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Chapter 18
Bacteria
DNA is super-coiled in part of the cell called the nucleoid. Some is in
small, self-replicating circles called plasmids.
Most reproduction is asexual through binary-fission. Some have slight
mutations, increasing diversity.
Other methods of Gene Transfer and Recombination in Bacteria:
1)Transformation- Bacterial cells take up naked foreign DNA From
environment.
2) Transduction- Phages carry bacterial genes from one host to
another during the phage reproductive cycle.
3) Conjugation- Bacterial “sex”. Bacterial with F factor from sex pili,
and it attaches to a neighboring bacterium, forming a mating bridge.
DNA is donated one-way. F+ cell convert the F – to F+. If the F factor
is in plasmid forms, only the F factor is transferred. If it I in the
chromosome, other DNA is transferred too, and bacteria can take up
and become recombinant.
R plasmids- carry antibiotic resistance genes. Many also carry
instructions for sex pili and gene transfer.
Transposable elements- DNA that undergoes recombination by
moving elsewhere in the chromosome during DNA folding, or but “
copying and pasting” elsewhere.
Insertion sequence- contain a single gene for tansposase, which
catalyzes insertion from one site to another. Gene is bracketed by
inverted repeats of nucleotides that the enzyme recognizes as the
end of the gene. Can cause mutation if it transposes into the coding
sequence if a gene or in an area regulation gene expression.
Transposons- have insertion sequence and other genes, such as
antibiotic resistance.
Operon- A cluster of regulated genes with related functions:
Includes:
An operator, located inside or after the promoter, that controls Two
whether or not RNA polymerase cane bind.
A promoter that helps position the RNA polymerase.
The enzyme- coding gene.
Two Types of Negative Gene Regulation
1) Repressible operons- if a repressor attaches to the operator,
the RNA polymerase can’t attach, and the mRNA coding for the
enzyme is not produced. Repressors are made by regulatory
genes. Repressors often allosteric proteins with two shapes. If
the molecules coded for by the operon it present, it acts as a
corepressor. It binds to the repressor thereby causing it to
change shape and attach to the operator, stopping
transcription, when corepressor levels drop, the reaction
reverses, and the operator allows transcription to continue
again.
2) Inducible operons- The operon is usually in the noncoding form,
with a repressor attached to the operator. Specific molecules
can then attach to the repressor, causing it to separate from the
operator, and allowing transcription to take place.
Positive gene regulation: A catabolitic activator protein (CAP) is an
activator of transcription. When a specific molecule is present, it
bonds to the CAP, which then binds to the CAP- Binding site. RNA
polymerase can only bond and start transcription after this happened