Prokaryotic Genetics

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Chapter 7

• Prokaryotic Genetics

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7.1 Prokaryotic DNA

• Prokaryotic DNA Is Organized within the Nucleoid
• Most of the genetic information in prokaryotic
  cells is contained in the chromosome
• The chromosome is located in the nucleoid

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• DNA within a Chromosome Is Highly Compacted
• DNA occupies around 1/3 of the total volume of
  the cell
• The chromosome is supercoiled twisted and
  tightly packed by nucleoid-associated proteins

Figure 7.3a, page 194
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• Many Prokaryotic Cells also Contain Plasmids
• Plasmids are stable extrachromosomal DNA elements
  that carry nonessential genetic information
• They replicate independently from the chromosome
• F plasmids allow genetic material to be
  transferred from a donor cell to a recipient
• R plasmids carry genes for antibiotic resistance

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7.2 DNA Replication

• DNA Replication Is a Highly Regulated Process
• The first stage of prokaryotic replication is
  initiation, where DNA unwinds and the strands
  separate
• The point where replication starts is the origin
  of replication (oriC)
• As the DNA unzips, two replication forks form and
  move in opposite directions away from the origin

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• The second stage is elongation, when enzymes
  synthesize a new strand to pair with each
  original strand
• Insertion of complementary nucleotides on the
  template strand is carried out by DNA polymerase
  III
• DNA polymerase III can move only in a 3 to 5
  direction, creating a leading strand and a
  lagging strand
• The lagging strand is synthesized in Okazaki
  fragments, which are joined by DNA ligase
• Mistakes that occur in DNA replication are called
  mutations

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• The third stage is termination, when the two DNA
  helices separate from each other
• When the replication forks reach the termination
  point (terC), terminator proteins block any
  further replication
• The pairing of one old strand with one new strand
  is called semiconservative replication

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Figure 7.4, page 196
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7.3 Protein Synthesis

• The Central Dogma Identifies the Flow of Genetic
  Information
• DNA is expressed as RNA through transcription
• RNA is translated into proteins through
  translation

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• Transcription Copies Genetic Information into RNA
• RNA polymerase is the enzyme that synthesizes RNA
  from the DNA template
• Only one of the two DNA strands is transcribed
• Messenger RNA (mRNA) carries the information of
  what protein will be synthesized

Figure 7.5, page 198
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• Ribosomal RNA (rRNA) are the framework of
  ribosomes
• Transfer RNA (tRNA) contains the anticodon
• a sequence of 3 nitrogenous bases complementary
  to the codon on the mRNA
• it corresponds to a specific amino acid

Figure 7.6, page 199
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• The Genetic Code Is Degenerate
• More than one codon specifies a particular amino
  acid
• AUG is the start codon
• Some codons do not code for amino acids
• They can terminate the addition of amino acids to
  a polypeptide (stop codon)

Table 7.3, page 200
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• Translation Is the Process of Making the
  Polypeptide
• Translation occurs as the ribosome moves along
  the mRNA
• the mRNA codons are exposed to tRNA binding sites
  in the ribosome
• Translation begins with the addition of the tRNA
  whose anticodon is complementary to AUG

Figure 7.8, page 201
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• Each amino acid in the chain is attached to its
  neighbors by a peptide bond
• Polypeptide elongation continues until the stop
  codon is reached
• releasing factors bind to the codon and trigger
  release of the polypeptide
• Chaperones help polypeptides fold into the
  correct shape
• Antibiotics Interfere with Protein Synthesis
• Many antibiotics disrupt transcription and
  translation

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• Protein Synthesis Can Be Controlled in Several
  Ways
• Segments of prokaryotic DNA are organized into
  operons
• Each operon consists of
• a cluster of structural genes that carry the
  genetic information for proteins involved in
  metabolic functions
• an operator, which controls transcription of
  structural genes
• a promoter to which RNA polymerase binds in
  initiation of structural gene transcription

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Figure 7.10, page 204
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• DNA also contains regulatory genes that code for
  a repressor protein
• The repressor protein binds to the operon
• This prevents transcription of structural genes
  (negative control)

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• Transcription and Translation Are
  Compartmentalized
• Most of the transcription presumably occurs
  within the nucleoid core where RNA polymerases
  are concentrated in Bacillus subtilis
• The ribosomes tend to be concentrated at the cell
  poles

Figure 7.11, page 206
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7.4 Mutations

• Mutations Are the Result of Natural Processes or
  Induced
• Spontaneous mutations are heritable changes to
  the base sequence in DNA
• They result from natural phenomena such as
  radiation or uncorrected errors in replication
• UV light is a physical mutagen that creates a
  dimer that cannot be transcribed properly

Figure 7.13, page 208
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• Nitrous acid is a chemical mutagen that converts
  adenine bases to hypoxanthine
• Hypoxanthine base pairs with cytosine instead of
  thymine
• Base analogs bear a close resemblance to
  nitrogenous bases and can cause replication errors

Figure 7.14, page 209
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Point Mutations Are a Result of Spontaneous or
Induced Mutations

• A point mutation affects just one base pair in a
  gene
• Base-pair substitutions result in an incorrect
  base in transcribed mRNA codons
• Base-pair deletion or insertion results in an
  incorrect number of bases

Figure 7.15, page 210
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• Repair Mechanisms Attempt to Correct Mistakes or
  Damage in the DNA
• Mismatch repair involves DNA polymerase
• proofreading the new strand
• removing mismatched nucleotides

Figure 7.16, page 211
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• Excision repair involves
• cutting out damaged DNA
• replacing it with correct nucleotides

Figure 7.17, page 211
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• Transposable Genetic Elements Can Cause Mutation
• Insertion sequences (IS) are small segments of
  DNA
• They carry no genetic information except for that
  required to insert themselves into a chromosome
• IS form copies of themselves, which move into
  other areas of the chromosome
• This disrupts the coding sequence

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• Transposons (jumping genes) can carry
  information such as antibiotic resistance
• They can jump between plasmids and chromosomes

Figure 7.18, page 213
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7.5 Identifying Mutants

• Plating Techniques Select for Specific Mutants or
  Characteristics
• Negative selection can be used to identify
  mutants
• Documenting lack of growth of his- mutants in a
  medium containing no histidine identifies
  auxotrophs

Figure 7.19, page 214
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• Positive selection can be used to identify
  mutants
• Tetracycline resistant strains are identified by
  the presence of cells in a medium containing
  tetracycline

Figure 7.20, page 214
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• The Ames Test Can Identify Potential Mutagens
• Carcinogens are cancer-causing compounds that
  also induce mutations in bacteria
• The Ames test helps identify carcinogens by
  observing whether an agent causes mutations in
  bacterial DNA

Figure 7.21, page 215