Download - Lecture 09 - Microbial Genetics
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7/31/2019 Lecture 09 - Microbial Genetics
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Chapter 9 - 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 referredto as the genome.)
Chromosome
Gene
Protein
Genotype
Phenotype
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Chromosome
Procaryotic
Histonelike proteins condense DNA
Eucaryotic
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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Representation of the flow of genetic information.
Fig. 9.9 Summary of the flow of genetic information in cell.
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DNA
Structure
Replication
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DNA is lengthy and occupies a small part of the cell by coiling up
into a smaller package.
Fig. 9.3 AnEscherichia coli cell disrupted to release its
DNA molecule.
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Structure
Nucleotide
Phosphate
Deoxyribose sugar
Nitrogenous bases ( adenine, guanine;
thymine, cytosine)
Double stranded helix
Antiparallel arrangement
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Purines and pyrimidines pair (A-T or G-C) and the sugars(backbone) are linked by a phosphate.
Fig. 9.4 Three views of DNA structure
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Replication
Semiconservative
Enzymes
Leading strand
Lagging strand
Okazaki fragments
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Semiconservative
New strands are synthesized in 5 to 3
direction
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Semiconservative replication of DNA synthesizes a new strand ofDNA from a template strand.
Fig. 9.5 Simplified steps to show the semiconservativereplication of DNA
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Enzymes
Helicase
DNA polymerase III
Primase
DNA polymerase I
Ligase
Gyrase
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The function of important enzymes involved in DNA replication.
Table 9.1 Some enzymes involved in DNA replication 16
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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The steps associated with the DNA replication process.
Fig. 9.6 The
bacterial
replicon: a model
for DNASynthesis
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Replication processes from other biological systems (plasmids,
viruses) involve a rolling cycle.
Fig. 9.8 Simplified model of rolling circle DNA
Replication20
RNA
Transcription
Message RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
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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The synthesis of mRNA from DNA.
Fig. 9.12 The major events in mRNA synthesis 24
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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Important structural characteristics for tRNA and mRNA.
Fig. 9.11 Characteristics of transfer and message RNA 26
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, enabling tRNAs to bind, followed
by protein synthesis.
Fig. 9.9 Summary of
the flow of genetics 28
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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The codons from mRNA specify a given amino acid.
Fig. 9.14 The Genetic code 30
Representation of the codons and their corresponding aminoacids.
Fig. 9.15 Interpreting the DNA code
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Protein
Translation
Protein synthesis have the following
participants
mRNA
tRNA with attached amino acid
Ribosome
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Participants involved in the translation process.
Fig. 9.13 The players in translation
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Translation
Ribosomes bind mRNA near the start codon(ex. AUG)
tRNA anticodon with attached amino acidbinds to the start codon
Ribosomes move to the next codon, allowinga new tRNA to bind and add another aminoacid
Series of amino acids form peptide bonds
Stop codon terminates translation
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The process of translation.
Fig. 9.16 The events in proteinsynthesis
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For procaryotes, translation can occur at multiple sites on the
mRNA while the message is still being transcribed.
Fig. 9.17 Speeding up the protein assembly line in bacteria36
Transcription and translation
in eucaryotes
Similar to procaryotes except
AUG encodes for a different form ofmethionine
mRNA code for one protein
Transcription and translation are notsimultaneous
Pre-mRNA
Introns Exons
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The processing of pre-mRNA into mRNA involves the removal ofintrons.
Fig. 9.18 The split gene of eucaryotes38
Regulation
Lactose operon
sugar
Repressible operon
Amino acids, nucleotides
Antimicrobials
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The regulation of sugar metabolism such as lactose involves
repression in the absence of lactose, and induction when lactose
is present.
Fig. 9.19 The lactose operon in bacteria40
The regulation of amino acids such as arginine involves
repression when arginine accumulates, and no repression when
arginine is being used.
Fig. 9.20 Repressible operon
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Antimicrobials
Ex. 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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Examples of chemical and radioactive mutagens, and their
effects.
Table 9.3 Selected mutagenic agents and their effects44
Repair of mutations involves enzymes recognizing, removing, and
replacing the bases.
Fig. 9.22 Excision repair of mutation by enzymes
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The Ames test is used to screen environmental and dietarychemicals for mutagenicity and carcinogenicity without using
animal studies.
Fig. 9.23 The Ames test.46
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+ (Ffactor) cell to a F- cell
An F+ bacterium possesses a pilus
Pilus attaches to the recipient cell andcreates pore for the transfer DNA
High frequency recombination (Hfr)donors contain the F factor in thechromosome
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Conjugation is the genetic transmission through direct contactbetween cells.
Fig. 9.24 Conjugation: genetic transmission through direct contact50
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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DNA released from a killed cell can be accepted by a live
competent cell, expressing a new phenotype.
Fig. 9.25 Griffiths classic experiment in transformation52
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 based on generalized transduction.
Fig. 9.26 Generalized transduction54
Genetic transfer based on specialized transduction.
Fig. 9.27 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.
Fig. 9.28 Transposons: shifting segments of the genome