Download Study Guide 8 - Bacterial Genetics Chptr 8

Independent Study Guide - Bacterial Genetics (Chapter 8)
I.
Genetic change in bacteria (p. 186-187)
a. Two general mechanisms of genetic change in bacteria (figure 8.1):
i. Mutation – alteration in existing DNA sequence
ii. DNA transfer (horizontal gene transfer) – acquisition of DNA from another source
b. Why study bacterial genetics?
i. Model system
c. Spontaneous mutations occur in all cells at a very low frequency (≈one per billion
nucleotides)
i. Bacteria quickly grow to high concentrations (109/ml) in culture, making it
possible to study rare occurrences
d. Terms
i. Phenotype – the observable characteristics of an organism
ii. Genotype – the sequence of nucleotides in the DNA of an organism
iii. Wild type – characteristics similar to the organism as it occurs in nature
iv. Prototroph – a strain that grows on minimal medium (glucose-salts)
v. Auxotroph – a strain that lacks the ability to synthesize a nutrient; as a
consequence, that compound must be supplied in the growth medium
II.
Spontaneous Mutation (section 8.2)
a. Mistakes made during replication
i. Base substitution (figure 8.3)
1. Silent
2. Missense
3. Nonsense
ii. Removal or addition of nucleotides
1. Frameshift mutation (figure 8.4)
a. Generates an entirely different set of triplets
b. Often, a stop codon is generated
III.
Transposable elements (transposons) (p. 189; p. 207)
a. Segments of DNA that can "jump or hop" from one site to another
i. Insertional inactivation of the gene in which the transposon lands
ii. A transposon can insert elsewhere in the same DNA molecule, or into an entirely
different DNA molecule
iii. Some transposons simply “jump or hop”; others replicate then jump
IV.
Induced Mutation (section 8.3)
a. Chemical mutagens
i. Chemically modify purines and pyrimidines
1. Alter the base-pairing properties
2. Example: nitrous acid strips the amino groups from nucleotides
ii. Base analogs
1. Resemble nucleotide bases; erroneously incorporated into DNA
2. Consequence of incorporation: analog base-pairs with a different
nucleotide
iii. Intercalating agents
1. Insert between base-pairs, pushing nucleotides apart; extra nucleotide
may then be erroneously added during replication
b. Biological mutagens
i. Transposons
c. Physical mutagens
i. Ultraviolet irradiation- causes formation of covalent bonds (thymine dimers)
between adjacent thymine bases
ii. X rays -causes double- and single-stranded breaks in DNA
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V.
DNA Repair (section 8.4)
a. Repairs of errors in base incorporation
i. DNA polymerase
1. Proofreading
ii. Mismatch repair
1. removal/replacement (figure 8.10)
b. Repairs of thymine dimers
i. Light repair (photoreactivation) (figure 8.11a)
ii. Dark repair/Excision repair (figure 8.11b)
c. Repair of modified bases
i. Glycosylase removes oxidized guanine
d. SOS system
i. Induction of SOS system
1. New DNA polymerase
VI.
Mutant Selection (section 8.5)
a. Direct selection (figure 8.13)
b. Indirect selection (replica plating) (figure 8.14)
i. Obtain auxotrophs
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Application of direct selection
i. The Ames Test - used to narrow down list of possible carcinogens (figure 8.16)
Review questions
Considering that mutations are rare events, why are they significant?
How is a missense mutation different from a nonsense mutation?
Which type of single base-pair change would change the amino acid sequence of the encoded
protein most significantly?
Can the genotype change without a change in phenotype?
Can the phenotype change without a change in genotype?
Describe the difference between a prototroph and an auxotroph.
What is a transposon, and how does it cause mutations?
How can a base substitution cause a mutation?
What types of mutations can base substitutions cause?
Explain how intercalating agents cause mutations.
How does UV light cause mutations?
How do X-rays cause mutations?
How are thymine dimers repaired?
What would the consequence be to a cell if it didn't have an SOS system?
What is the purpose of adding liver extract to the substance being examined in the Ames test?
Why is replica plating required to isolate an auxotroph from a prototrophic strain, but not the
reverse?
How is mutation different from horizontal gene transfer?
Why would it be more important for DNA polymerase, rather than RNA polymerase, to have
"proofreading" ability?
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