Microbial Genetics

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Chapter 9

• Microbial Genetics

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Some definitions

• Genetics-the science of heredity
• Genome-genetic information inside a cell
• Genome includes chromosomes and plasmids
• Chromosomes-contain DNA
• Genes are segments of DNA that code for
  functional products
• Genotype-genetic make up
• Phenotype-expressed properties (eye color in
  humans)

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DNA-deoxyribonucleic acid

• Long strands of nucleotides (the base unit)
  twisted into a double helix
• Alternating sugar/phosphate groups called the
  sugar/phosphate backbone
• Nucleotide bases are adenine, guanine, cytosine,
  thymine
• Grouped in complimentary base pairs (A-T and C-G)
• Sugar is a pentose called deoxyribose

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DNA-structure discovered by Watson and Crick

• Bacterial DNA is circular (loop) shaped
• E. coli (highly studied) has 4.6 million base
  pairs and is 1 mm long
• This is longer than the cell, but it is
  super-coiled and takes up only 10 of the cells
  volume
• In DNA replication, one parent DNA is converted
  to two identical daughter DNAs

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DNA replication

• Replication begins with gyrase enzyme which
  relaxes the helix and helicase which unwinds the
  DNA a little at a time
• The DNA strands separate and exposed bases pair
  up with new ones from cytoplasm (complimentary
  base pair rule applies)
• New nucleotide bases hook up by DNA polymerase
  enzyme

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Replication continued

• Original strand and new daughter strand rewind
  (semi-conservative replication)
• DNA polymerase can add new nucleotides in at the
  3 end only
• One strand will be leading strand and goes in
  5--? 3 direction
• The other is lagging strand is made in pieces
  called Okazaki fragments

http//distancelearning.ksi.edu/demo/bio378/lectur
e.htm A good web site to go to for more
information and history
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DNA complimentary base pairs
http//www.pbs.org/saf/1202/teaching/teaching3.htm
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Replication
http//www.ba-education.demon.co.uk/for/science/dn
amain.html
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RNA and Protein Synthesis

• Transcription-genetic information from DNA is
  copied (transcribed) into a complimentary
  sequence of RNA
• Translation-cell uses information encoded in RNA
  to make protein
• 3 types RNA are messenger (mRNA), transfer
  (tRNA), and ribosomal (rRNA)

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RNA

• Messenger RNA is codon
• Transfer RNA is anticodon
• RNA is single stranded, has ribose sugar
• RNA has sugar/phosphate backbone and 4 bases
  (adenine, cytosine, guanine, and uracil)
• Adenine links with uracil this time
• Chargaff is the scientist who figured out base
  pairs for both RNA and DNA

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Transcription

• A piece of DNA (sense strand) is used as the
  template
• Begins with initiation at the promotor region,
  recognized by RNA polymerase
• Elongation is the copying of the DNA,
• Proceeds in a 5 ? 3 direction until termination
• Adenine pairs with uracil, cytosine pairs with
  guanine

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Translation

• mRNA codons are matched to tRNA anticodons (in
  groups of 3)
• Each codon codes for a specific tRNA which brings
  in a specific amino acid (page 267)
• Start with start codon (AUG) which also codes for
  methionine (INITIATION)
• There are 20 amino acids that can be used
• There are 64 codons, including 3 stop codons
  which do not code (UAA, UAG, UGA)

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Translation continued

• Must have a ribosome, tRNA, amino acids, and mRNA
  to make the protein
• Continue matching triplet codon/anticodons
  (elongation) until termination when stop codon is
  reached
• Bacteria have the ability to start another
  protein before one is finished (polyribosome)
• It takes less than 30 seconds to construct a
  protein with 400 amino acids

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http//www.contexo.info/DNA_Basics/Protein_synthes
is.htm
15
http//abbysenior.com/biology/dna_protein_synthesi
s.htm
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Mutation

• Any change in DNA (an adaptation is a good
  change)
• A mutation may be silent and have no effect.
• These are usually when a single base pair is
  substituted (especially if it is in the 3rd
  position (see the previous table)

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Types of mutations

• Base substitution is the most common
• Also called a point mutation
• This is where a single base in DNA is substituted
  for a different one
• When the mRNA is translated into a protein, it
  may cause the insertion of an incorrect amino
  acid
• If this happens, it is referred to as a missense
  mutation

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continued

• If the base substitution creates a stop codon in
  the middle of the mRNA, it will prevent synthesis
  of the complete protein
• This is called a nonsense mutation
• There are also frameshift mutations caused by
  deleting or inserting one or more nucleotides in
  the DNA
• Spontaneous mutations occur because of mistakes
  in replication

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continued

• Mutagens-environmental agents like uv light or
  chemicals or radiation may cause mutations
• UV light causes thymine dimers to form in the DNA
  (AATTTAGGCCG) Where 2 Thymines are together,
  they are read as one.

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Transformation

• Discovered by Griffiths experiment with S
  (smooth) and R (rough) Streptococcus
• Mice injected with S (virulent strain) died
• Mice injected with R (harmless) lived
• Mice injected with killed S lived
• Mice injected with killed S and live R died
• Bacteria were transformed into lethal strain
  because DNA entered live cells

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Conjugation

• Another method of genetic transfer
• A plasmid from a donor (F cell), is transferred
  into a recipient (F- cell) using Pili

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Transduction

• A third method of genetic transfer
• DNA transferred from donor to recipient inside a
  virus that infects bacteria (bacteriophage)
• Experiment from Hershey and Chase using phages
  marked with radioactive Phosphorus helped prove
  DNA was the unit of heredity, not protein