Z-Micro Exam 3- Mutations

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Definition of mutation

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1. (physical or chemical) change in heritable genetic material
2. change in base sequence of DNA

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Effects of Mutations

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1. codon changed
2. may change aa (specified by codon)
3. Result: protein or enzyme with a different amino acid

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Result of a protein/enzyme with an amino acid

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a. (may be) inactive or less active
b. (may possibly ,rarely, have some) new function
i. ex. nylonase!!
4. changing genotype can change phenotype

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Changing genotype can change _________.

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phenotype

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Base substitution or point mutation

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single base at one position is replaced with a different
(silence, missense, nonsense mutations)

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missense mutation

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amino acid substitution
codon change:
UUG---> UCG
(a.a. specified) Leu--> Ser

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silent mutation (synonymous mutation)

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codon change: UUG -->UUA
(a.a. specified) Leu -->Leu

“For every change which alters an amino acid (a "non-synonymous" change), there are 8 that do not(i.e. "synonymous" changes).”

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nonsense mutation

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change to termination codon before end of protein
codon change: UUG--> UAG
(a.a. specified) Leu -->Stop

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Frameshift mutation


OP (Fig 8.17d)

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a. 1 or 2 base pairs are inserted or deleted from DNA

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What is an indel? Effect? Consequences?

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i. indel: (An indel) is a single insertion or deletion of a stretch of nucleotides in a gene
b. (this) shifts the triplet reading frame changing all codons “downstream”
c. consequences:
i. leads to long stretch of missense
ii. almost always disastrous
iii. exception: ex. nylonase bacteria that can now digest nylon

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Are mutations always bad?

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Most mutations are neutral, some are harmful, and a smaller number are beneficial. The whole point of Darwin's great idea, though, is that there are mechanisms which can select for and increase the frequency of the beneficial mutations over time, while winnowing out the harmful ones.”

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What's Hodge's favorite example of a mutation producing new information?

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"My favorite example of a mutation producing new information involves a “Japanese” bacterium that suffered a frame shift mutation that just happened to allow it to metabolize nylon waste. The new enzymes are very inefficient (having only 2% of the efficiency of the regular enzymes), but do afford the bacteria a whole new ecological niche. They don't work at all on the bacterium's original food - carbohydrates. And this type of mutation has even happened more than once!"

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Cause of Mutations

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mutagenesis (a good way to kill bugs)
1. (some are) spontaneous = without obvious cause
2. chemical mutagens (good way to kill bugs)
3. radiation

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Types of Chemical Mutagens

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nitrous acid (HNO2) (used in chemical warfare)

base analogs

frameshift mutagens = aflatoxin

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What does nitrous acid (HNO2) (used in chemical warfare, a chemical mutagen)

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i. convert A into a form that acts like G
ii. this is a base pair mutagen

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What are some base analogs (a chemical mutagen) ?

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AZT (drug used to treat AIDS patients)
 works by competitive inhibition on reverse transcriptase (RNA DNA)
 AZT is a nucleoside analog

ii. 5 bromouracil

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Example of Frameshift Mutation (a chemical mutagen)

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Aflatoxin
i. Aspergillus flavus attacks peanuts & grain (moldy)
ii. (at 1 time it) was mass produced in Iraq; fear is only immediate effect

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How is radiation useful?

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Radiation is useful as control agent as effects are mutagenic.

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Types of radiation

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Ionizing = x-rays & gamma rays cause physical breaks in DNA


Ultraviolet light (nonionizing)
i. causes formation of thymine dimers OP (Fig 8.20)
 covalent bonds between 2 adjacent thymines in DNA strand
 can be cut out & repaired
ii. some bacteria are capable of photoreactivation = process in which visible light triggers enzyme mediated repair of UV damage

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Ways to Identify Mutants

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positive (direct) selection
negative (indirect) selection

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Positive (direction) selection of mutants

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a. detect mutant cells by elimination of non-mutated parent cells
b. (penicillium or) amicillin resistant mutants – how to select (for these)?

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Negative (indirect) selection of mutants

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a. detect a cell that has lost a certain metabolic function
b. replica plating OP (Fig 8.21)
example: parent His+ makes histidine; mutant His– can't make histidine
i. start with master plate of hundreds of colonies growing on complete medium with His
ii. using sterile velvet pad transfer to medium without His & incubate
iii. growth on plate with histidine but no growth on plate without histidine = mutant

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2 ways to demo mutagenicity:

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a. “normal” convert to mutant (or)

b. mutant convert to “normal”

c. which is easier to ID? Why?

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Ames Test

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(developed by Brue Ames at UC Berkeley)
1. (One simple) test for determining whether or not a particular chemical agent is carcinogenic
2. (Since) most carcinogens are mutagens, (and mutagens are easier to ID than carcinogens) once a mutagen is identified, (you can) presume it is (likely a) carcinogen

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What's the easy way and the hard way to identify a mutagen?

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a. if a chemical converts a specific mutant back to normal at higher than background rate, that chemical is a mutagen (easy)
b. alternative: expose normal cell to chemical mutant (hard)

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prototroph

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wild type cell with normal metabolism

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auxotroph

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mutant of wild type (prototroph) that has some nutritional deficiency.

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example of auxotroph

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(Examples
i. His– (genotype) = histidine auxotroph (because it) can’t synthesize the a.a. histidine
 can His– grow without supplement of His? Why?
Mutations –Pg. 4
ii. Since we are Vit C–, can we thrive without vitamin C? (interesting pseudogene...)

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If chemical increases rate of conversion of auxotrophs to prototrophs, chemical is ____

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a mutagen & is a suspect carcinogen

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See details of AMES test

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Look at Mutation notes Page 4