The Bacterial Genome
Bacterium DNA:
1. singular and circular
2. found in the nucleoid region of the bacterial cell
3. contain a small double strand ring of DNA called PLASMIDS.
Most bacteria reproduce by binary fission. There is only one origin of replication.
2 replication forks move in opposite directions. Bacteria can completely divide in 20 minutes under optimal conditions.
Transfer and recombination of Bacterial genes:
There are three natural processes of gene transfer in bacterium:
1. Transformation: Bacteria picks up DNA from its environment. Avery's experiment.
2. Transduction: Gene transfer from one bacterium to another using a bacteriophage.
a). General transduction: random pieces of host DNA are packaged in within a phage capsid during the lytic cycle.
b). Restricted transduction: when a temperate phage takes some of the host's DNA with it when it forms a new virus.
3. Conjugation: transfer of genetic material between 2 cells that are temporarily joined.
a). The cell donating the DNA extends an external appendage called a sex pili.
b). This pili attaches to the cell receiving the DNA.
c). A cytoplasmic bridge forms which the DNA is transferred.
The ability for the sex pili to form is conferred by genes in a plasmid called the F factor. (fertility factor). Cells with the F factor are designated F+ and cells without it are designated F-. During conjugation between an F+ and an F- bacterium both cells finish up as F+ forms. Sometime the the F factor inserts into the circular chromosome. Integrated F factor is still expressed. It is designated Hfr. (high frequency of recombination).
Transfer of genetic material by conjugation can help scientists map the chromosomes of bacteria.
1. Specific strains of Hfr bacteria always transfer genes in the same sequence.
2. The duration of conjugation determines the number of genes transferred.
Episomes: plasmids that can integrate into the bacterial chromosome. (F factor). Can exist independently of the circular chromosome or integrate and replicate with it. Some carry up to 7 genes for resistance for certain antibiotics.
Transposons: DNA sequences that can move from one chromosomal site to another. Serve as natural agents of genetic change. Occur in both prokaryotic and eukaryotic cells.
Control of gene expression in Prokaryotes:
Cells have 2 ways of controlling metabolism: Regulating enzyme activity and regulating enzyme synthesis.
1. Regulating enzyme activity: The end product of an anabolic pathway may turn off its own production by inhibiting activity of an enzyme at the beginning of the pathway. (feedback inhibition).
2. Regulating enzyme synthesis: Accumulation of products triggers a mechanism that inhibits mRNA production by genes that code for an enzyme at the beginning of the pathway. (gene repression).
Operons and their control:
Francois Jacob and Jacques Mond proposed a mechanism for gene regulation, the operon concept.
Structural gene: gene that codes for a polypeptide.
Operon: a regulated cluster of regulated structural genes with related functions.
a). common in bacteria and phages
b). has a single promotor region, so an RNA polymerase will transcribe all structural genes on an all-or-none basis.
c). contains a single operator, a DNA segment between the operon's promotor and structural genes. It is a binding site for the operon's repressor protein. Acts as the on /off switch for movement of RNA polymerase and transcription of the structural genes.
d). Repressor: specific protein that binds to an operator and blocks transcription of the operon. It blocks the attachment of RNA polymerase to the promotor.
e). some operons are switched on by activators. DNA-binding regulatory protein which activates transcription of the operon.
f). Regulatory genes: genes that code for repressor and activator proteins.
Repressible Operon: trp Operon.
If there is no tryptophan in the cell, the regulatory gene produces a repressor protein which remains inactive,allowing the operator to proceed,by allowing the RNA polymerase to complete the formation of the m-RNA and the issuing polypeptides making tryptophan.
If there is tryp in the cell, the repressor protein binds to the promotor with trp as an allosteric inhibitor. This stops the production of the polypeptide.
An Inducible Operon:
Lac operon: If lactose is absent from a cell the enzymes will not be made. If the sugar enters the cell, the operon will be switched on and produce the enzymes needed to digest the lactose. Lactose metabolism in E. coli is programmed by the lac operon which has three structural genes. lac.Z codes for B-glactosidase which hydrolyzes lactose. lac. Y codes for permease, a membrane protein that transports lactose into the cell, and lac A codes for transacetylase, an enzyme that has no known action in lactose metabolism. Lactose acts as an inducer to turn on the operon.
Repressible Enzymes Inducible Enzymes
1. Their genes are switched on until 1. There genes are switched off specific metabolite activates the repressor until a specific metabolite inactivates the repressor.
2. Generally function in an anabolic pathway. 2. Functions in catabolic pathways.
3. Pathway end product switches off its own 3. Enzyme synthesis is switched on production by repressing enzyme synthesis. by the nutrient the pathway uses.
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