Unit 19 - DNA Mutation and Repair

Describe how to test for mutagenicity of a compund.

- Inject mouse or rat with arochlor to stimulate liver enzymes (detoxify or toxify compounds). - Isolate the liver enzymes to mimic a mammalian system. - Mix together the suspected mutagen, a rat liver extract , and a Salmonella strain that cannot synthesize histidine. - The suspected mutagen is omitted from the control sample. - Plate the mixtures onto petri plates that lack histidine. - Incubate overnight to allow bacterial growth. - A large number of colonies suggests that the suspected mutagen causes mutation.

Induced mutations include:

-Deamination -Base altering agents -Base analogs -Intercalating agents -Physical damage

Spontaneous mutations can arise by:

1.Depurination 2.Deamination 3.Tautomeric shift 4. Oxidative DNA Damage 5. Replication Slippage

Base analogs include _________ and ___________.

2-aminopurine and 5-bromouracil

Assume a tautomeric shift occurred, which was followed by two rounds of DNA replication. If the mutation is not repaired, what would be the result?

AT to GC mutation in one of the four daughter molecules

The _________ uses a His- strain of Salmonella typhimuriu and monitors the rate of the reverse mutations of His- to His+.

Ames test

Which of the following types of DNA repair would involve the enzyme AP endonuclease?

BER (base excision repair)

The Ames test was developed by _________.

Bruce Ames

A _________ changes the wild-type genotype into some new variation.

Forward mutation

What are examples of triplet disorders?

Fragile X, Huntington, myotonic dystrophy

___________ is a somatic mutation that results in different colored eyes.

Heterochromia

Discuss the consequences of a germ-line versus a somatic mutation.

If a mutation within the germ line is passed to an offspring, all the cells of the offspring's body will carry the mutation. A somatic mutation affects only the somatic cell in which it originated and all of the daughter cells that the somatic cell produces. If a somatic mutation occurs early during embryonic development, it may affect a fairly large region of the organism. Because germ-line mutations affect the entire organism, they are potentially more harmful (or beneficial), but this is not always the case. Somatic mutations can cause quite harmful effects such as cancer.

Base altering agents (ex. alkylating agents) include:

Mustard gas, EMS

_________ are alterations in genetic material that provide allelic variations.

Mutations

Which of the following enzymes participate in the base excision repair system of humans?

N-glycosylase Flap endonuclease AP endonuclease

Why is rat liver extract used in the Ames test for mutagenicity?

Rat liver contains enzymes that can activate the mutagen.

In the Ames test for mutagenicity, the control sample contains what?

Rat liver extract + Salmonella bacteria that cannot synthesize histidine

__________ is the formation of a Hairpin in a Trinucleotide Repeat.

Replication slippage

A ___________ changes a mutant allele back to the wild-type.

Reserve mutation

__________ result from abnormalities in cellular/biological processes.

Spontaneous mutations

Mutations can be caused in two main ways: (1) _____________ mutations are the result of natural biological or chemical processes; and (2) ____________ mutations are produced by environmental agents.

Spontaneous; induced

Tautomeric shifts can result in ____________ base pairing.

TG/CA

Base altering agents results in TS or TV?

TS

Tautomeric shifts cause TS or TV?

TS

Does oxidative DNA damage result in TS or TV?

TV

Intercalating agents result in TS or TV?

TV

A _________ is a temporary change in base structure.

Tauntomeric shift

In the mismatch repair system, how is the newly-made daughter strand distinguished from the parental strand?

The parental strand is methylated, while the daughter strand is not.

You are using the Ames test to assay the mutagenicity of a compound. However, you forgot to add the rat liver extract. What can you say about the results?

The results are less relevant to mammals

Which of the following human diseases is associated with a high predisposition to skin cancer because of an inability to repair UV-induced lesions?

Xeroderma pigmentosum

The common, stable form of adenine and cytosine is the ________ form. Rarely, A and C can convert to an _______ form.

amino, imino

Some trinucleotide repeat disorders get progressively more severe in future generations. This phenomenon is termed ___________.

anticipation

Nucleotide excision repair (NER) is the main system used in the repair of ______.

bulky, helix distorting lesions

Tautomeric shifts are when a nucleotide can change ____________ .

conformation

UV light and other ionizing radiations damage DNA molecules by __________.

creating thymine dimers between adjacent thymines in the DNA chain

Breakage of a glycosidic bond is called ___________.

depurination

Alkylating agents cause mutations by ______.

disrupting the normal pairing between nucleotides within the DNA

A chemical agent is being assayed for mutagenicity using the Ames test. This agent would be judged to be a mutagen if the number of bacterial colonies on the test plate is ______.

significantly higher than that of the control plate

Errors in DNA replication are examples of ______ mutations, while mutations caused by ultraviolet light are ______ mutations.

spontaneous ; induced

Describe the steps in the base excision repair (BER) system.

DNA glycosylase recognize abnormal bases in DNA and removes it, leaving an AP site. AP endonuclease recognizes AP site and cuts the backbone to remove it. It is filled in by DNA polymerase and ligase

The removal of an amino group from a base is called ___________.

Deamination

Hairpin loop of template results in ___________.

Deletion

The removal of a purine from DNA is called __________.

Depurination

Hairpin loop of new strand results in ____________.

Insertion

A new mutagen is able to cause frameshift mutations. Which kind of mutagen would be capable of reversing this mutation?

Intercalating agent

___________ result in the distortion of double helix and include ethidium bromide and proflavin.

Intercalating agents

Alkylating agents covalently attach _________ or _________ groups to DNA bases, and thus disrupt their normal base pairing properties.

Methyl or ethyl

Individuals with Xeroderma pigmentosum have defects in which repair pathway?

NER

Human disorders result from mutations of DNA repair enzymes. Many are mutations in ___________.

NERs

__________ mutations are much more likely to be harmful than beneficial to the individual and often are the cause of diseases.

Negative side

A __________ mutation is when a new mutation simply results in a variation with no impact on the organism.

Neutral

What are the three types of mutations?

Neutral, positive side, and negative side

The enzyme that can recognize thymine dimers and split them is called _________.

Photolyase

___________ mutations are the foundation for evolutionary change needed for a species to adapt to changes in the environment.

Positive side

For a tautomeric shift to cause a mutation, it must occur ______.

Prior to DNA replication

Is a random mutation more likely to be beneficial or harmful? Explain your answer.

Random mutations are more likely to be harmful than beneficial. The genes within each species have evolved to work properly. They have functional promoters, coding sequences, terminators, and so on, that allow the genes to be expressed. Mutations are more likely to disrupt these sequences.

Ultraviolet light causes the formation of crosslinked _____________ in DNA.

Thymine dimers

A(n) ___________ is a type of base substitution that involves a change of a pyrimidine to another pyrimidine, or a purine to another purine.

Transition

A base substitution in which a purine and a pyrimidine are interchanged is called a(n) ___________ mutation.

Transverse

The phenomenon in which a repeated sequence of three nucleotide bases increases in number generation after generation is called ____________.

Trinucleotide repeat expansion

__________ is a group of (mostly autosomal dominant) inherited disorders with variable traits including white hair patch, hearing loss, different color eyes. The inherited gene is pax3.

Waardenburg syndrome

If an individual possesses a germ-line mutation, then ______ produced by the individual will carry the mutation. If an individual possesses a somatic mutation, then ______ produced by the individual will carry the mutation.

half of the gametes ; none of the gametes

Trinucleotide repeat expansions are repeated sequences of three nucleotide bases which ______.

increase generation after generation

The protein alkyltransferase can only be used once because it ______.

is inactivated by the addition of a methyl group

The common, stable form of thymine and guanine is the _______ form. Rarely, T and G convert to an ________ form.

keto, enol

You wish to test a new pesticide to determine its potential as a carcinogen using the Ames test. You detect significantly more bacterial colonies on the treatment plate than the control plate. This indicates that _____.

the chemical tested is mutagenic and probably also carcinogenic

During mismatch repair, the template strand must be distinguished from the newly synthesized strand. In prokaryotes the template strand is __________.

the methylated strand of a hemimethylated duplex

The strand that is repaired in the mismatch repair system is ______.

the nonmethylated strand

Deamination of 5-methyl cytosine results in ____________ .

thymine

Mutations can involve:

•Changes in chromosome structure •Changes in chromosome number •Changes in DNA of a single gene