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Advantages of genetic transfer

• The most important aspect from a clinical
  perspective is the transfer of antibiotic
  resistance from resistant bacteria to sensitive
  ones
• Plasmids and bacteriophges can also give other
  factors (eg virulence genes such as toxins) to
  bacteria who previously didnt have them