© 2004 Wadsworth – Thomson Learning Chapter 6 The Genetics of Microorganisms

© 2004 Wadsworth – Thomson Learning

Chapter 6Chapter 6The Genetics of The Genetics of MicroorganismsMicroorganisms


Structure of DNA• Two strands• Nucleotides

– Hydrogen bonds between strands

– Neighboring deoxyribose connected

• 3’ of one deoxyribose to 5’ of next deoxyribose

• Phosphate in between

• Double helix• Base pairing

– G and C– A and T

Figure 6.1


Roles of DNA

• Replication– cell division– need accurate copy

• Gene expression– DNA– RNA– Protein

Figure 6.2


DNA Replication

• Semi-conservative– old strand-template– new strand-

complementary

• Replication fork– multiple enzymes– DNA unwinds– exposes nucleotides– synthesize new strand– one direction: 5’ to 3’

Figure 6.3


DNA Replication

• Complementary nucleotides match (A=T; G=C)• DNA polymerase III binds nucleotides releasing

pyrophosphate

Figure 6.3


Bacterial chromosomes• Replication of circular

chromosome• Origin of replication

– bubble forms– DNA unwinds

• Replication occurs in both directions

• Two replication forks• Continues until

replication forks meet• Strands separate

Figure 6.4


Replication fork

Figure 6.5


DNA Replication

• Leading strand– replication is continuous (5’ to 3’)

• primase makes primer• DNA added to primer• fork opens and replication continues

• Lagging strand– polymerization in only one direction

• can’t go 3’ to 5’

– short segments synthesized (Okazaki fragments)• when fork opens, new primer is made• synthesis in direction away from fork• fragments are joined together by DNA ligase


Transcription

• RNA polymerase binds DNA at site of promoter

Figure 6.6


Transcription

• DNA unwinds• nucleotide bases are

exposed

Figure 6.6


Transcription

• ribonucleotides pair with exposed bases– uracil in RNA

replaces thymine– U binds A

• ribonucleotides are polymerized into growing RNA chain

Figure 6.6


Transcription

• Termination sequence

• Release of transcript• single strand RNA• DNA closes

Figure 6.6


Transcription

• Role of RNA from transcription– mRNA

• template which encodes the protein

– tRNA• transfer amino acids used to build the protein

– rRNA• part of ribosome which is the site of protein

synthesis

• All used for translation


Translation• Production of proteins• Based on genetic

information of DNA• Genetic code

– Codon has three nucleotide

– Four different nucleotides

– 64 possible combinations

– 20 amino acids• Redundancy• Nonsense codons

Figure 6.8


Translation

• tRNA– binds an amino acid

• specific amino acid for each tRNA

– Anticodon• recognizes codon• three nucleotide

sequence in mRNA which encodes a specific amino acid

– activated with ATP

Figure 6.7


Translation

• Ribosome binds to mRNA– specific region– start codon

• Methionine

– Ribosome binding region

• Shine-Dalgarno sequence

Figure 6.9


Translation• tRNA with appropriate

anticodon and specific amino acid binds to the codon on the mRNA– A site

• second tRNA binds in similar fashion– P site

• two amino acids are joined in a peptide bond

Figure 6.9


Translation

• Ribosome moves along mRNA

• first tRNA without amino acid is removed

• second tRNA with both amino acids moves to P site

Figure 6.9


Translation

• New tRNA enters A site• Growing amino acid chain is transferred to

new amino acid

Figure 6.9


Translation

• steps repeat– ribosome moves– one codon at a time

• protein chain– one amino acid

added for every codon

Figure 6.9


Translation

• Continues until nonsense (stop) codon is reached

• no tRNA matches• ribosome is removed• protein chain is

released

Figure 6.9


Transcription and Translation

• Simultaneous transcription and translation

• mRNA chain is transcribed

• translation begins• multiple ribosomes on

single mRNA– polysome

Figure 6.10


Regulation of genes

• Transcription– Production of

regulatory proteins• Bind DNA near the

promoter• Example: Lactose

operon

– Interruption of transcription

• Attenuation

• Translation– Ribosomal proteins

• Global regulation– Catabolite

repression– Nitrogen regulation– Phosphorus

regulation– Stringent response– Heat shock proteins


Transcriptional regulation

lac operon• lacZ• lacY• lacA

– regulated by lacI

• Lactose absent– repressor binds– stops transcription

Figure 6.11


Transcriptional regulation

• Lactose present– repressor bound

by product of lactose

• allolactose

– transcription occurs

– gene products of all genes are made

Figure 6.11


Attenuation

• Histidine operon– Histidine present– Leader protein made

• Translation occurring simultaneously with transcription

• Requires histidine

– Attenuator loop forms on mRNA

• Displaces RNA polymerase• Stops transcription

Figure 6.12a


Attenuation

• Histidine absent– Leader protein not

made• Not enough histidines to

complete protein

– Antiterminator loop forms

• Prevents attenuator loop from forming

• RNA polymerase continues

Figure 6.12b


Regulation of translation

• Expression of ribosomal proteins– Unused proteins bind to encoding mRNA– Inhibit translation

Figure 6.13


Two component regulation

• Phosphorylation of sensor

• Phosphate passed to response regulator

• Response regulator reacts with DNA changing gene expression– Increase– decrease

Figure 6.14


Genetic Information• Genome

– total DNA of a cell– most have single circular chromosome– some have linear chromosome

• Plasmids– small, circular, extrachromosomal DNA

• encode beneficial factors• resistance factors (antibiotic)• conjugative plasmids

– transfer to other cells

• Genotype: genetic makeup• Phenotype: appearance and function


Changes in Genetic Information

• Mutations– chemical change

in DNA• chemical

mutagens– Bind DNA– Change in DNA

• physical mutagens– UV light– Ionizing radiation

• biological mutagens– Transposable

elements» Insertion

sequences» transposons

Figure 6.16


Consequence of mutations• Types of mutations

– base substitution• wrong nucleotide

– deletion mutation• nucleotides deleted

– Inversion• reverses order of a

segment

– Transposition• moves a segment of

DNA

– Duplication• identical new segment

• Results– Lethal mutation– Conditional expressed

mutations

Figure 6.15


Physical mutagen--UV damage

• UV light– stimulates neighboring

bases to form dimers• thymine dimers

– activate repair systems

Figure 6.17



• Thymine dimers distort the DNA structure

• Enzymes remove the damaged nucleotides

Figure 6.17



• Repairs may result in incorrect nucleotide replacement

• Mutation is result

Figure 6.17


Selecting and identifying mutants• Direct selection

– Conditions favor growth of desired mutant– Growth of bacteria in presence of antibiotic– Only successful growth are mutants

• Indirect selection– Prevent growth of mutant– Kill growing cells– Desired mutants larger percentage of population– Isolate mutants

• Site-directed mutagenesis– Recombinant DNA manipulation


Selecting and identifying mutants

• Brute strength– Screen large numbers– Replica plating

• Transfer large numbers of colonies• Track growth

Figure 6.18


Ames Test

Figure 6.19


Transformation

• DNA exits one cell, taken up by another cell– Natural

• few bacteria take up DNA fragments

– Artificial--induced in laboratory• useful tool for recombinant DNA technology

Figure 6.20


Conjugation

• Conjugative plasmids– plasmids transfer– genetically encoded– F plasmid in E. coli– sex pilus connect two

cells• one cell F+

• one cell F-

Figure 6.21


Conjugation– One strand of

plasmid DNA is broken (nicked)

– replication begins– synthesized linear

strand enters F- cell– linear strand is

copied forming a complete plasmid

– both cells are F+

Figure 6.21


Transduction

• Bacteriophage– virus that infects

bacteria– reproduce in bacteria– some phages contain

bacterial DNA• rare event• transducing particle

– cell lysis and release• normal phage• transducing particles

Figure 6.23


Transduction– Transducing particles

• infect other bacteria• inject bacterial DNA

into new cell

– genetic exchange• one bacteria cell to

another

– integration into chromosome

Figure 6.23


Eukaryotic Microorganisms

• Genetic exchange– Similar to plants

and animals– Haploid

gametes fuse

Figure 6.23

Documents

© 2004 Wadsworth – Thomson Learning Chapter 6 The Genetics of Microorganisms