Microbial Genetics Part 1

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Microbial Genetics Part 1

• Genetics can be a challenge to understand. Use
  the McGraw Hill website to supplement this
  lecture.
• www.mhhe.com/cowan1
• Please do not spend any time studying the Lactose
  Operon.

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Over view of Microbial Genetics

• 1. Replication of DNA occurs before each cell
  division is complete.
• 2. Transcripton DNA is converted to RNA and
  occurs to carry on life processes.
• 3. Translation RNA is converted to protein
  (enzymes).
• 4. Genetic Transfer and Recombination How do
  we get genetic diversity (antibiotic resistance
  for example) within the microbial community?

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Replication

• Each organism has its own genome. A genome is
  all of the cells genetic information. Included
  in the genome are chromosomes and plasmids, as
  well as other DNA that is sometimes found within
  microbes.
• Chromosomes are structures made up of DNA that
  carry hereditary information. (Remember that
  they are circular in bacteria.)
• Genes are segments of DNA within chromosomes,
  that code for functional products. For example,
  the insulin gene codes for the final insulin
  product.
• Each organism has a genotype and a phenotype.
• The Genotype is the genetic make up of the
  organism. In other words, all the genes that it
  has.
• The Phenotype is the manifest characteristics due
  to the genes it has. In other words, do you have
  blue eyes or green eyes?

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• DNA is composed of 4 nucleotides Adenine (A),
  Thymine (T), Cytosine (C), and Guanine (G).
• AT are complementary base pairs and CG are
  complementary base pairs.
• To give you an idea of the size of DNA in
  bacteria, E-coli has 4 million bps. 4,000 Kb
  in its one chromosome.
• Each chromosome consists of 2 strands of DNA
  bases bound together. They are not identical to
  each other but are complementary to each other.
• In order to know which end is which, they are
  labeled 3 and 5. (See Fig. 9.4 in your
  textbook)
• Note that the 3side of the nucleotide has a
  different chemical group from that of the 5
  side.

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Steps of Replication

• 1. DNA is partially unwound with the help of an
  enzyme called a helicase. The point where the
  helicase pauses the unwinding is called the
  replication fork.
• 2. A molecule, called an RNA primer, is place on
  the DNA to help the nucleotides begin to bind.
  The complementary bases are then added to the
  template (parent) strand using an enzyme called
  polymerase.
• DNA can only replicate in the 5to 3 direction.
  The reason is because the chemical group on 3
  side of the nucleotide acts like a hand that can
  grab onto the next nucleotide on its 5side.
• Since the DNA strands are complementary, (also
  called antiparallel) only one strand can
  replicate quickly and easily in the 5 to 3
  direction. This is called the leading strand.
• .

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• A little help is needed for the opposite strand
  so that it too can be replicated.
• The first step consists of multiple RNA primers
  placed along the template strand. These primers
  provide the necessary hand for the nucleotides to
  grab onto. Then they can replicate the strand 5
  to 3 for a short distance. These fragments of
  DNA are called Okazaki Fragments.
• Once the strand has been replicated, the RNA
  primers are cut out and replaced by the missing
  nucleotides. This strand is called the lagging
  strand because it takes longer for it to be
  replicated.
• 3. Once the strands are replicated up to the
  replication fork, the helicase unwinds the DNA
  some more and the replication fork moves down
  strand to a new location.
• 4. The newly replicated DNA rewinds. One new
  strand winds together with one old strand.
• This process of replication is called
  Semi-conservative Replication because one half of
  the template DNA is kept with one half of the new
  DNA.

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• In Prokaryotes, replication begins at a specific
  site in the chromosome called the origin of
  replication.
• Because prokaryotes have a circular chromosome
  replication can proceed bi-directionally or
  rolling circle.
• Bi-directional means that replication starts at
  the origin of replication and proceeds right and
  left on both strands. See Fig. 9.6
• Rolling Circle means that replication only occurs
  right or left from the origin of replication but
  as it proceeds, the DNA comes off of the
  chromosome in a motion similar to a tape
  dispenser. When replication is complete, the new
  chromosome is stitched into a circle using an
  enzyme called ligase.
• The replication speed for E.coli is estimated to
  be 1000 nucleotides/sec.

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Transcription

• Formation of RNA from DNA
• The nucleotides are essentially the same as the
  DNA nucleotides. The main difference is that
  Uracil (U) replaces Thymine (T) in RNA. In other
  words, anytime T would have been placed in the
  new strand, U is put in that spot instead.
• RNA is single stranded, not double stranded.
• 3 types of RNA are formed
• mRNA, messenger RNA is the template for protein
  synthesis.
• tRNA, transfer RNAs are taxis for amino acids
  during protein synthesis.
• rRNA, ribosomal RNAs are the site of protein
  synthesis. They put the protein together.

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• 1. RNA synthesis begins at a place on DNA called
  the promoter.
• 2. DNA is unwound.
• 3. A primer is put in place and complementary
  bases are added replacing T with U.
• 4. As the RNA strand is synthesized it comes off
  of the template and the DNA strands rewind.
• Once the RNA strand is finished, it folds into a
  shape that gives it its final function.

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Translation

• mRNA codes for functional proteins
• 1. Ribosomes bind to the mRNA.
• Each ribosome has 2 assembly sites amino acids.
• Each protein is made up of a string of amino
  acids bound together.
• 3 nucleotides of mRNA is called a codon
• A codon codes for 1 amino acid.
• There is also a start codon, which signals to the
  ribosomes and tRNA that translation starts here,
  and a stop codon. The stop codon doesnt code
  for any amino acids. It is like a bump in the
  road that bumps the ribosomes off when
  translation is done.

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• 2. tRNA binds to an amino acid and takes it to
  the ribosome.
• Each tRNA has one that binds to a specific amino
  acid. It can never bind to any other kind of
  amino acid.
• The other end of the tRNA has an anti-codon. The
  anti-codon is complimentary to a specific codon
  on the mRNA.
• So, the tRNA binds to its specific amino acid,
  then goes to the ribosome. It then enters the
  open assembly site and if the mRNA codon and the
  tRNA anti-codon match, then the amino acid is
  bound to its neighboring amino acid in the
  adjacent assembly site. (See figure 9.13)

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• 3. Amino acid elongation
• As the amino acids are synthesized, the ribosome
  moves down the mRNA one codon at a time. This
  frees up one assembly spot in the ribosome for a
  new tRNA to bring a new amino acid.
• Amino acid elongation occurs until the ribosomes
  have traveled down the length of the mRNA. Then
  the new amino acid chain is release and the
  ribosomes fall off of the mRNA.
• 4. Protein folding
• The newly formed chain of amino acids has many
  different charges on it due to the variety of
  chemical structures of the amino acids. Once the
  chain is released from the ribosome, it then
  folds into a functional shape based upon the
  charges and shapes of the amino acids. (Remember
  that negative repels negative, positive repels
  positive, and negative and positive attract.)