Comparison of Genetic Material and Replication for Eukaryotes and Prokaryotes Bacteria Archaea Eukaryotes

Genome haploid; circular

haploid; circular

diploid; linear

Histones Absent Present; nucleosome

Present; nucleosome

Rate Faster Faster Slower

Point of origin Single Multiple Multiple

Telomeres Absent Absent Present

# DNA Polymerase

~5 ~5 ~15

Comparison of Transcription for Eukaryotes and Prokaryotes

Bacteria Archaea Eukaryotes

# RNA Polymerase

1 1 – similar to eukaryotic

3

# genes on transcript

polycistronic polycistronic monocistronic

Post-transcription modification

None Introns Introns, cap and tail

Transcription factors

No

Sigma Factor

Yes Yes

Promoter Unique Similar Similar

Enzymes are common feature of biochemical pathways

Constitutive enzymes (60-80%) Inducible enzymes

Default position off Repressible enzymes

Default position on

Regulation of Gene Expression

Operon model of gene expression Regulatory gene, operator, promoter and series of

structural genes divided into three regions:

Regulatory gene – codes for regulatory protein Control region - operator and promoter Structural genes - genes being transcribed

Operon structure

Promoter – Binding site for RNA polymerase

Operator – binding site for the repressor protein

Structural Genes – DNA sequence for proteins of interest

Operator

Gene 1 Gene 3Gene 2

Promoter

Regulatorygene

Regulatory gene – DNA sequence for repressor protein

Control region

Operon controlled by regulatory region Protein acts as “on/off” switch

Can act as repressor or inducer

Operon model based on studies of induction of the enzymes of lactose catabolism on E. coli

Inducible enzyme

Default position is off

Enzymes not made until needed

Catabolite Repression glucose represses enzymes for lactose degradation Low glucose levels corresponds to high cAMP cAMP binds to catabolite activating protein (CAP)

alarmone CAP binds to promoter and induces RNA polymerase

to bind

2-step diauxic growth caused by catabolite repression

E.coli grows on either substrate

Repressible enzyme

Default position is on

Enzymes made until no longer needed

Operons rare in eukaryotes Function differently

Eukaryotes utilize transcription factors or alternate splicing of exons

Expression may be regulated at translation level

Unsure of regulation of expression in archaea May be more similar to eukaryotes than bacteria

Many microbes adapt to changing environments by altering level of gene expression Global Regulatory Systems

Signal transduction Transmits information from external environment to

inside cell Allows cell to respond to environmental changes

Two-component regulatory systems Sensors recognize change in environment

Kinase protein in membrane Response regulators activate or repress gene

expressionDNA binding protein

Quorum sensing Based on density of cell population Activation of genes beneficial only when produced by

multiple cells Vibrio fisheri Biofilm formation

Natural selection Antigenic variation

Alteration in characteristics of certain surface proteins

Ex. Neisseria gonorrhoeae varies pilin gene at expression locus

Regulation may occur at the translation level Riboswitches Antisense RNA

Bacterial Genetics and Genetic Transfers

Eukaryotes - sexual reproduction Gametes have various genetic combinations

Prokaryotes - asexual reproduction All offspring are clones of parent cell No genetic variation

Genetic Diversity

Diversity in Bacteria

Bacterial mechanisms for genetic diversity Mutation Gene transfer

Change in genotype Wild type vs. mutant

May or may not cause phenotypic changes silent, beneficial, or harmful

Passed vertically to all offspring Selective pressure can lead to evolution through

natural selection

Mutations

Point Mutation (base substitution)

Missense

Types of Mutations•Change in one base

• Results in change of amino acid

Nonsense • Results in a stop codon

Frame-shift mutation •Insertion or deletion of one or more bases

Mutagen Agent that induces mutations Physical or chemical agents

Spontaneous mutations Occur in the absence of a mutagen May be due to error or transposons

Transposable Elements (Transposons) May disrupt proper gene function Contain insertion sequences (transposase) Complex (composite) transposons carry other genes

•Nucleotide excision repair

•Endonuclease, DNA ligase & DNA Polymerase

•Light repair

•Direct repair

•Photoactivation of enzymes (photolyase)

Induced Mutations

Mutations are essential for understanding genetics Intentionally produced (induced) to demonstrate

function of particular gene or set of genes

Mutations can be induced via Chemical mutagens Transposition Radiation

•Ames Test

•Mutational reversion assay

•Tests mutagenicity of compounds

•Utilizes a histidine auxotroph

Mutations followed by selection may produce microbes with desirable traits

Positive (direct) selection detects mutant cells because they grow or appear different Ex. Penicillin resistant mutants growing on penicillin

containing agar – non mutants will not grow Eliminates wild type

Negative (indirect) selection detects mutant cells because they do not grow Replica plating to isolate mutants requiring a specific

growth factor – auxotroph Selects for wild type

Replica Plating

Figure 8.21