Ap biology viruses and bacteria

Browse Subjects. Study with Hours. SAT Exam Prep. ACT Exam Prep. College Resources. AP Trivia. Sign in Sign up. Hfr cells function as males during conjugation.

Random movements almost always disrupt conjugation long before an entire copy of the Hfr chromosome can be passed to the F? In the partially diploid cell, the newly acquired DNA aligns with the homologous region of the F? Recombination exchanges segments of DNA. The resulting recombinant bacterium has genes from two different cells. In the s, Japanese physicians began to notice that some bacterial strains had evolved antibiotic resistance.

Some of these genes code for enzymes that specifically destroy certain antibiotics, like tetracycline or ampicillin. The genes conferring resistance are carried by plasmids, specifically the R plasmid R for resistance. When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population. Because R plasmids also have genes that encode for sex pili, they can be transferred from one cell to another by conjugation.

Unlike plasmids or prophages, transposable elements never exist independently but are always part of chromosomal or plasmid DNA. In bacteria, the movement may be within the chromosome, from a plasmid to a chromosome or vice versa , or between plasmids. In other words, the transposable element is added at a new site without being lost from the old site. Most transposable elements can move to many alternative locations in the DNA, potentially moving genes to a site where genes of that sort have never before existed.

The simplest transposable elements, called insertion sequences, exist only in bacteria. An insertion sequence contains a single gene that codes for transposase, an enzyme that catalyzes movement of the insertion sequence from one site to another within the genome. The insertion sequence consists of the transposase gene, flanked by a pair of inverted repeat sequences. The 20 to 40 nucleotides of the inverted repeat on one side are repeated in reverse along the opposite DNA strand at the other end of the transposable element.

The transposase enzyme recognizes the inverted repeats as the edges of the transposable element. Transposase cuts the transposable elements from its initial site and inserts it into the target site. Insertion sequences cause mutations when they happen to land within the coding sequence of a gene or within a DNA region that regulates gene expression. Insertion sequences account for 1. This is about the same rate as spontaneous mutations from external factors. Transposable elements longer and more complex than insertion sequences, called transposons, also move about in the bacterial genome.

In some bacterial transposons, the extra genes are sandwiched between two insertion sequences. While insertion sequences may not benefit bacteria in any specific way, transposons may help bacteria adapt to new environments.

For example, a single R plasmid may carry several genes for resistance to different antibiotics. This is explained by transposons, which can add a gene for antibiotic resistance to a plasmid already carrying genes for resistance to other antibiotics. The transmission of this composite plasmid to other bacterial cells by cell division or conjugation can spread resistance to a variety of antibiotics throughout a bacterial population.

In an antibiotic-rich environment, natural selection factors bacterial clones that have built up R plasmids with multiple antibiotic resistance through a series of transpositions.

Transposable elements are also important components of eukaryotic genomes. First, cells can vary the number of specific enzyme molecules they make by regulating gene expression.

Second, cells can adjust the activity of enzymes already present for example, by feedback inhibition. The tryptophan biosynthesis pathway demonstrates both levels of control. If tryptophan levels are high, some of the tryptophan molecules can inhibit the first enzyme in the pathway. If the abundance of tryptophan continues, the cell can stop synthesizing additional enzymes in this pathway by blocking transcription of the genes for these enzymes.

The five genes coding for these enzymes are clustered together on the bacterial chromosome, served by a single promoter. Transcription gives rise to one long mRNA molecule that codes for all five enzymes in the tryptophan pathway. The mRNA is punctuated with start and stop codons that signal where the coding sequence for each polypeptide begins and ends.

When an E. The switch is a segment of DNA called an operator. The operator, located between the promoter and the enzyme-coding genes, controls the access of RNA polymerase to the genes.

The operator, the promoter, and the genes they control constitute an operon. By itself, an operon is on and RNA polymerase can bind to the promoter and transcribe the genes. Each repressor protein recognizes and binds only to the operator of a certain operon. Regulatory genes are transcribed continuously at low rates.

Binding by the repressor to the operator is reversible. The number of active repressor molecules available determines the on or off mode of the operator. Repressors contain allosteric sites that change shape depending on the binding of other molecules. In the case of the trp, or tryptophan, operon, when concentrations of tryptophan in the cell are high, some tryptophan molecules bind as a corepressor to the repressor protein.

This activates the repressor and turns the operon off. At low levels of tryptophan, most of the repressors are inactive, and the operon is transcribed. The trp operon is an example of a repressible operon, one that is inhibited when a specific small molecule binds allosterically to a regulatory protein.

In contrast, an inducible operon is stimulated when a specific small molecule interacts with a regulatory protein. In inducible operons, the regulatory protein is active inhibitory as synthesized, and the operon is off. Allosteric binding by an inducer molecule makes the regulatory protein inactive, and the operon is turned on. The lac operon contains a series of genes that code for enzymes that play a major role in the hydrolysis and metabolism of lactose milk sugar.

In the absence of lactose, this operon is off, as an active repressor binds to the operator and prevents transcription. Lactose metabolism begins with hydrolysis of lactose into its component monosaccharides, glucose and galactose.

Only a few molecules of this enzyme are present in an E. The regulatory gene, lacI, located outside the operon, codes for an allosteric repressor protein that can switch off the lac operon by binding to the operator. Unlike the trp operon, the lac repressor is active all by itself, binding to the operator and switching the lac operon off. An inducer inactivates the repressor.

When lactose is present in the cell, allolactose, an isomer of lactose, binds to the repressor. This inactivates the repressor, and the lac operon can be transcribed. Repressible enzymes generally function in anabolic pathways, synthesizing end products from raw materials. When the end product is present in sufficient quantities, the cell can allocate its resources to other uses.

Inducible enzymes usually function in catabolic pathways, digesting nutrients to simpler molecules. By producing the appropriate enzymes only when the nutrient is available, the cell avoids making proteins that have nothing to do. Both repressible and inducible operons demonstrate negative control because active repressors switch off the active form of the repressor protein. Positive gene control occurs when an activator molecule interacts directly with the genome to switch transcription on.

Even if the lac operon is turned on by the presence of allolactose, the degree of transcription depends on the concentrations of other substrates. The regulatory protein catabolite activator protein CAP is an activator of transcription.

When cAMP is abundant, it binds to CAP, and the regulatory protein assumes its active shape and can bind to a specific site at the upstream end of the lac promoter. The attachment of CAP to the promoter directly stimulates gene expression. Thus, this mechanism qualifies as positive regulation.

The cellular metabolism is biased toward the use of glucose. The lac operon will be transcribed but at a low level. CAP works on several operons that encode enzymes used in catabolic pathways. If glucose is present and CAP is inactive, then the synthesis of enzymes that catabolize other compounds is slowed. If glucose levels are low and CAP is active, then the genes that produce enzymes that catabolize whichever other fuel is present will be transcribed at high levels.

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