Lesson 9 Gene Mutation and DNA Repair Flashcards Preview

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A change of a pyramidine to another pyramidine (C to T) or a purine to another purine (A to G)



When a Purine is interchanged with a pyramidine


What can point mutations include

1. Base substitution
2. A short sequence of DNA may be deleted or added to the chromosomal DNA


Suppressors or suppressor mutations

Second mutation that affects the phenotypic expression of a first mutation. Acts to suppress the phenotypic effects of another mutation.


What are the two common reasons for position effects?

1. Gene has been relocated next to regulatory sequences from another
2. Gene has been relocated to a chromosome that has a different packing structure.


What three chemical changes can result in spontaneous mutations?

Depurination, damnation, and tautomeric shifts



Covalent bond between a purine base breaks and creates an apurinistic site. It this occurs just prior to DNA replication, it can cause a mutation.



Primarily occurs on the cytosine base. If it occurs on a methylcytosine, it creates rhyming, making it difficult for DNA repair enzymes to distinguish the correct base from an altered base.


tautomeric shifts

Changes between Kato and Enola, or between amino and imino forms of the bases. Though these shifts are rare and transient, they may cause mutations if they occur just prior to DNA replication.


Experiment 16A: environmental agents as mutagens

The exposure of flies to X rays will increase the rate of mutation.


Experiment 16A: environmental agents as mutagens
Starting material

Female flies contain one normal X chromosome and a CIB X chromosome. The male flies contain a normal X chromosome.


Experiment 16A: environmental agents as mutagens

1. Expose male flies to X rays. Have control group that is not exposed to X rays.
2. Mate the male flies to female flies carrying one normal X chromosome and one CIB X chromosome
3. Save about 1000 daughters with bar eyes (these have CIB X from mother and X from father that may or may not have a recessive lethal mutation)
4. Mate each bar-eyed daughter with normal (nonirradiated) meals. This is done in 1,000 individual tubes.
5. Count the number of crosses that do not contain any male offspring. These crosses indicate that the bar-eyed female parent contained an X-linked lethal recessive mutation on the nonCIBX chromosome.


Experiment 16A: environmental agents as mutagens
Interpreting the data

In the absence of X ray treatment, only 1 cross in approximately 1,000 was unable to produce male offspring. This means that the spontaneous rate for any X-linked lethal mutation was relatively low. By comparison, X-ray treatment of the fathers that gave rise to these CIB females produced 91 crosses without male offspring. These females inherited their non-CIB chromosome from irradiated fathers, the results indicate that X rays greatly increase the rate of X-linked, recessive lethal mutations.


pericentric inversion

If the centromere lies within the inverted region of the chromosome, this is the inverted region


Parametric inversion

If centromere is found outside the inverted region, the inverted region is parametric


Two types of mutagens

Chemical and physical


What can activate a mutagen inside the body?

Cellular enzymes such as oxidases


Nitrous acid mutations

Nitrous acid replaces amino groups with keto groups (-NH2 to =O). This changes cytosine to uracil and adenine to hypoxanthine. When this mutated DNA replicates, the modified bases do not pair with the appropriate nucleotides in the newly made strand. Instead, uracil pairs with adenine, and hypoxanthine pairs with cytosine.


What does nitrous acid change cytosine to?



What does uracil pair with?



What does nitrous acid change adenine to?



What does hypoxanthine pair with?



alkylating agents

alkylate bases within DNA. During alkylation, methyl or ethyl groups are covalently attached to the bases.


acridine dyes

Interfere with DNA replication process. Contain flat planar structures that interchelate into the double helix by sandwiching between adjacent base pairs, thereby distorting the helical structure. When this DNA is replicated, single-nucleotide additions and/or deletions can be incorporated into the newly made daughter strand.


5-bromouracil and 2-aminopurine

nucleotide base analogues that become incorporated into daughter strands during DNA replication. 5BU is a thymine analogue that can be incorporated into DNA instead of thymine. 5BU can base pair with adenine, but will also tautomerize and base-pair with guanine. This cause a mutation in which an AT base pair is changed to a G-5BU pair.


UV light

nonionizing radiation. Penetrates only the surface of material such as skin. Known to cause DNA mutation. Causes formation of cross-linked thymine diners. A thymine diner within a DNA strand may cause a mutation when that DNA strand is replicated.


How does Ames test work?

1. Suspected mutagen mixed with a rat liver extract and bacterial cells. Rat liver provides a mixture of enzymes that may potentially activate a mutagen.
2. After incubation period, large no of bacteria are plated on a minimal growth medium that does not contain histidine.
3. Salmonella is not expected to grow on these plates unless a mutation has occurred that allows a bacterium to synthesize histidine. To estimate the mutation rate, colonies that grown on the minimal media are counted and compared with the total no. Of bacterial cells that were originally streaked on the plate.



Yeast cells use this enzyme to repair thymine diners. It can split the diner to restore DNA to its original condition.


O6-alkylguanine alkyltransferase

Removes the methyl or ethyl groups from guanine bases that have been mutagenized by agents such as nitrogen mustards and ethyl methane-suffocate. Alkyltransferase is permanently inactivated so it can only be used one.



Involved in base excision repair. Can recognize an abnormal base and cleave the bond between it and the sugar in the DNA backbone. This releases base and leaves behind an apyrimidinic nucleotide.