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Dr. Attya Bhatti
The Modern Concept
 Charles Darwin recognized that variation existed in populations and
suggested natural selection as a mechanism for choosing some variants
over others resulting in survival of the fittest and gradual changes in
populations of organisms.
 Without a mechanism for generation of new variation, populations
would be selected into a corner where only one variation would survive
and new species could never arise.
 The Modern Synthesis combines the mechanism of mutation in DNA to
generate variation with natural selection of individuals in populations to
produce new species.
Mutations
 A heritable alterations or change in the genetic material.
 Biologists use the term “mutation” when talking about any change in
the genetic material. Not all result in a change in phenotype.
 Polymorphism
Mutations
 Somatic Mutations; occur in somatic cells and only affect the
individual in which the mutation arises. can not transmitted to
offspring.
 Gonadal/ Gamete Mutations; Germ-line mutations alter gametes
and passed to the next generation.
Mutations are quantified in two ways:
 Mutation rate = probability of a particular type of mutation per unit
time (or generation).
 Mutation frequency = number of times a particular mutation occurs
in a population of cells or individuals.
Mutation Rates
 The frequency with which a gene changes from the wild type to a mutant
is referred to as the mutation rate.
 Generally expressed as the number of mutations per biological unit,
which may be mutations per cell division, per gamete, or per round of
replication.
 For example, the mutation rate for achondroplasia (a type of
hereditary dwarfism) is about four mutations per 100,000 gametes,
usually expressed more simply as 4 x 10-5.
Mutation frequency is defined as the incidence of a specific type
of mutation within a group of individual organisms.
 For achondroplasia, the mutation frequency in the United States
is about 2 X 10-4, which means that about 1 of every 20,000
persons in the U.S. population carries this mutation.
Factors affecting Mutation rate
1. They depend on the frequency with which primary changes take place in
DNA. Primary change may arise from spontaneous molecular changes in
DNA or it may be induced by chemical or physical agents in the
environment.
2. A second factor influencing the mutation rate is the probability that,
when a change takes place, it will be repaired.
Most cells possess a number of mechanisms to repair altered DNA;
so most alterations are corrected before they are replicated.
 If these repair systems are effective, mutation rates will be low; if
they are faulty, mutation rates will be elevated.
 Some mutations increase the overall rate of mutation at other
genes; these mutations usually occur in genes that encode
components of the replication machinery or DNA repair enzymes.
3. A third factor, one that influences our ability to calculate
mutation rates, is the probability that a mutation will be
recognized and recorded.
 When
DNA is sequenced, all mutations are potentially
detectable. In practice, however, sequencing is expensive; so
mutations are usually detected by their phenotypic effects.
 Some mutations may appear to arise at a higher rate simply
because they are easier to detect.
Causes of Mutations
Mutation arises in two ways

Errors in DNA Replication

Mutagens
Causes of Mutations
1.Spontaneous mutations (natural) and rare: 2-12X 10-6 (per generation / gene)
2. Induced mutations caused by mutagens.
Tautomerism - A base is changed by the repositioning of a hydrogen atom.
An example is 5-bromo-deoxyuridine (5BU), which can exist in two
tautomeric forms: typically it exists in a keto form (T mimic) that pairs
with A, but it can also exist in an enol form (C mimic) that pairs with G.
Depurination - Loss of a purine base (A or G).
Deamination - Changes a normal base to an atypical base; C → U, (which can
be corrected by DNA repair mechanisms),
Transition - A purine changes to another purine, or a pyrimidine to a
pyrimidine.
Transversion - A purine becomes a pyrimidine, or vice versa.
Errors in DNA Replication
Induced mutations on the molecular level can be caused by:
•Exposure to X-ray, UV light (Radiations)
•Chemical treatment: base analogs 5’-bromouracil (=T or rarely C)
hydroxylating agent (add OH-group to C)
alkylating agent such as EMS (ethylmethane sulfonate)
deaminating agent such as nitrous acid
intercalating agent such as Acridine Orange
•Transposons that insert into a gene and disrupt the normal reading
frame
Ionizing radiations
X-rays and gamma rays damage DNA by dislodging electrons from
atoms; these electrons then break phosphodiester bonds and
alter the structure of bases.
Ionizing radiation causes three types of damage to DNA
 Single-strand breaks - mostly sealed by DNA ligase
 Double-strand breaks - often lethal because can't be resealed by
ligase so degraded by nucleases
 Alteration of bases - this type of oxidative damage is usually lethal
because forms a replication barrier at that site
Radiation
 UV radiation – 260-280 nm is wavelength at which maximum
absorption occurs for DNA.
 Ultraviolet radiation (non-ionizing radiation) excites electrons to
a higher energy level. DNA molecules are good absorbers of
ultraviolet light.
 Two nucleotide bases in DNA - cytosine and thymine-are most
vulnerable to excitation that can change base-pairing properties.
 UV light can induce adjacent thymine bases in a DNA strand to
pair with each other, as a bulky dimer.
 DNA has so-called hotspots, where mutations occur up to 100
times more frequently than the normal mutation rate. A hotspot
can be at an unusual base, e.g., 5-methylcytosine.
Mutation rates also vary across species.
Chemical mutagens
 Compounds that increase the frequency of some types of
mutations.
 They vary in their potency since this term reflects;
 Their ability to enter the cell,
 Their reactivity with DNA,
 Their general toxicity, and
 The likelihood that the type of chemical change they introduce
into the DNA will be corrected by a repair system
Chemical Mutagens
 Nitrosoguanidine (NTG), Hydroxylamine NH2OH
 Base analogs (e.g. BrdU)
 Simple chemicals (e.g. acids)
 Alkylating agents (e.g. N-ethyl-N-nitrosourea (ENU)) These agents can mutate both
replicating and non-replicating DNA. In contrast, a base analog can only mutate the
DNA when the analog is incorporated in replicating the DNA. Each of these classes
of chemical mutagens has certain effects that then lead to transitions,
transversions, or deletions.
 Methylating agents (e.g. ethyl methanesulfonate (EMS)
 Polycyclic hydrocarbons (e.g. benzopyrenes)
 DNA intercalating agents (e.g. ethidium bromide)
 DNA crosslinker (e.g. platinum)
 Oxidative damage caused by oxygen (O) radicals
 A major problem with chemically-induced and irradiation-induced
mutations, however, is that they are generated essentially at
random. In order to identify a mutant phenotype of interest, a
laborious screen for mutants needs to be conducted by close
examination of the phenotypes following mutagenesis.
 Transposons that insert into a gene and disrupt the normal
reading frame
Ames test
 The Ames test is based on the principle that both cancer and
mutations result from damage to DNA.
 The Ames test uses his- strains of bacteria to test chemicals for
their ability to produce his-__his+ mutations. Because mutagenic
activity and carcinogenic potential are closely correlated, the Ames
test is widely used to screen chemicals for their cancer-causing
potential.