Microbiology Ch 8

Two bacterial genes are transduced simultaneously. What does this suggest about the location of the two genes relative to each other?

The two genes must be close together on the bacterial chromosome. Recall that the bacterial chromosome is cut into fragments during the virus life cycle. For two genes to be transferred together, they must both be found on one of these fragments

UV light exposure forms:

*covalent bonds* between adjacent thymine nucleobases on the same strand of *DNA*

Transformation is facilitated by

*proteins* on the cell wall that bind DNA.

BT toxin gene

This is a plasmid gene. The bacterium B. thuringiensis can produce a toxin that kills insects. It is not essential for survival.

Chloramphenicol-resistance gene

This is an antibiotic-resistance gene found on a plasmid.

Which of the following requires that cells be competent?

Transformation Bacteria that undergo transformation must be competent (able to take up DNA from the environment).

UV light and other non-ionizing radiation damage DNA molecules by

creating thymine dimers between adjacent thymine nucleobases in the DNA chain.

During the entry of the donor DNA into the recipient cell,

one strand is broken down by nucleases.

A recombinant organism is

one that that contains genes from another organism. transferred via conjugation or some other method of horizontal gene transfer.

Transposons can cause mutations by

"jumping" into a gene and disrupting its function.

The first step in conjugation is

contact between the donor and recipient

These dimers are repaired by

the enzyme DNA photolyase *and* exposure to light.

You are analyzing a bacterial genome. You find that the G-C content is 60%. However, in one part of the genome, the G-C content is 37%. What can you conclude?

the organism has a genomic island

After the conjugation of an Hfr cell with an F- cell,

the recipient remains F-

The main advantage for bacteria that have the ability to carry out conjugation and other forms of genetic exchange is that

they become genetically diverse Binary fission is essentially asexual, so these recombination mechanisms help bacteria vary their genetic makeup and respond better to changes in their environment.

Preventing the function of light repair (photoreactivation) interferes with the repair of which of the following?

thymine dimers

A __________________________ is generated when a fragment of bacterial DNA is accidentally packaged into a newly synthesized bacteriophage.

transducing particle

Which of the following statements regarding mutations is TRUE?

A prototroph can grow on glucose-salts media Prototrophs can synthesize all the nutrients they require for growth. Glucose-salts medium lacks nutrients - only prototrophs will grow on this medium.

Mechanisms that repair errors in nucleotide incorporation are 1. mismatch repair 2. proofreading by DNA polymerase 3. light repair 4. SOS repair 5. excision repair

1, 2

An F pilus or its equivalent is essential for 1. DNA-mediated transformation 2. chromosome transfer by conjugation 3. plasmid transfer by conjugation 4. generalized transduction 5. cell movement

2, 3

You are trying to isolate a mutant of wild type E. coli that requires histidine for growth. This can best be done using 1. direct selection 2. replica plating 3. penicillin enrichment 4. the Ames test 5. reversion

2, 3

DNA polymerases use their ______ activity to remove a mismatched base pair.

3' -> 5' exonuclease

All plasmids 1. carry genes for antimicrobial resistance 2. encode mechanisms for transmission to other bacteria 3. occur in multiple copies in the cells 4. code for non-essential functions 5. are replicons

4, 5

On which of the following DNA strands will UV light have the most effect?

ACTTGCTTAC Recall that UV light causes *thymine dimers* in DNA—bonds between adjacent T's. One of the strands shown has two sites where thymine dimers may be formed.

Which of the following are true about CRISPR?

CRISPR loci can reveal a historical record of phage infections of a bacterial cell. - Because the spacers of the CRISPR system are incorporated in sequential order, CRISPR loci reflect the history of viral infection of the host. CRISPR systems act as a barrier to horizontal gene transfer. - Recall that one mechanism of horizontal gene transfer is by transduction: transfer of bacterial genes from one cell to another by a phage. If a bacterial cell inherits immunity to phage infection, transduction cannot occur.

Which of the following genes are likely to be found on a plasmid rather than on the bacterial chromosome?

BT toxin gene Chloramphenicol-resistance gene Oil degradation enzyme genes

Which of the following pairs about mutation is NOT correctly matched? A. Mutagen—induces mutations B. Ethidium bromide—chemical mutagen C. Nonsense mutation—incorrect amino acid D. Transposons—"jumping genes" E. Phenotype—observable characteristics

C. Nonsense mutation results in a stop codon

Which of the following statements about the *Ames test* is FALSE? A. It relies on *direct selection*. B. *Auxotrophs* are used in the test. C. It is used to determine *whether a substance is a mutagen*. D. Liver extract is added to provide nutrients. E. *Revertants are counted*.

D. Animal liver extract may also be added as part of the Ames test because enzymes produced by the liver modify certain chemicals, and in some cases this inadvertently transforms the chemicals into a form that is mutagenic... that is, *the chemical would not normally be mutagenic, but once inside the body, the liver enzymes convert into a form that causes mutations*

How does DNA polymerase use the structure of DNA to catch errors?

DNA polymerase moves along a single strand of DNA, building the complementary strand as it goes. The two-stranded molecule passes through a proofreading DNA polymerase molecule after synthesis is complete. If the wrong base has been inserted then the bond is unstable. The DNA polymerase can detect and replace the incorrect, mismatched nucleotide.

Adding DNase to a mixture of donor and recipient cells will prevent gene transfer via

DNA-mediated transformation

Which of the following best describes vertical gene transfer?

Duplicating the entire genome and passing a copy to a daughter cell.

true or false: A missense mutation has a greater impact on a cell than does a frameshift mutation.

FALSE

true or false: Chemical agents can cause mutations by inducing ethylation of guanine residues in DNA.

FALSE

true or false: The enzyme ethylguanine-DNA ethyltransferase prevents DNA mutations from occurring.

FALSE

true or false: Transposons are composed of amino acids.

FALSE

true or false: After conjugation of an Hfr cell and an F- cell, the entire genome of the Hfr cell is usually transferred to the recipient cell.

FALSE It is theoretically possible for the complete genome and the F plasmid to be transferred to the recipient cell, however, this does not happen because *the donor and recipient cells will separate prior to the complete transfer of the donor chromosome*

true or false: All mutagens are carcinogens but not all carcinogens are mutagens.

FALSE MOST carcinogens are mutagens, and not all mutations are carcinogens

What process involves the transfer of DNA from one bacterial cell to another via a sex pilus?

Horizontal gene transfer by conjugation

Oil degradation enzyme genes

Most organism cannot degrade oil. Those that can have specific plasmid-encoded genes for the enzyme(s) required.

If a Gram-positive competent cell takes up DNA containing a penicillin resistance gene, what would be added to a selective medium to isolate the transformants?

Penicillin Addition of *penicillin* to the medium would *prevent the growth of non-transformants*, thus selecting for the cells that have acquired the new DNA.

Please match each mobile genetic element with its characteristic. Phage DNA Transposon Genomic island Plasmid

Phage DNA-Virus genome that sometimes carries additional genes. Transposon-DNA sequence that moves from one place to another in a genome. Genomic island-DNA segment in a cell's genome that originated in another species. Plasmid-Generally encodes only non-essential genetic information.

Distinguish between genotype and phenotype.

Phenotype = The organisms observable characteristics. Ex: presence of flagellum Genotype = The genetic makeup of the organism, in other words, the DNA sequence

true or false: A transposon can move from the host chromosomal DNA to a plasmid.

TRUE

true or false: Following base removal, DNA polymerase can add nucleotides in the 5' to 3' direction.

TRUE

true or false: Improper base-pairing during DNA replication causes a pause in chain elongation.

TRUE

true or false: In addition to *transformation*, the other two forms of recombination are *conjugation* (DNA transfer involving cell-cell contact) and *transduction* (the transfer of DNA using phage vectors).

TRUE

true or false: The donor cell DNA is integrated into the recipient cell's DNA by homologous recombination.

TRUE

What does a competent cell mean?

That the cell is able to take up DNA from the environment Transformation requires competent bacteria

In direct selection methods used to identify possible carcinogens (example: Ames testing), what does it mean when a chemical can cause a bacterium to develop resistance to some environmental condition?

The chemical is a mutagen.

How can transposons cause evolutionary changes?

Transposons are segments of the genome that have the ability to both replicate themselves and to insert themselves elsewhere in the genome. The function of transposons is merely to replicate themselves. However, they can potentially play an important role in evolution by copying and transferring other genetic material. There is evidence that transposons and their host genomes have co-evolved over long periods and that transposon activity may increase in response to environmental factors. The movement of transposons is a source of increased genetic variability. In particular, transposons contain regulatory genes that control transposon activity. It is thought that these regulatory genes may evolve to control other genes as well.

Generalized transduction occurs when

a bacteriophage packages a random piece of bacterial DNA during replication. this is a packaging error by the bacteriophage.

Hfr refers to

a cell in which the F plasmid has been integrated into the cell chromosome.

During maturation or formation of phage particles,

a few phage heads may surround fragments of host bacterial DNA instead of viral DNA

The F pilus is

a protein appendage that attaches the two cells together. The F pilus is a protein appendage or tube that attaches the donor and recipient cells.

Plasmids are

able to replicate independently of the chromosome.

The frequency of transfer of an F' molecule by conjugation is closest to the frequency of transfer of

an F plasmid by conjugation

The plasmid is mobilized for transfer when

an endonuclease cleaves it at the origin of transfer.

The basic parts of a bacteriophage are

an icosohedral head, tail, tail pins, and fibers. which bind to the host bacteria.

Mismatch repair is activated by

any difference between the nucleotide sequence in the donor and recipient DNAs

A virus that infects bacterial cells is called a ______.

bacteriophage

After entry of the bacteriophage into the host cell, a phage enzyme

breaks down host DNA into fragments

A plasmid that can replicate in E. coli and Pseudomonas is most likely a/an

broad host range plasmid

When the bacteriophage transfers bacterial DNA into a recipient bacterial cell, that DNA

can integrate into the chromosome and provide new genetic information

As a result of mismatch repair during transformation:

cells may have either original DNA or donor DNA in them.

Cells that are capable of bringing DNA from their environment into the cell through their cell wall are ______.

competent

Horizontal gene transfer can occur via

conjugation

In addition to transformation, the other two forms of horizontal gene transfer are

conjugation and transduction

Bacterial cells that have been genetically modified to carry an antibiotic resistance gene can be separated from susceptible (non-resistant) microbes by using a ______ selection method using agar plates with antibiotic.

direct antibiotic resistant mutants can be easily selected directly by inoculating cells onto a medium containing the antibiotic... only the resistant cells will form colonies

In the beginning of transformation, ______ binds to the target cell.

double-stranded DNA

A culture of E. coli is irradiated with UV light. The UV light affects the cell's DNA by

forming covalent bonds between thymine bases on the same strand of DNA.

Transformation is a form of horizontal gene transfer that involves homologous recombination. Homologous recombination is

genetic recombination between similar or identical nucleotide sequences

The classic transformation experiment done by Griffith used

harmless and virulent strains of Streptococcus pneumoniae. The classic transformation experiment done by Griffith used mice and harmless (rough) and virulent (smooth) strains of Streptococcus pneumoniae.

F+ cells

have a plasmid and are donors in conjugation.

The proofreading function of DNA polymerase reduces the error rate from about one in a million base pairs to about one in a ______ base pairs.

hundred million

These dimers weaken ______ between bases.

hydrogen bonds

Plasmid DNA is transferred

in single-stranded form Each single strand is copied to produce a new double-stranded plasmid in each cell.

Hfr refers to a cell that has

integrated the F plasmid into its genome.

What do you know about an E. coli cell that is F+?

it carries a fertility plasmid it is able to conjugate with F- cells it is able to make a sex pilus

In transduction, bacterial DNA is transferred to a new cell when

it is injected by the virus carrying bacterial DNA.

Penicillin enrichment of mutants works on the principle that the antibiotic...

kills only growing cells

During the process of transformation in the lab setting, cells are plated on selective media to

make sure that only transformants grow. since only the transformants contain a certain gene needed to survive.

Bacteriophages adsorb or attach

only to members of the same bacterial species Typically, bacteriophages adsorb or attach specifically to one bacterial species. Because of that, transduction occurs between members of the same species.

The donor DNA

pairs with a homologous region of the recipient DNA

After host cell DNA is broken down by a viral enzyme,

phage DNA is replicated and phage coat proteins are produced.

A culture of E. coli is irradiated with UV light. The highest frequency of mutations would be obtained if after the irradiation, the cells were immediately

placed in the dark

If cells were exposed to UV light, the highest frequency of mutations would be obtained if the cells were immediately...

placed in the dark

A difference between transferring plasmids and DNA fragments between bacterial cells is

plasmids can replicate independently. A difference between transferring plasmids and fragments of DNA between cells is that plasmids can replicate independently while DNA fragments must integrate to be replicated.

If you find that a particular plasmid has an R factor, the bacteria that has it will be

resistant to certain antibiotics. R factors can make bacteria resistant to antibiotics such as *penicillins, sulfonamides, or tetracycline.*

Transposons are

segments of DNA

Genes from the donor chromosome are transferred as

single stranded DNA

During conjugation, the donor chromosome is transferred as

single-stranded DNA

In transformation, genes from a donor chromosome are transferred as

single-stranded DNA

The F pilus binds to

specific receptors on the cell wall of the recipient. This specificity means conjugation takes place between members of the same or similar species.

Examples of advantageous genes that can be transferred by transduction are genes for

sugar fermentation. toxin production. drug resistance.

The F plasmid encodes for

the F pilus therefore, F+ cells can make the pilus and serve as donors in conjugation.

F+ refers to a cell containing

the F plamid

After the transfer of the F plasmid is complete

the F- cell becomes F+ because it now contains an F plasmid and can make a pilus.