BIOL 400 Ch. 16

Match each type of mutation with the appropriate example. (1) auxotroph (2) conditional (3) morphological (4) resistance

(1) mutant cannot synthesize an essential molecule such as an amino acid (2) phenotype is only affected at high temperatures phenotype is only affected at high temperatures (3) mutation affects the appearance of colonies (4) mutant is not affected by a particular pathogen or chemical

Conjugation

(1) the form of gene transfer and recombination in bacteria and archaea that requires direct cell-to-cell contact. (2) a complex form of sexual reproduction commonly employed by protists. -depends on the presence of a conjugative plasmid

Simple transposition

(also called cut-and-paste transposition) transposase catalyzes exision of the transposable element, followed by cleavafe of a new target site and ligation of the element into this site.

Spontaneous mutations

- mutations that occur due to errors in replication or the occurrence of lesions in the DNA (e.g., apurinic sites) - arise occasionally in all cells in the absence of any added agent

Wild type

-The phenotype most commonly observed in natural populations; also refers to the individual with that phenotype. -In general, the more prevalent form of a gene and its associated phenotype.

The diagram illustrates the genetic changes that result from different types of mutations. Match the letter of each type of mutation with the appropriate name.

A = silent; B = missense; C = nonsense, D = frameshift

Missense mutation

A base-pair substitution that results in a codon that codes for a different amino acid. Missense mutations play a very important role in providing new variability to drive evolution because they often are not lethal and therefor remain in the gene pool.

SOS response

A complex, inducible process that allows bacterial cells with extensive DNA damage to survive, although often in a mutated form; it involves cessation of cell division, upregulation of several DNA repair systems, and induction of translesion DNA synthesis.

RecA

A protein that functions in DNA repair and recombination processes. Homologues of RecA have been found in all three domains of life.

Insertion sequences (IS elements)

A transposable element that contains genes only for those enzymes, such as transposase, that are required for transposition.

Direct repair

A type of DNA repair mechanism in which a damaged nitrogenous base is returned to its normal form (e.g., conversion of a thymine dimer back to two normal thymine bases).

Translesion DNA synthesis

A type of DNA synthesis that occurs during the SOS response. DNA synthesis occurs without an intact template and generates mutations as a result.

F' plasmid

An F plasmid that carries some bacterial genes and transmits them to recipient cells when the F' cell carries out conjugation. - A cell containing an F' plasmid retains all of its genes, although some of them are on the plasmid.

Which three of these statements are correct about nonsense mutations?

By definition, they always produce stop codons; Effects range from mild to complete loss of normal gene function; They cause the early termination of translation.

Hfr conjugation

Conjugation involving an Hfr strain and an F- strain.

Which two statements correctly describe missense mutations?

Effects range from no change to complete loss of normal gene function; They involve a single base substitution that changes a codon for one amino acid into another.

Recombinases

Enzymes that catalyze integration and excision of DNA segments during site-specific recombination.

F+ x F- Conjugation

F+ (fertile) plasmid contains genes required for conjugation process. Bacteria without this plasmid are termed F- (nonfertile). Plasmid is replicated and transferred through pilus from F+ cell. Plasmid DNA only.

transposable elements

Genetic element that has the ability to move (transpose) from one site on a chromosome to another. - "specific DNA segments that can repeatedly insert one or more sites or into one or more genomes"

Which two of the following are disadvantages to DNA proofreading?

It does not correct every error in replication; It cannot correct induced mutations.

Base analogues

Molecules that resemble normal DNA nucleotides and can substitute for them during DNA replication, leading to mutations. Also used as antiviral agents.

Transition mutations

Mutations that involve the substitution of a different purine base for the purine present at the site of the mutation or the substitution of a different pyrimidine for the normal pyrimidine.

What effect would you expect from a mutation in the -35 region of the lac operon promoter?

No expression regardless of the carbon source available

Recombinants

Organisms produced following a recombination event.

site-specific recombination

Recombination between two DNA molecules that does not involve extensive areas of homology between the two molecules. It is catalyzed by enzymes called recombinases distinct from RecA, and it is the mechanism by which transposition occurs.

F' conjugation

Result when the F factor incorrectly leaves the host chromosome Some of the F factor is left behind in the host chromosome Some host genes have been removed along with some of the F factor these genes can be transferred to a second host cell by conjugation

The diagram shows which of the following?

Slippage leading to an insertion

F factor

The fertility factor, a plasmid that carries genes for bacterial conjugation and makes its Escherichia coli host the gene donor during conjugation. The F factor has several IS elements that assist plasmid integration into the host cell's chromosome.

What would be the effect of a mutation in the operator sequence of the lac operon, so that the lac repressor protein could not bind?

The operon would be continuously transcribed.

Horizontal (lateral) gene transfer (HGT)

The process by which genes are transferred from one mature, independent organism to another. In bacteria and archaea, transformation, conjugation, and transduction are the major mechanisms by which HGT can occur. - HGT is distinctive from vertical gene transfer because genes from one independent, mature organism are transferred to another mature organism, often creating a stable recombinant having characteristics of both the donor and the recipient.

Transposable elements are of interest for many reasons:

Their movement into a chromosome can alter gene function either by causing mutations in the genes into which they move or by providing regulatory sites that change the expression patterns of the adjacent genes.

Why are transversion mutations more rare than transition mutations?

There are steric problems with pairing purines with purines.

Which two of these statements are correct about frameshift mutations?

They usually yield mutant phenotypes; They usually result in the synthesis of nonfunctional proteins.

True or false: A mutation can prevent a gene from being expressed, even if the coding sequence is completely unchanged.

True

True or false: DNA proofreading enzymes do not always correct replication mistakes.

True

True or false: Lethal mutants can be recovered only if they are conditional mutants.

True

Hfr strain

Where an F plasmid is integrated into the bacterial chromosome at a specific site

Competent cell

a cell that can take up free DNA fragments and incorporate them into its genome during transformation

plasmid

a double-stranded DNA molecule that can exist and replication independently of the chromosome. A plasmid is stably inherited but is not requires for the host cell's growth and reproduction.

Prototroph

a microorganism that requires the same nutrients as the majority of naturally occurring members of its species.

Transposon

a mobile genetic sequence that contains the genes for transposition as well as one or more other genes not related to transposition.

Rolling-circle replication

a mode of DNA replication in which the replication fork moves around a circular DNA molecule, displacing a strand to give a 5' tail that is also copied to produce a new double-stranded DNA

Suppressor mutation

a mutation at a different site that overcomes the effect of a forward mutation and produces the normal phenotype. The different site can be within the same gene (intragenic suppressor) or different gene (extragenic suppressor) as the forward mutation.

Select two examples of mutations that affect regulatory sequences of microbial genes.

a mutation in a promoter so that RNA polymerase cannot bind; a mutation in the lac operator so the lac repressor cannot bind

Conjugative plasmid

a plasmid that carries genes that enable its transfer to other bacteria during conjugation

transversion mutations

a pyrimidine is replaced by a purine or vice versa

Replica plating

a technique for isolating mutants by transferring cells from an agar plate with nonselective medium to a piece of velvet and then onto plates with selective media or environmental conditions.

Mismatch repair

a type of DNA repair in which a portion of a newly synthesized strand of DNA containing mismatched base pairs is removed and replaced, using the parental strand as a template.

Excision repair

a type of DNA repair mechanism in which a section of a strand of damaged DNA is excised and replaced, using the complementary strand as a template. - corrects damage that causes distortions in the DNA double helix.

Oxygen free radicals convert guanine to 8-oxo-7,8-dihydrodeoxyguanine, which then forms an abnormal base pair with ____.

adenine

Auxotroph

an organism with a mutation that causes it to lose the ability to synthesize an essential nutrient; because of the mutation, the organism must obtain the nutrient or a precursor from its surroundings.

Induced mutations

are the result of exposure to a mutagen, which can be either a physical or a chemical agent

Nucleotide excision repair

can remove thymine dimers and repair almost any other injury that produces a detectable distortion in DNA

Forward mutation

change from wild type to mutant

Silent mutations

change the nucleotide sequence of a codon but do not change the amino acid encoded by that codon. This is possible because the genetic code exhibits degeneracy.

Reverse mutation

changes a mutant phenotype back to the wild-type phenotype

Mutations in a promoter region that affect the ability of RNA polymerase to initiate transcription include _____.

changing the sequence of the -35 region; changing the spacing between the -35 and -10 regions

Recombinational repair

corrects damaged DNA in which both bases of a pair are missing or damaged, or where there is a gap opposite a lesion; a piece of DNA from a sister molecule is used.

Intercalating agents

distort DNA to induce single nucleotide pair insertions and deletions. These mutagens are planar and insert themselves (intercalate) between the stacked bases of the helix.

Base excision repair

employs enzymes called DNA glycosylases. These enzymes remove damaged or unnatural bases yielding apurinic or apyrimidinic (AP) sites.

Conditional mutations

expressed only under certain environmental conditions (e.g., temperature)

Mutations that arise from the insertion or deletion of base pairs within the coding region of a gene are specifically called _____ mutations.

frameshift

Mutations that result from exposure to a physical or chemical agent (mutagen) are known as _____ mutations.

induced

Transformations

is the uptake of circular or linear DNA from the environment outside the cell and maintenance of the DNA in the recipient cell in a heritable form.

Which of these types of point mutation is most likely to drive evolution because they are not lethal and remain in the gene pool?

missense

The diagram illustrates which type of DNA repair?

missmatch repair

Transition mutations, which result in substitution of the alternate purine for the original purine, or the alternate pyrimidine for the original pyrimidine, are __________ common than transversion mutations.

more

Generalized transduction

most often occurs during the lytic cycle of virulent phages but sometimes happens during the lytic cycle of temperate phages. Any part of the bacterial genome can be transferred after being partially degraded as the virus takes control of its host.

DNA modifying agents

mutagens that change a base's structure and therefore alter its base-pairing specificity

nonsense mutation

mutation that converts a sense codon (one that codes for an amino acid) to a nonsense codon (a stop codon; one that does not code for an amino acid). This causes the early termination of translation and therefore results in a shortened polypeptide.

Frameshift mutation

mutation that involves the insertion or deletion of a nucleotide in the coding region of a gene. Frameshift mutations are usually very deleterious and yield mutant phenotypes resulting from the synthesis of nonfunctional proteins.

Specialized transduction

only specific portions of the bacterial genome are carried by transducing particles

Which type of chemical agent converts guanine to 8-oxo-7,8-dihydrodeoxyguanine?

oxygen free radicals

Episomes

plasmids that can exist either with or without being integrated into host chromosomes.

Silent, missense, nonsense, and frameshift are all common types of _____ mutations.

point

Transposase

recognizes the ends of the IS and catalyzes transposition

homologous recombination

recombination involving two DNA molecules that are very similar in nucleotide sequence

A mutation in the operator site of the lac operon that prevents the lac repressor protein from binding, is considered a mutation in a _____.

regulatory sequence

Resistance mutant

resistance to pathogen, antibiotic or chemical agent

Point mutations that change the nucleotide sequence of a codon but do not change the amino acid encoded by that codon are called _____ mutations.

silent

A mutation that caused the codon CGU to change to CGC, which does not affect the amino acid encoded, could be described by which two of these terms?

silent and point

Most common types of point mutations?

silent, missense, nonsense, frameshift

Which of the following is depicted in the diagram?

slippage leading to a deletion

Mutations that arise in the absence of any added agent are _____ mutations

spontaneous

Mutations that result from the activity of mobile genetic elements called transposons are considered to be which type of mutations?

spontaneous

Which two of these missense mutations would be likely to have a significant effect on the function of the protein encoded by the mutated gene?

substitution of an amino acid located at the active site of an enzyme; replacement of a nonpolar amino acid with a polar amino acid

Proofreading

the ability of enzymes to check their products to ensure that the correct product is made. For instance, DNA polymerase checks newly synthesized DNA and replaces an incorrect nucleotide with the correct one prior to further synthesis.

Recombination

the genetic process by which one chromosome breaks off and attaches to another chromosome during reproductive cell division

Transposition

the movement of a piece of DNA around the chromosome

When the mismatch repair system of E. coli detects mismatched bases, it is able to determine the strand that needs repair, because _____ is methylated.

the old strand

Replicative transposition

the original transposon remains at the parental site on the chromosome and a copy is inserted at the target DNA site

Photoreactivation

the process in which blue light is used by an enzyme to repair thymine dimers in DNA.

Transduction

the transfer of genes between bacterial or archaeal cells by viruses

Vertical gene transfer

the transfer of genes from a parent organism to its offspring

Missense mutations provide variation within a population and drive evolution because _____.

they are not lethal and remain in the population gene pool

Nucleotide substitutions that result in purine for purine or pyrimidine for pyrimidine mutations are known as _____ mutations.

transition

Possible causes of spontaneous mutation include:

transposons; errors in DNA replication; spontaneous lesions in DNA

Nucleotide substitutions that result in purine for pyrimidine, or pyrimidine for purine mutations, are known as _____ mutations.

transversion

Sex pilus

used to establish contact between the F+ and F- cells

The most prevalent form of a gene, and its associated phenotype, is called the _______.

wild type