chapter 7 microbial genetics - brazosport...

Chapter 7 Microbial Genetics

MDufilho 10/1/2017 1

The Structure and Replication of Genomes

• Genetics

• Study of inheritance and inheritable traits as expressed

in an organism's genetic material

• Genome

• The entire genetic complement of an organism

• Includes its genes and nucleotide sequences

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• The Structure of Prokaryotic Genomes

• Prokaryotic Chromosomes

• Main portion of DNA, along with associated proteins and

RNA

• Prokaryotic cells are haploid (single chromosome copy)

• Typical chromosome is a circular molecule of DNA in the

nucleoid



• The Structure of Prokaryotic Genomes

• Plasmids

• Small molecules of DNA that replicate independently

• Not essential for normal metabolism, growth, or

reproduction

• Can confer survival advantages

• Many types of plasmids:

• Fertility factors

• Resistance factors

• Bacteriocin factors

• Virulence plasmids




• DNA Replication

• Other Characteristics of Bacterial DNA Replication

• Bidirectional

• Gyrases and topoisomerases remove supercoils in DNA

• DNA is methylated

• Control of genetic expression

• Initiation of DNA replication

• Protection against viral infection

• Repair of DNA


Figure 7.7 The bidirectionality of DNA replication in prokaryotes.

Origin Parental strand

Daughter strand

Replication forks

Replication proceeds in both directions Termination

of replication


• The Relationship Between Genotype and

Phenotype

• Genotype

• Set of genes in the genome

• Phenotype

• Physical features and functional traits of the organism

• Genotype determines phenotype

• Not all genes are active at all times

Gene Function


• The Transfer of Genetic Information

• Transcription

• Information in DNA is copied as RNA

• Translation

• Polypeptides synthesized from RNA

• Central dogma of genetics

• DNA transcribed to RNA

• RNA translated to form polypeptides

Gene Function


Figure 7.8 The central dogma of genetics.

3′

5′

3′

3′

5′

5′

Transcription

Translation

by ribosomes

NH2 Methionine Arginine Tyrosine Leucine Polypeptide

mRNA

DNA

(genotype)

Phenotype


Figure 7.9 The events in the transcription of RNA in prokaryotes.

RNA polymerase attaches

nonspecifically to DNA and

travels down its length until

it recognizes a promoter

sequence. Sigma factor

enhances promoter

recognition in bacteria.

Upon recognition of the

promoter, RNA polymerase

unzips the DNA molecule

beginning at the promoter.

Unzipping of DNA, movement of RNA polymerase

"Bubble"

Attachment of RNA polymerase

Sigma factor

RNA polymerase

Promoter Terminator

DNA 3′

5′

3′

5′

Template

DNA strand

5′

3′

5′

3′

Initiation of transcription

"Bubble"

5′

3′

Triphosphate ribonucleotides

align with their DNA

complements, and RNA

polymerase links them

together, synthesizing RNA.

No primer is needed. The

triphosphate ribonucleotides

also provide the energy

required for RNA synthesis.

Growing RNA molecule

(transcript) 5′

3′

5′

3′

3′

5′

Template

DNA

strand

Elongation of the RNA transcript

5′

3′ 5′

3′

Terminator 3′ 5′

RNA transcript

released

Self-termination: transcription of GC-rich terminator

region produces a hairpin loop, which creates tension,

loosening the grip of the polymerase

on the DNA.

Rho-dependant termination: Rho pushes between polymerase

and DNA. This causes release of polymerase, RNA transcript,

and Rho.

RNA polymerase

Rho termination

protein

Rho protein moves

along RNA

Terminator 3′

Template

strand

Terminator

Termination of transcription: release of RNA polymerase

GC-rich

hairpin

loop

3′

5′

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Figure 7.11 Processing eukaryotic mRNA.

Exons (polypeptide coding regions)

Template

DNA strand

Introns (noncoding regions)

Transcription

Exon 1 Exon 2 Exon 3 Exon 4 Pre-mRNA

Poly-A tail Intron 3 Intron 2 Intron 1 cap 5′

Processing

Spliceosomes

mRNA

splicing

3′

Exon 4

Exon 3 Exon 1

Exon 2

5′

5′

Intron 1

mRNA (codes for

one polypeptide) 3′

Nuclear envelope

Nuclear pore Nucleoplasm

Cytosol

mRNA

5′ 3′

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• Translation

• Events in Translation

• Three stages of translation:

• Initiation

• Elongation

• Termination

• All stages require additional protein factors

• Initiation and elongation require energy (GTP)

Gene Function

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Translation: The Process

Translation: The Process PLAY

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translation_process_subtitle.mp4

Gene Function

• Regulation of Genetic Expression

• Most genes are expressed at all times

• Other genes transcribed and translated when cells need

them

• Allows cell to conserve energy

• Regulation of polypeptide synthesis:

• Typically halts transcription

• Can stop translation directly

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Mutations of Genes

• Mutation

• Change in the nucleotide base sequence of a genome

• Rare event

• Almost always deleterious

• Rarely leads to a protein that improves ability of

organism to survive

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Figure 7.24 The effects of the various types of point mutations.

Normal DNA

Normal mRNA

Normal

polypeptide

mRNA

Template DNA strand

Mutated template

DNA strand

Mutated mRNA

Mutated template

DNA strand

Mutated mRNA

Mutated template

DNA strand

Mutated mRNA

STOP

CODON

Insertion

Mutated template

DNA strand

Mutated mRNA

Mutated template

DNA strand

Mutated mRNA

Phe Leu His Val

Phe

Phe

Phe Phe Ile Cys Thr Tyr Ala Arg

Arg Tyr Gly

Phe Tyr Ala Arg

Normal

Silent mutation No change in amino acid

sequence of polypeptide

Missense mutation Slightly different amino

acid sequence

Frameshift mutations:

Frameshift insertion Major difference in

amino acid sequence

Major difference in

amino acid sequence

Frameshift deletion

Nonsense mutation Polypeptide synthesis

ceases

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Mutations of Genes

• Mutagens

• Radiation

• Ionizing radiation

• Nonionizing radiation

• Chemical mutagens

• Nucleotide analogs

• Disrupt DNA and RNA replication

• Nucleotide-altering chemicals

• Result in base-pair substitutions and missense

mutations

• Frameshift mutagens

• Result in nonsense mutations

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Mutations of Genes

• Frequency of Mutation

• Mutations are rare events

• Otherwise organisms could not effectively reproduce

• Mutagens increase the mutation rate by a factor of 10 to

1000 times

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• Vertical gene transfer

• Organisms replicate their genomes and provide

copies to descendants

• Horizontal Gene Transfer Among Prokaryotes

• Horizontal gene transfer

• Donor cell contributes part of genome to recipient cell

• Three types:

• Transformation

• Transduction

• Bacterial conjugation

Genetic Recombination and Transfer

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Figure 7.33 Transformation of Streptococcus pneumoniae.

Observations of Streptococcus pneumoniae

Live cells Injection

Capsule

Heat-treated dead cells of strain S Injection

Strain R live cells (no capsule)

Injection

Mouse dies

Mouse lives

Mouse lives

Griffith's experiment:

Living

strain R

Heat-treated dead cells of strain S

Injection

Mouse dies

Culture of Streptococcus from dead mouse

Living cells with capsule (strain S)

+

In vitro transformation

Heat-treated dead cells of strain S

DNA broken into pieces

DNA fragment from strain S

Living strain R

Some cells take up DNA from the environment and incorporate it into their chromosomes

Transformed cells acquire ability to synthesize capsules

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Figure 7.34 Transduction.

Cell 1

Bacteriophage

Host bacterial cell

(donor cell)

Bacterial chromosome

Phage injects its DNA.

Phage enzymes

degrade host DNA.

Cell synthesizes new

phages that incorporate

phage DNA and, mistakenly,

some host DNA.

Phage

DNA Phage with donor cell's

DNA (transducing phage)

Transducing phage

Recipient host cell

Cell 2

Transduced cell

Transducing phage

injects donor DNA.

Donor DNA is incorporated

into recipient's chromosome

by recombination.

Inserted DNA

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Figure 7.35 Bacterial conjugation.

Pilus

Cell 2 Cell 1

F plasmid Origin of

transfer Chromosome

Donor cell attaches to a recipient cell with

its pilus.

Pilus draws cells together.

One strand of F plasmid DNA transfers

to the recipient.

The recipient synthesizes a complementary

strand to become an F+ cell with a pilus; the

donor synthesizes a complementary strand,

restoring its complete plasmid.

Now F+ cell F+ cell

Origin of

transfer

F+ cell F– cell

Pilus

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Deadly Horizontal Gene Transfer

• Patient Details?

• What infection? MDR?

• Define healthcare-associated infection

• Source of the infection?

• Three ways by which Enterococcus faecium might

have acquired genes for drug resistance.

• How can hospital personnel prevent the spread of

resistant E. faecium throughout the hospital?

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chapter 7 microbial genetics - brazosport...

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