Lecture 09 - Microbial Genetics

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

Lecture 09 - Microbial Genetics

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