microbial genetics

Foundations in Microbiology

Chapter9

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 traits3. structure & function of genetic material4. 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/or plasmids)– 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 trait– site on the chromosome that provides information for

a certain cell function– segment of DNA that contains the necessary code to

make 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 nucleotides similar to how proteins are made of amino acids

• each nucleotide consists of 3 parts– a 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 3’to 5’ and 5’to 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.

DNA replication is semiconservative 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 double

helix• An RNA primer is synthesized• DNA polymerase III adds nucleotides in a 5’ to 3’

direction• Leading strand – synthesized continuously in 5’

to 3’ direction• Lagging strand – synthesized 5’ to 3’ in short

segments; 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) specifies a particular amino acid.

2. A protein’s primary structure determines its shape & function.

3. Proteins determine phenotype. Living things are what their proteins make them.

4. DNA is mainly a blueprint that tells the cell which kinds of proteins to make and how to make 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

Translationribosomes

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Transcription

1. RNA polymerase binds to promoter region upstream of the gene

2. RNA polymerase adds nucleotides complementary to the template strand of a segment of DNA in the 5’ to 3’ direction

3. Uracil is placed as adenine’s complement4. At termination, RNA polymerase recognizes

signals 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 mRNA transcript

• Ribosome scans the mRNA until it reaches the start codon, usually AUG

• A tRNA molecule with the complementary anticodon 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 anticodon fills the A site

• A peptide bond is formed• The first tRNA is released and the ribosome slides

down to the next codon.• Another tRNA fills the A site & a peptide bond is

formed.• This process continues until a stop codon is

encountered.

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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 the cytoplasm.

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

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

4. Eucaryotic DNA contains introns – intervening sequences of noncoding DNA- which have to be spliced 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

Regulation of protein synthesis & metabolism

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Operons

• a coordinated set of genes, all of which are regulated as a single unit.

• 2 types– inducible – operon is turned ON by substrate:

catabolic operons- enzymes needed to metabolize a nutrient are produced when needed

– repressible – genes in a series are turned OFF by the 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:• Regulator- gene that codes for repressor• Control locus- composed of promoter and

operator• Structural locus- made of 3 genes each coding

for an enzyme needed to catabolize lactose – β-galactosidase – hydolyzes lactose permease - brings lactose across cell membraneβ-galactosidase transacetylase – uncertain function

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Lac operon

• Normally off– In the absence of lactose the repressor binds

with the operator locus and blocks transcription of downstream structural genes

• Lactose turns the operon on– Binding of lactose to the repressor protein

changes its shape and causes it to fall off the operator. 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 to the 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 mRNA

chain elongation• Erythromycin & spectinomycin – interfere with

attachment 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 or substitution of a few bases

• Missense mutation – causes change in a single amino acid

• Nonsense mutation – changes a normal codon into a stop codon

• Silent mutation – alters a base but does not change the amino acid

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Excision repair

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Ames Test

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Types of intermicrobial exchange

DNA transfer mediated by bacterial virus

transduction

transfer of naked DNAtransformation

requires the attachment of two related species & formation of a bridge that can transport DNA

conjugation

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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