Download Topic 12 (Ch9/7) – Microbial Genetics Genetics Chromosome

Topic 12 (Ch9/7) –
Microbial Genetics
Topics
- Genetics
- Flow of Genetics
- Regulation
- Mutation
- Recombination
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Genetics
• Genome (The sum total of genetic material of a cell is referred
to as the genome.)
– Chromosome
– Gene
– Protein
• Genotype
• Phenotype
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Chromosome
• Prokaryotic
– Histone like proteins condense DNA
• Eukaryotic
– Histone proteins condense DNA
• Subdivided into basic informational
packets called genes
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Genes
• Three categories
– Structural
– Regulatory
– Encode for RNA
• Genotype
– sum of all gene types
• Phenotype
– Expression of the genotypes
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Flow of Genetics
• DNA =>RNA=>Protein
– Replication
– Transcription
– Translation
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Genetic Information Flow
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DNA
• Structure
• Replication
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Nucleic Acids
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Prokaryotic Genome
– Plasmids
• Small molecules of DNA that replicate
independently
• Not essential for normal metabolism, growth, or
reproduction
• Can confer survival advantages
• Many types of plasmids
–
–
–
–
Fertility factors
Resistance factors
Bacteriocin factors
Virulence plasmids
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3
DNA is long and occupies a small part of the cell by coiling up into
a smaller package.
An Escherichia coli cell disrupted to release its DNA molecule.
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Another Look
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Eukaryotic Genome
– Nuclear chromosomes
• Typically have more than one chromosome per
cell
• Chromosomes are linear and sequestered
within nucleus
• Eukaryotic cells are often diploid (two
chromosome copies)
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4
Eukaryotic Nuclear Packaging
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Structure
• Nucleotide
– Phosphate
– Deoxyribose sugar
– Nitrogenous bases (purines- adenine, guanine;
pyramidines – thymine, cytosine)
• Double stranded helix
– Antiparallel arrangement
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Purines and pyrimidines pair (A-T or G-C),
backbone sugars linked by phosphate.
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Replication
•
•
•
•
Semiconservative
Enzymes
Leading strand
Lagging strand
– Okazaki fragments
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Replication is Semiconservative
• Means each new molecule is made
from one new and one original
• New strands are always synthesized in
5’ to 3’ direction
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Semiconservative replication of DNA synthesizes a new strand of
DNA from a template strand.
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Enzymes
•
•
•
•
•
•
Helicase
DNA polymerase III
Primase
DNA polymerase I
Ligase
Gyrase
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Function of some enzymes involved in DNA replication
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Leading strand
• RNA primer initiates the 5’ to 3’
synthesis of DNA in continuous manner
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Lagging strand
• Multiple Okazaki fragments are
synthesized
• Okazaki fragments are ligated together
to form one continuous strand
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DNA replication process steps.
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Prokaryotic Bi-direction!
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What is the Process?
– Transcription
• Information in DNA is copied as RNA
– Translation
• Polypeptides synthesized from RNA
– Central dogma of genetics
• DNA transcribed to RNA
• RNA translated to form polypeptides
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The Dogma!
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RNA
• Transcription
– Message RNA (mRNA)
– Transfer RNA (tRNA)
– Ribosomal RNA (rRNA)
– Others… (see website info)
• Codon
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Transcription
• A single strand of
RNA is transcribed
from a template
strand of DNA
• RNA polymerase
catalyzes the
reaction
• Synthesis in 5’ to 3’
direction
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mRNA
• Copy of a structural gene or genes of
DNA
– Can encode for multiple proteins on one
message
• Thymidine is replaced by uracil
• The message contains a codon (three
bases)
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mRNA synthesis from DNA
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tRNA
• Copy of specific regions of DNA
• Complimentary sequences form hairpin
loops
– Amino acid attachment site
– Anticodon
• Participates in translation (protein
synthesis)
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Structural Characteristics for tRNA and mRNA
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rRNA
• Consist of two
subunits (70S)
• A subunit is
composed of rRNA
and protein
• Participates in
translation
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Ribosomes bind to the
mRNA. This enables
tRNAs to bind, followed
by protein synthesis.
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Codons
• Triplet code that specifies a given amino
acid
• Multiple codes for one amino acid
• 20 amino acids
• Start codon
• Stop codons
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mRNA Codons Specify Amino Acids
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How DNA codes lead to amino acids
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Protein
• Translation
– Protein synthesis have the following
participants
• mRNA
• tRNA with attached amino acid
• Ribosome
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Participants in the translation process
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Ribosome Structures
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Assembled Ribosome
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3 Stages of Translation
• Initiation
Ribosomes bind mRNA near the start codon
(ex. AUG)
tRNA anticodon with attached amino acid
binds to the start codon
• Elongation
Ribosomes move to the next codon, allowing
a new tRNA to bind and add another amino
acid
Series of amino acids form peptide bonds
• Termination
Stop codon terminates translation
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The process of translation
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For prokaryotes, translation can occur at multiple sites on the
mRNA while the message is still being transcribed = speedy!
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Transcription and translation
in eukaryotes
• Similar to prokaryotes except
– AUG encodes for a different form of
methionine
– mRNA code for one protein
– Transcription and translation are not
simultaneous
– Pre-mRNA
• Introns
• Exons
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Processing pre-mRNA into mRNA requires removal of introns
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Regulation
• Lactose operon
– sugar
• Repressible operon
– Amino acids,
nucleotides
• Antimicrobials
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About Gene Regulation...
– 75% of genes are expressed at all times
– Other genes transcribed and translated
when cells need them
• (energy conservation)
– Regulation of protein synthesis
• Typically halts transcription
• Can stop translation directly
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Control of Translation
• Genetic expression can be regulated at
level of translation
– Riboswitch
• mRNA molecule that blocks translation of the
polypeptide it encodes
– Short interference RNA (siRNA)
• RNA molecule complementary to a portion of
mRNA, tRNA, or a gene that binds and renders
the target inactive
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What an Operon Looks Like
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Operon Overview
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Lac operon example for regulation of sugar metabolism
 Repression in the
absence of lactose
Induction in the
presence of lactose 
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Example for regulation of amino acids, like arginine
 No repression when
product is being used
Repression when
product accumulates 
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Antimicrobials
• Antibiotics and drugs can inhibit the
enzymes involved in transcription and
translation
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Mutations
• Changes made to the DNA
–
–
–
–
Spontaneous – random change
Induced – chemical, radiation.
Point – change a single base
Nonsense – change a normal codon into a stop
codon
– Back-mutation – mutation is reversed
– Frameshift – reading frame of the mRNA changes
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Point Mutation Effects
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Mutation repairs -- enzymes that recognize, remove, replace
bases
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Excision repair
The Ames test
screens
environmental
and dietary
chemicals for
mutagenicity and
carcinogenicity.
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Effects of mutations
• Positive effects for the cell
– Allow cells to adapt
• Negative effects for the cell
– Loss of function
– Cells cannot survive
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Recombination
• Sharing or recombining parts of their
genome
– Conjugation
– Transformation
– Transduction
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Conjugation
• Transfer of plasmid DNA from a F+ (F
factor) cell to a F- cell
• An F+ bacterium possesses a pilus
• Pilus attaches to the recipient cell and
creates pore for the transfer DNA
• High frequency recombination (Hfr)
donors contain the F factor in the
chromosome
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Conjugation
(genetic transmission through direct contact between cells)
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Transformation
• Nonspecific acceptance of free DNA by
the cell (ex. DNA fragments, plasmids)
• DNA can be inserted into the
chromosome
• Competent cells readily accept DNA
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Griffith’s classic experiment in transformation
DNA released from
a killed cell can be
accepted by a live
competent cell, that
expresses a new
phenotype.
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Transduction
• Bacteriophage infect host cells
• Serve as the carrier of DNA from a donor
cell to a recipient cell
– Generalized
– Specialized
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Genetic transfer
(generalized transduction)
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Genetic transfer
(specialized transduction)
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Transposon
• “Jumping genes”
• Exist in plasmids and chromosomes
• Contains genes that encode for
enzymes that remove and reintegrate
the transposon
• Small transposons are called insertion
elements
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Movement of transposons can occur in plasmids and
chromosomes
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