chapter 9 power point le

8/9/2019 Chapter 9 Power Point Le

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Foundations in Microbiology

Chapter

9

PowerPoint to accompany

Fifth Edition

Talaro

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


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Microbial Genetics

Chapter 9


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Genetics the study of heredity

1. transmission of biological traits from

parent to offspring

2. expression & variation of those traits

3. structure & function of genetic material

4. how this material changes


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Levels of genetic study


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Levels of structure & function of the

genome genome sum total of genetic material of an organism

(chromosomes + mitochondria/chloroplasts and/orplasmids)

genome of cells DNA

genome of viruses DNA or RNA chromosome length of DNA containing genes

gene-fundamental unit of heredity responsible for

a given traitsite on the chromosome that provides information fora certain cell function

segment of DNA that contains the necessary code tomake a protein or RNA molecule


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Genomes vary in size

smallest virus 4-5 genes

E. coli single chromosome containing

4,288 genes; 1 mm; 1,000X longer than cell

Human cell 46 chromosomes containing

31,000 genes; 6 feet; 180,000X longer than

cell


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Nucleic acids are made of nucleotidessimilar to how proteins are made of amino

acids

each nucleotide consists of 3 partsa 5 carbon sugar (deoxyribose or ribose)

a phosphate group

a nitrogenous base (adenine, thymine, cytosine,guanine, and uracil)


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DNA structure 2 strands twisted into a helix

sugar -phosphate backbone

nitrogenous bases form steps in ladder

constancy of base pairing

A binds to T with 2 hydrogen bonds

G binds to C with 3 hydrogen bonds

antiparallel strands 3to 5 and 5to 3

each strand provides a template for the exact

copying of a new strand

order of bases constitutes the DNA code


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Significance of DNA structure

1. Maintenance of code during reproduction.

Constancy of base pairing guarantees that

the code will be retained.

2. Providing variety. Order of bases

responsible for unique qualities of each

organism.


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DNA replication issemiconservative because each

chromosome ends up with one

new strand of DNA and one old

strand.


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Semi-conservative replication of DNA


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DNA replication

Begins at an origin of replication

Helicase unwinds and unzips the DNA doublehelix

An RNA primer is synthesized

DNA polymerase III adds nucleotides in a 5 to 3direction

Leading strand synthesized continuously in 5to 3 direction

Lagging strand synthesized 5 to 3 in shortsegments; overall direction is 3 to 5


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Bacterial replicon


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Flow of genetic information


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What are the products that genes encode?

RNAs and proteins

How are genes expressed?

transcription and translation


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Gene expression

Transcription DNA is used to synthesize

RNA

RNA polymerase is the enzyme responsible

Translation making a protein using the

information provided by messenger RNA

occurs on ribosomes


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Genotype - genes encoding all the potential

characteristics of an individual

Phenotype -actual expressed genes of an

individual (its collection of proteins)


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DNA-protein relationship

1. Each triplet of nucleotides (codon) specifiesa particular amino acid.

2. A proteins primary structure determines itsshape & function.

3. Proteins determine phenotype. Living thingsare what their proteins make them.

4. DNA is mainly a blueprint that tells the cellwhich kinds of proteins to make and how tomake them.


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DNA-protein relationship


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3 types of RNA

messenger RNA (mRNA)

transfer RNA (tRNA)

ribosomal RNA (rRNA)


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DNA

RNA

PROTEINS

TranscriptionRNA polymerase

Translation

ribosomes


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Transcription

1. RNA polymerase binds to promoter regionupstream of the gene

2. RNA polymerase adds nucleotidescomplementary to the template strand of asegment of DNA in the 5 to 3 direction

3. Uracil is placed as adenines complement

4. At termination, RNA polymerase recognizessignals and releases the transcript

100-1,200 bases long


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Transcription


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Translation

Ribosomes assemble on the 5 end of a mRNAtranscript

Ribosome scans the mRNA until it reaches thestart codon, usually AUG

A tRNA molecule with the complementaryanticodon and methionine amino acid enters the

P site of the ribosome & binds to the mRNA


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Translation


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Interpreting the DNA code


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Translation elongation

A second tRNA with the complementary anticodonfills the A site

A peptide bond is formed

The first tRNA is released and the ribosome slidesdown to the next codon.

Another tRNA fills the A site & a peptide bond isformed.

This process continues until a stop codon isencountered.


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Translation termination

Termination codons UAA, UAG, and

UGA are codons for which there is no

corresponding tRNA. When this codon is reached, the ribosome

falls off and the last tRNA is removed from

the polypeptide.


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Polyribosomal complex


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Eucaryotic transcription &

translation differs from procaryotic1. Do not occur simultaneously. Transcription

occurs in the nucleus and translation occurs in thecytoplasm.

2. Eucaryotic start codon is AUG, but it does not useformyl-methionine.

3. Eucaryotic mRNA encodes a single protein,unlike bacterial mRNA which encodes many.

4. Eucaryotic DNA contains introns interveningsequences of noncoding DNA- which have to bespliced out of the final mRNA transcript.


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Split gene of eucaryotes


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Multiplication of dsDNA viruses


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Multiplication of+ssRNA


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Regulation of protein synthesis &

metabolism


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Operons

a coordinated set of genes, all of which areregulated as a single unit.

2 typesinducible operon is turned ON by substrate:

catabolic operons- enzymes needed to metabolizea nutrient are produced when needed

repressible genes in a series are turned OFF bythe product synthesized; anabolic operon enzymes used to synthesize an amino acid stop

being produced when they are not needed


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Lactose operon: inducible operon

Made of 3 segments:

1. Regulator- gene that codes for repressor

2. Control locus- composed of promoter andoperator

3. Structural locus- made of 3 genes each codingfor an enzyme needed to catabolize lactose

-galactosidase hydolyzes lactosepermease - brings lactose across cell membrane

-galactosidase transacetylase uncertain function


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Lac operon

Normally offIn the absence of lactose the repressor binds

with the operator locus and blocks transcriptionof downstream structural genes

Lactose turns the operon onBinding of lactose to the repressor protein

changes its shape and causes it to fall off theoperator. RNA polymerase can bind to the

promoter. Structural genes are transcribed.


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Lactose operon


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Arginine operon: repressible

Normally on and will be turned off when

nutrient is no longer needed.

When excess arginine is present, it binds tothe repressor and changes it. Then the

repressor binds to the operator and blocks

arginine synthesis.


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Repressible operon


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Antibiotics that affect gene

expression Rifamycin binds to RNA polymerase

Actinomycin D - binds to DNA & halts mRNAchain elongation

Erythromycin & spectinomycin interfere withattachment of mRNA to ribosomes

Chloramphenicol, linomycin & tetracycline-bind

to ribosome and block elongation Streptomycin inhibits peptide initiation &

elongation


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Mutations changes in the DNA

Point mutation addition, deletion orsubstitution of a few bases

Missense mutation causes change in asingle amino acid

Nonsense mutation changes a normalcodon into a stop codon

Silent mutation alters a base but does notchange the amino acid


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Excision repair


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Ames Test


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Types of intermicrobial exchange

conjugation requires the attachment of tworelated species & formation of a

bridge that can transport DNA

transformation transfer of naked DNA

transduction DNA transfer mediated by bacterialvirus


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conjugation


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transformation


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Generalized transduction


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Specialized transduction


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Transposons DNA segments that shift

from one part of the genome to another

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