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Microbial Genetics
Chapter 9
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Flow of Genetic Information
Figure 8.2
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DNA

1. Polymer of nucleotides adenine, thymine,
   cytosine, guanine
2. Double helix associated with proteins
3. "Backbone" is deoxyribose-phosphate
4. Strands held together by hydrogen bonds between
   AT and CG
5. Strands are antiparallel

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DNA replication is semiconservative because each
chromosome ends up with one new strand of DNA and
one old strand.
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Bacterial replicon
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DNA
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Flow of genetic information
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• What are the products that genes encode?
• RNAs and proteins
• How are genes expressed?
• transcription and translation

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Gene expression

• Transcription DNA is used to synthesize RNA
• RNA polymerase is the enzyme responsible
• Translation making a protein using the
  information provided by messenger RNA
• occurs on ribosomes

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• Genotype - genes encoding all the potential
  characteristics of an individual
• Phenotype -actual expressed genes of an
  individual (its collection of proteins)

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DNA-protein relationship

1. Each triplet of nucleotides (codon) specifies a
   particular amino acid.
2. A proteins primary structure determines its
   shape function.
3. Proteins determine phenotype. Living things are
   what their proteins make them.
4. DNA is mainly a blueprint that tells the cell
   which kinds of proteins to make and how to make
   them.

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DNA-protein relationship
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3 types of RNA

• messenger RNA (mRNA)
• transfer RNA (tRNA)
• ribosomal RNA (rRNA)

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DNA
Transcription RNA polymerase
RNA
Translation ribosomes
PROTEINS
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Transcription

1. RNA polymerase binds to promoter region upstream
   of the gene
2. RNA polymerase adds nucleotides complementary to
   the template strand of a segment of DNA in the 5
   to 3 direction
3. Uracil is placed as adenines complement
4. At termination, RNA polymerase recognizes signals
   and releases the transcript
5. 100-1,200 bases long

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Transcription
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Transcription
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Transcription

1. DNA is transcribed to make RNA (mRNA, tRNA, and
   rRNA)
2. Transcription begins when RNA polymerase binds to
   the promoter sequence
3. Transcription proceeds in the 5? ? 3? direction
4. Transcription stops when it reaches
   theterminator sequence

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Translation

1. Translation information in the nucleotide base
   sequence of mRNA is used to dictate the amino
   acid sequence of a protein
2. mRNA is translated in codons (3 nucleotides)
3. Translation of mRNA begins at the start codon
   AUG
4. Translation ends at a STOP codons UAA, UAG, UGA

Figure 8.2
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Translation

• mRNA a/w ribosome that are made of rRNA and
  protein
• 64 codons
• Sixty-one sense codons amino acids
• Three nonsense codons stop signals
• Specific amino acids are attached to one end of
  tRNA
• Anticodon other portion or end of tRNA is a base
  triplet that is specific for base sequence code
  of mRNA

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Translation
Figure 8.10.1
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Translation

• Ribosomes assemble on the 5 end of a mRNA
  transcript
• Ribosome scans the mRNA until it reaches the
  start codon, usually AUG
• A tRNA molecule with the complementary anticodon
  binds to the mRNA

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Translation
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Interpreting the DNA code
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Translation elongation

• A second tRNA with the complementary anticodon
  fills the A site
• A peptide bond is formed
• The first tRNA is released and the ribosome
  slides down to the next codon.
• Another tRNA fills the A site a peptide bond is
  formed.
• This process continues until a stop codon is
  encountered.

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Translation termination

• Termination codons UAA, UAG, and UGA are
  codons for which there is no corresponding tRNA.
• When this codon is reached, the ribosome falls
  off and the last tRNA is removed from the
  polypeptide.

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Polyribosomal complex
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Eucaryotic transcription translation differs
from procaryotic

1. Do not occur simultaneously. Transcription occurs
   in the nucleus and translation occurs in the
   cytoplasm.
2. Eucaryotic start codon is AUG, but it does not
   use formyl-methionine.
3. Eucaryotic mRNA encodes a single protein, unlike
   bacterial mRNA which encodes many.
4. Eucaryotic DNA contains introns intervening
   sequences of noncoding DNA- which have to be
   spliced out of the final mRNA transcript.

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Split gene of eucaryotes
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RNA processing in Eukaryotes
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Regulation of protein synthesis metabolism
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Operons

• a coordinated set of genes, all of which are
  regulated as a single unit.
• 2 types
• inducible operon is turned ON by substrate
  catabolic operons- enzymes needed to metabolize a
  nutrient are produced when needed
• repressible genes in a series are turned OFF by
  the product synthesized anabolic operon enzymes
  used to synthesize an amino acid stop being
  produced when they are not needed

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Lactose operon inducible operon

• Made of 3 segments
• Regulator- gene that codes for repressor
• Control locus- composed of promoter and operator
• Structural locus- made of 3 genes each coding for
  an enzyme needed to catabolize lactose
• b-galactosidase hydolyzes lactose
• permease - brings lactose across cell membrane
• b-galactosidase transacetylase uncertain
  function

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Lac operon

• Normally off
• In the absence of lactose the repressor binds
  with the operator locus and blocks transcription
  of downstream structural genes
• Lactose turns the operon on
• Binding of lactose to the repressor protein
  changes its shape and causes it to fall off the
  operator. RNA polymerase can bind to the
  promoter. Structural genes are transcribed.

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Lactose operon
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Arginine operon repressible

• Normally on and will be turned off when nutrient
  is no longer needed.
• When excess arginine is present, it binds to the
  repressor and changes it. Then the repressor
  binds to the operator and blocks arginine
  synthesis.

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Repressible operon
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Antibiotics that affect gene expression

• Rifamycin binds to RNA polymerase
• Actinomycin D - binds to DNA halts mRNA chain
  elongation
• Erythromycin spectinomycin interfere with
  attachment of mRNA to ribosomes
• Chloramphenicol, linomycin tetracycline-bind to
  ribosome and block elongation
• Streptomycin inhibits peptide initiation
  elongation

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Mutations changes in the DNA

• Point mutation addition, deletion or
  substitution of a few bases
• Missense mutation causes change in a single
  amino acid
• Nonsense mutation changes a normal codon into a
  stop codon
• Silent mutation alters a base but does not
  change the amino acid

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Excision repair
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Ames Test
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Types of intermicrobial exchange
conjugation requires the attachment of two related species formation of a bridge that can transport DNA
transformation transfer of naked DNA
transduction DNA transfer mediated by bacterial virus
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conjugation
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transformation
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Generalized transduction
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Specialized transduction
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Transposons DNA segments that shift from one
part of the genome to another