DNA, AND IN SOME CASES RNA, IS THE PRIMARY SOURCE OF HERITABLE INFORMATION
Noneukaryotic Genetic Information
Bacterial DNA
Two types of DNA in bacteria. The main form of genetic
material in bacteria is a single circular chromosome made of DNA. The chromosome replicates via binary fission. In binary fission, the chromosomes replicates and the cell divides into two cells, with each cell gets an identical copy of the chromosome.
Bacteria also contain plasmids, small, circular DNA molecules outside the chromosome. Plasmids replicate independently of the chromosome. Plasmids are not always necessary to the survival of the bacteria but can be beneficial to the survival of the bacteria.
Binary Fission
Genetic Variation in Bacteria Conjugation is a process of DNA
exchange between bacteria. Transduction occurs when DNA is
introduced into the genome of a bacterium by a virus.
Transformation occurs when bacteria absorb DNA from their surroundings and incorporate it into their genome,
Mutation occurs when there is a random change in the DNA
Conjugation
Transduction
Transformation
Summary of Sources of Genetic VariationMutations also results in genetic variation
Regulation of Gene Expression
Gene expression in bacteria is controlled by the operon model.
An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control. Natural selection has favored bacteria that produce only the products needed by that cell.
A bacteria cell can regulate the production of enzymes by feedback inhibition or gene regulation via an operon.
Parts of an Operon promoter - region is a sequence
of DNA to which the RNA polymerase attaches to begin transcription
operator - region can block the action of the RNA polymerase if the region is occupied by a repressor protein
structural genes - contain DNA sequences that code for several related enzymes that direct the production of some particular end product
regulatory genes - produces proteins that either (1) bind to the operator and block transcription(repressor proteins) or (2) bind to the repressor which causes it to release the operator and allow transcription to take place (activator proteins).
Repressible Operons – always ON
Repressible operons are always turned on - meaning they produce their protein product until they are turned off.
The trp operon is an example. By default the trp operon is on and the genes for tryptophan synthesis are transcribed. When tryptophan is present, it binds to the trp repressor protein, which turns the operon off. The repressor is active only in the presence of its corepressor - tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high. Repressible enzymes usually function anabolic pathways; their synthesis is repressed by high levels of the end product
Inducible Operons – always OFF
Inducible operons are usually turned off - meaning they don't produce the protein until a molecule called an inducer inactivates the repressor and turns on transcription.
The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose. By itself, the lac repressor is active and switches the lac operon off. A molecule called an inducer inactivates the repressor to turn the lac operon on. When there is no lactose present - there is not need for the enzymes that break it down. Inducible enzymes usually function in catabolic pathways; like digestion; their syntesis is induced by a chemical signal (the item that is meant to be digested).
Positive Gene RegulationPromoter
DNA
CAP-binding site
lacZlacI
RNApolymerasebinds andtranscribes
Operator
cAMPActiveCAP
InactiveCAP
Allolactose
Inactive lacrepressor
(a) Lactose present, glucose scarce (cAMP level high):abundant lac mRNA synthesized
Promoter
DNA
CAP-binding site
lacZlacI
OperatorRNApolymerase lesslikely to bind
Inactive lacrepressor
InactiveCAP
(b) Lactose present, glucose present (cAMP level low):little lac mRNA synthesized
The compounds present determine which operons are turned on. For example: positive control of the lac operon by catabolite activator protein (CAP). RNA polymerase has high affinity for the lac promoter only when CAP is bound to a DNA site at the upstream end of the promoter. CAP attaches to its DNA site only when associated with cyclic AMP (cAMP), whose concentration in the cell rises when glucose concentration falls.
Operon Assignment
Due 1/20/15
Explain the concept of an operon and the function of the operator, repressor, and corepressor. Be sure to state the adaptive advantage of grouping bacterial genes into an operon. Discuss how repressible and inducible operons differ and how those differences reflect differences in the pathways they control.
