1. Define the following terms and
utilize these terms in an appropriate context:
acridine dye alkylating
agent base analogue
DNA ligase
DNA polymerase excision repair
frameshift mutation induced mutation
lethal mutation mismatch repair
nutritional mutation photoreactivation
enzyme
point mutation regulatory mutation
tautomeric shift
temp-sensitive mutation
transposon
2. Distinguish mutations according
to tissues being affected, somatic mutations versus
gametic mutations.
3. Relate the type of mutation to
the effects on the individual as well as those effecting gene
frequencies and the
gene pool.
4. Classify mutations according to effects
on the person receiving such genes as
morphological, biochemical,
lethal, conditional and regulatory.
5. Differentiate between a chromosomal mutation and a gene mutation.
6. Explain why a random mutation is more likely to be deleterious than beneficial?
7. Explain why mutations in a diploid organism are usually recessive.
8. Describe the occurance of spontaneous
mutations that arise naturally as a result of chemical
rearrangements,
tautomeric shifts or as the result of errors occurring during DNA replication.
9. Describe tautomerism and the way in which this chemical event may lead to mutation.
10. Describe and illustrate a point mutation and its effects on a gene sequence.
11. Describe how base analogues and
alkylating agents and tautomeric shifts cause chemical
changes in nucleotides
leading to base-pairing problems resulting in base substitution
mutations (point mutations)
arising following DNA replication.
12. Describe and illustrate a frame shift mutation and its effects on a gene sequence.
13. Catagorize missense and nonsense modifications of a gene sequence.
14. Describe the effect of agents
,such as radiation and acridine dyes, on a gene sequence
with relation to their
ability to cause frame shift mutations.
15. Explain why frameshift mutations are likely to be more detrimental than a point mutation.
16. Contrast the various types of
DNA repair mechanisms known to counteract the effects of
UV light and other DNA damage.
17. Differentiate between excision repair and photoreactivation mechanisms of repair.
18. If given a short wild-type protein
sequence and several mutated versions of this sequence:
-be able to identify
likely mRNA codons relating to the given protein
-be able to identify
the specific and most likely mutational event that created the new mutated
sequence.
Resources: Text Chapter 14,
Cartoon Guide pgs. 79-83; 158-163; 179-184
Mutation and Mutagenesis
Mutation- failure to store genetic
info faithfully; change in stored info
reflected in expression
Mutation includes chromosomal changes and changes within single genes
*Gene mutation- Hugo DeVries 1901 Attention to: classification, origin, induction
Genetics would not be if it weren't for mutation
Classification of
Mutations
Various schemes, not mutually exclusive
-depend on which aspect of mutation being investigated
1. Spontaneous vs. Induced Mutation
All mutations either:
A. spontaneous- no specific agent other than natural forces
-arise randomly, normal chemical processes, background radiation, alterations
in chemical structure
B. Induced- result of influence of an artificial factor
-X-rays, wide range of chemical mutagens
2. Gametic vs. Somatic mutations
A. Gametic- mutation in sex cell (germ-line, germinal mutations)
- hemophilia perhaps best known (sex-linked recessive)
B.Somatic- non sex cells
-changes not passed on to future generations
-mutation in single cell may not impare the organism even if mutation is
detrimental
a. mutation may occur in cell not active or essential
b. even if critical gene affected, thousands of others remain
-greatest impact; if dominant and occur early in devel.
-autosomal recessives require 2 genes for expression
3. Classification on basis of their effect on organism
A. morphological mutation (visible)
-variations deviate from normal or wild-type phenotype
B. Nutritional or biochemical variations
-bacteria, fungi (AMP+), synthesis of amino acids or vitamins
-Hemophilia, sickle cell anemia
*not visible and do not usually affect specific morphological characters;
more general effect on well being
C. Behavioral mutations
-affect behavior patterns of an organism (mating behavior, circadian rhythms)
-often difficult to discern
Ex. mating behavior of fruit fly impared because it can't beat its wings
??flight muscle, nerve ending, brain???
D. Regulatory mutations
-may disrupt transcription, permanently activate or inactivate a gene
E. Lethal mutations
-Huntingtons disease, Tay Sachs
-severity does not allow individual to survive
-inactivating gene for RNA polymerase
Haploids die immediately- no wild type gene to mask
Diploids can carry lethal if masked by dominant (most are recessive,
however
Huntingtons dom.)
F. Conditional mutations
-affect mutant under certain conditions
Ex. Temperature sensitive mutation
-Permissive temps: mutant gene functions normally
-Restrictive temps: lose functional ability
Effects of Mutation on DNA
1. Point mutation
A. missense- alters base creating a change of an amino acid (missense because
not
the same original message)
B. nonsense- original codon converted to termination codon
-simplest change is single base (nucleotide)
Transition- purine replaces purine or pyrimidine replaces pyrimidine
Transversion- purine replaces pyrimidine or visa versa (*more damaging)
Changes in third pair of codon:
if pur to pur or pyrm to pyrm- likely to code for same amino acid
if purine to pyrimidine- likely to be different AA
2. Frame shift Mutation
-insertion or deletion of single nucleotide at any point in gene
-severe, cause change
in reading frame after mutation
THE CAT SAW THE DOG
POINT MUTATION
FRAME SHIFT MUTATION
THE BAT SAW THE DOG
Gain: THE CAT SAW THE ZDO G
THE CAT SAW THE HOG
(addition of Z)
THE CAT SAT THE DOG
THE CMA TSA WTH EDO G
(addition of M)
*missense
Loss: THE CAT SAW TED OD
(loss of original H)
THE ATS AWT HED OG
(loss of original C)
*Nonsense
Molecular basis of Mutation (Chemical)
1. Replication machinery
Mutators- proofreading function lost (alteration of DNA poly III)
2. Tautomeric Shift
(Tautomerization)
-bases of DNA occur in 2 or more forms, differ in proton shift
-Result in base pair changes (Point mutation)
-Biologically important tautomers involve:
Keto-enol pairs for thymine and guanine
Amino-imino pairs for adenine and cytosine
*infrequent tautomer capable of bonding with normally noncomplementary
base.
However, pairing still purine/pyrimidine
End result A=T becomes G=C, of course next round of replication incorporates
the mutation
*A ordinarily pairs w/ T, in its rare tautomeric form A pairs w/ C
3. Deamination- loss of amino group
Cytosine --> deaminated --> carbonyl replaces amino and converts cytosine
to uracil,
therefore, pairs with A not G
*usually corrected by polymerase because U not usually component of DNA
5-methylcytosine - common modified base, pairs w/ G deaminated form pairs
w/ T;
more serious because T is normal component of DNA, therefore, not corrected
"Hot spots"
4. Base analogues
-mutagenic molecules that can substitute for purines or pyrimidines
during nucleotide
and DNA synthesis
-resembles Thymine
-causes A=T to G=C transition
5. Nitrous Acid, Hydroxylamine
6. Alkylating Agents
electrophilic- attracted to negative charges
DNA- each nucleotide contains full and partial negative charges
Electrophiles attack and add alkyl groups (CH3 or CH2-CH3)
Mustard Gases, most environmental carcinogens (breaks in nucleotides, mispairing)
7. Acridine Dyes --->
frame shifts
-about same dimension as nitrogenous bases
-intercalate, wedge between purines and pyrimidines of intact DNA. Cause
additions
and deletions
Proflavin, acridine orange
-mispairing- looping
Radiation induced mutation
High energy- Gamma radiation, X-ray (ionizing radiation)
U.V.- shorter wavelength than visible light
Ultraviolet light
-purines and pyrimidines absorb UV most intensly at 260nm
-major effect on pyrimidines; addition of H2O to ring
1. pyrimidine dimers formed, usually between 2 thymines
2. dimers distort DNA conformation and inhibit normal replication
-UV ordinarily not strong enough to penetrate layers of skin
-Xeroderma pigmentosum- autosomal recessive in humans, predisposes
individuals to epidermal pigment abnormalities
1. exposure to sunlight results in malignant growths on skin
High Energy- Gamma, X-ray, cosmic rays
(very short wavelength)
-strong enough to penetrate
deeply into tissues causing ionization of molecules
encountered along the way.
-as X-ray penetrates,
stable molecules become free radicals trail of ions can initiate
chemical reactions that can directly or indirectly effect genetic material
-frequently causes single or double strand breaks
-linear relationship
between radiation dose and induction of mutation
*each doubling of dose produces doubling of mutations
General observations
-intensity of the dose administered seems to make little difference in mutagenic rate.
Ex. 2000-roentgen exposure, either in a single dose or cumulative w/many
smaller
doses produces same effect
(1 roentgen- 2 billion ionizations/cm3)
-certain portions of cell cycle more susceptible
X-rays---> breaks, deletions, translocations, chromosome fragmentation
damage
occurs most readily when chromosomes are greatly condensed:
Mitosis
*also reasoning behind its use as a cancer treatment
DNA Repair
1. Reversion- repair
through mutation pg 286-287
2. Photoreactivation-
-UV induced damage could be partially reversed if cells were briefly exposed
to light in
blue range.
-temp dependent, involves enzymatically controlled mechanism
Photoreactivation enzyme (PRE)- cleaves bonds between thymine dimers.
Enzyme
must absorb a photon of light to cleave dimer.
3. Excision repair-
activated in response to any DNA damage thatdistorts helix
-apurinic or apyrimidinic site where base removed from sugar
-endonuclease recognizes damage and clips out DNA
-DNA polymerase fills gap; DNA ligase seals final nick
4. Proofreading and
mismatch repair
-spontaneous errors occur ~ 1/100,000 ; immediately subject to proofreading.
Thought to increase fidelity to 1/1,000,000
Mismatch repair
1. mismatch must be detected
2. incorrect nucleotide removed
3. replacement w/correct base
*special problem: no distortion of the helix to signal that problem exists like UV
*if detected how is the mismatched base discriminated from the "correct"
DNA Methylation
-methyl group (CH3) is added to adenine on parent DNA
-newly synthesized DNA remains temporarily unmethylated, therefore, new
can be
discerned from old
-methyl added to adenines in sequence 5'GATC 3'
3'CTAG 5'
these occur every 250 bases, usually not far away
5. Recombinational Repair-
responds when damaged DNA has escaped repair
-As DNA replicated polymerase stalls and then skips damage creating gap
-protein directs recombination; segment homologous to damaged segment inserted
into
gap
-resulting gap on donor strand repaired by polymerase and ligase action