Mutations:
While mutations are considered in the media to be gigantic monster producing phenomenon
resulting from extreme nuclear radiation and the likes, the reality of mutation is discrete
phenotypic changes, like in the color of an ant
These changes are sometimes involved in color, or slight morphological changes, but we
have yet to create a Godzilla-like mutant organism
Genetic variation has two main causes: mutation and recombination
Mutation causes the rise of new alleles, which are new variants of genes; they slightly
differ from the wild-type in their aspects and resulting phenotype
Recombination is when alleles are shuffled differently during meiosis, which can cause
the passing out of maternal of paternal chromosomes
The balance between mutation and selection allows for adaptation to occur, and is in fact
the basis of evolution
Mutations are changes in a DNA sequence which can lead to an altered coding region or a gene
regulatory sequence
This usually results in an altered gene product and therefore an altered phenotype
o This could mean an altered
protein product, but is could
also cause an altered functional
RNAs; in this course, we will
focus in proteins
Note that the wild-type variant is
arbitrarily defined; for example, the Arabidopsis plant was originally discovered in
Columbia, and therefore the Columbian plant is the wild type, while everything else is a
mutant version
o Therefore, the determine of the wild type merely depends on context
The word mutant is an adjective, so you cant say something is a mutant
Some mutations can be passed on to the next generation while others cannot; it depends on the
type of cells that are mutated
Somatic mutations are on body tissue cells, which means that they will NOT be
heritable
o An example is UV exposure
causing brown patches on the
skin; these skin cells will
replicate and the path will
become bigger, but the
persons children will not be
born brown
Germline mutations fall within the cells that make gametes, and therefore have a chance
of being passed on to the next generation
o Since we are diploid organisms, usually only one chromosome will acquire the
mutation, and therefore only half of the gametes will have the mutant allele o Therefore, only about half of the progeny will acquire the mutation, while the
others will be wild-type
There are some general types of mutations which we will later break down into more specific
groups:
Mutations can either be spontaneous or induced
o Spontaneous mutations are thought to happen in the background, through things
like DNA replication errors, breakage of DNA through oxidative stress, and
background UV that we cant avoid
The majority of these are fixed by cell repair mechanisms
o Induced mutations happen upon exposure to mutation-inducing agents, or
mutagens
These can either be chemical or through radiation
The scale, or size, of the mutation is also important:
o Point mutations only change one or a few nucleotides through addition/deletion
We will focus on these
o Insertion mutations is when large chunks of DNA are inserted into a sequence
through, for example, transposable elements
o Chromosomal mutations involve loss or gain of large bits of an entire
chromosome
In the past, we used to wonder is spontaneous mutations even occur; it was thought that mutation
only occurred to adapt to environmental changes
The Luria-Delbruck test for spontaneous mutation was able to show that mutations do in
fact happen randomly
o 20 samples of bacteria were
placed in a happy medium
and allowed to grow
o All 20 samples were infested
with bacteriophage, and the
experiment was to see how
many bacterial colonies
developed a resistance
If mutation wasnt
random, then similar
resistance patterns should have been seen in all colonies, and the
mutations should have only happened after the addition of phage
If mutation is random, then the mutations could happen at any time
(before or after addition of phage), and the amount of resistance between
samples should vary a lot
o Indeed, after the experiment was taken out, random mutation was observed
Some samples developed the resistance early on, meaning that a large
number of bacteria survived, while other colonies didnt develop any
resistance at all
It has also been seen that the rate of spontaneous mutation increases with the complexity
of the organismo This is thought to happen simply because more complex organisms have more
DNA, i.e. more to go wrong
There are various types of spontaneous mutations:
Replication errors involve tautomeric shifts in
bases allow mispairing between them; electrons
shuffle around randomly, which could cause
temporary chemical shifts in the bases
o This could fool a DNA polymerase into
adding the wrong base randomly, but
since the shift is very short, the wrong
base is usually picked up by proofreading
enzymes
Depurination causes the DNA double helix to
literally lose a purine base, and if left with a hole
known as a apurinic site (AP site)
o Some human cells can lose as many as
10,000 purines per day, but there are
efficient repair mechanisms that repair
these very quickly
o This can also be induced by some
reactive oxygen species, in which the
mutation rate increases dramatically
Deamination occurs upon the removal of an
amine group from a base
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