Ch8 microbial genetics

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CH8: Microbial Genetics

E. coli is found naturally in the human large intestine, and there it is beneficial. However, the strain designated E. coli O157:H7 produces Shiga toxin. How did E. coli acquire this gene from Shigella?


Terminology Genetics: The study of what genes are, how they

carry information, how information is expressed, and how genes are replicated

Gene: A segment of DNA that encodes a functional product, usually a protein

Chromosome: Structure containing DNA that physically carries hereditary information; the chromosomes contain the genes

Genome: All the genetic information in a cell

Structure and Function of Genetic Material


Terminology Genomics: The molecular study of genomes

(sequencing and molecular characterization of genomes).

Genotype: The genes of an organism Phenotype: Expression of the genes

Copyright © 2010 Pearson Education, Inc.Figure 8.2

The Flow of Genetic Information

Copyright © 2010 Pearson Education, Inc.Figure 8.3b

DNA

Polymer of nucleotides: Adenine, thymine, cytosine, and guanine

Double helix associated with proteins

"Backbone" is deoxyribose-phosphate

Strands are held together by hydrogen bonds between AT and CG

Strands are antiparallel

Copyright © 2010 Pearson Education, Inc.Figure 8.3a

Semiconservative Replication


DNA Synthesis


DNA Synthesis

DNA is copied by DNA polymerase In the 5' 3' direction Initiated by an RNA primer Leading strand is synthesized continuously Lagging strand is synthesized discontinuously Okazaki fragments RNA primers are removed and Okazaki fragments joined

by a DNA polymerase and DNA ligase


DNA Synthesis


Replication of Bacterial DNA

Copyright © 2010 Pearson Education, Inc.Table 8.1

Important Enzymes DNA GyraseDNA Helicase DNA polymerase

Copyright © 2010 Pearson Education, Inc.Table 8.1


RNA and Protein SynthesisTranscription DNA is transcribed to make RNA (mRNA, tRNA, and

rRNA) Transcription begins when RNA polymerase binds to

the promoter sequence Transcription proceeds in the 5' 3' direction Transcription stops when it reaches the

terminator sequence


Transcription


The Process of Transcription


ANIMATION Transcription: Overview

ANIMATION Transcription: Process

The Process of Transcription


RNA Processing in Eukaryotes


Translation

mRNA is translated in codons (three nucleotides)

Translation of mRNA begins at the start codon: AUG

Translation ends at nonsense codons: UAA, UAG, UGA


The Genetic Code

64 total codons of which 61 sense codons encode the 20 amino acids

3 are non-sense codons or stop codons.

The genetic code is degenerate

tRNA carries the complementary anticodon


The Genetic Code


Simultaneous Transcription & Translation


Figure 8.9-1


Figure 8.9-2


Regulation Constitutive genes are expressed at a fixed rate

(60-80%genes) Other genes are expressed only as needed

Repressible genes: Always active , can be turned off (repressed).

Inducible genes: Inactive , needs to be turned on or induced.

Catabolite repression: Inhibition of the metabolism of alternative carbon sources by Glucose.

Regulation of protein synthesis saves (conserve) energy for the organism.

The Regulation of Gene Expression


Operon

Control of TranscriptionConsists of a promotor, an operator and series of genes


• Inducible operons – must be activated by inducers

• Regulate catabolic pathway

• Lactose Operon

• Repressible operons – transcribed continually until deactivated by repressors

• Regulate anabolic pathway.

• Tryptophan Operon

Operons


Induction


Induction


Repression


Repression


Lactose present, no Glucose

Lactose + glucose present

Figure 8.15

Catabolite Repression


Figure 8.14b


Mutation

A change in the genetic material Mutation is a change in the base sequence of DNA. Mutations may be neutral, beneficial, or harmful Mutagen: Agent that causes mutations Spontaneous mutations: Occur in the absence of a

mutagen


Base substitution (point mutation)

Missense mutation

Mutation

Change in one base Result in change in

amino acid

Figure 8.17a, b


Nonsense mutation

Mutation

Results in a nonsense codon

Figure 8.17a, c


Mutation

Frameshift mutation Insertion or deletion of one or more nucleotide pairs

Figure 8.17a, d


Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes

Mutagens increase to one in 105(10-5) or one in 103(10–3) per replicated gene

Point mutations concept map

The Frequency of Mutation

Copyright © 2010 Pearson Education, Inc.Figure 8.19a

Chemical Mutagens

Nucleoside analogs


Chemical mutagens

Frameshift mutagens Cause small deletions or insertions Ex:Benzopyrene in smoke Aflatoxins Acridine orange

Frameshift mutagens are often carcinogens.


Radiation

Ionizing radiation (X rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose-phosphate backbone

Causes physical breakdown of DNA


Radiation

UV radiation causes thymine dimers


Repair Photolyases separate thymine dimers Nucleotide excision repair (can repair other

mutations also)


Selection

Positive (direct) selection detects mutant cells because they grow or appear different

Negative (indirect) selection detects mutant cells because they do not grow Replica plating


Replica Plating


Ames Test for Chemical Carcinogens

90% of the chemicals found to be mutagenic also have been shown to be carcinogenic.


• Exchange of DNA segments composed of homologous sequences

• Recombinants – cells with DNA molecules that contain new nucleotide sequences

• Vertical gene transfer – organisms replicate their genomes and provide copies to descendants

• Horizontal gene transfer – donor contributes part of genome to recipient; three types

• Transformation

• Transduction

• Bacterial Conjugation

Genetic Recombination and Transfer


Vertical gene transfer: Occurs during reproduction between generations of cells.

Horizontal gene transfer: The transfer of genes between cells of the same generation.

Genetic Recombination


Genetic Recombination

Exchange of genes between two DNA molecules Crossing over

occurs when two chromosomes break and rejoin


Genetic TransformationGriffith’s Experiment


• Griffiths experiment concluded transforming agent was DNA; one of conclusive pieces of proof that DNA is genetic material

• Cells that take up DNA are competent; results from alterations in cell wall and cytoplasmic membrane that allow DNA to enter cell

Transformation


Bacterial ConjugationTransfer of DNA by cell to cell contact


Transduction by a Bacteriophage• Generalized

transduction – transducing phage carries random DNA segment from donor to recipient

• Specialized transduction – only certain donor DNA sequences are transferred


Plasmids

Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid

Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds

Bacteriocins plasmids R factors: Encode antibiotic resistance


R Factor, a Type of Plasmid

Carry genes that confer resistance to antibiotics.

Copyright © 2010 Pearson Education, Inc.Figure 8.30a, b

Transposons

Segments of DNA that can move from one region of DNA to another

Contain insertion sequences for cutting and resealing DNA (transposase)

Complex transposons carry other genes


Genes and Evolution

Mutations and recombination provide diversity Fittest organisms for an environment are selected by

natural selection

Science

Ch8 microbial genetics