MUTATION AND MISCELLANY
Updated Summer 2015
by J. D. Hendrix
MUTATIONS: DEFINITIONS
Lethal mutation: results in death of the cell, and therefore cannot be
propagated or studied
Conditional mutation: One that is expressed only under certain
environmental conditions; for example, a temperature-sensitive mutation
Biochemical mutations: result in change in a biochemical pathway of the cell;
for example, an auxotrophic mutation
Spontaneous mutation: one that arises spontaneously due to error during
DNA replication
Induced mutation: one that has been caused by damage resulting from
chemical or radiation treatment (mutagen)
MUTATIONS: DEFINITIONS
Point mutations exchange a single nucleotide for another
These changes are classified as transitions or transversions. Most common is
the transition that exchanges a purine for a purine (A ↔ G) or a
pyrimidine for a pyrimidine, (C ↔ T). Less common is a transversion, which
exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/T ↔
A/G). An example of a transversion is the conversion of adenine (A) into a
cytosine (C).
Silent mutations, which code for the same amino acid.
Missense mutations, which code for a different amino acid.
Nonsense mutations, which code for a stop and can truncate the protein.
MUTATIONS: DEFINITIONS
A frameshift mutation is a mutation caused by insertion or deletion of a number of nucleotides that is not evenly
divisible by three from a DNA sequence. Due to the triplet nature of gene expression by codons, the insertion or
deletion can disrupt the reading frame, or the grouping of the codons, resulting in a completely different translation
from the original
An in-frame mutation is any insertion or deletion that is evenly divisible by three
A nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a nonsense
codon in the transcribed mRNA, and possibly a truncated, and often nonfunctional protein product.
Missense mutations or nonsynonymous mutations are types of point mutations where a single nucleotide is changed to
cause substitution of a different amino acid
A neutral mutation is a mutation that occurs in an amino acid codon that results in the use of a different, but chemically
similar, amino acid. The similarity between the two is enough that little or no change is often rendered in the protein.
For example, a change from AAA to AGA will encode arginine, a chemically similar molecule to the intended lysine.
Silent mutations are mutations that do not result in a change to the amino acid sequence of a protein
MUTATIONS: MECHANISMS
Mechanisms of Induced Mutation
 Base analogs
 A chemical similar in structure to one of the bases and can substitute for it but doesn’t
base pair normally
 For example, 5-bromouracil substitutes for thymine, but it frequently pairs with a G
instead of A
 Specific Mispairing
 When a mutagen changes a base’s structure and thereby alters its base pairing
 For example: nitrosoguanidine adds a methyl group to guanine, causing it to pair with
T instead of C.
MUTATIONS: MECHANISMS
Mechanisms of Induced Mutation (cont.)
Intercalating agents
Examples include acridine orange and ethidium bromide
Flat, planar, aromatic molecules that can fit between the stacked
bases (intercalate) in the center of a DNA double helix and
distort its geometry
The distortion somehow induces single nucleotide insertions or
deletions
MUTATIONS: MECHANISMS
Mechanisms of Induced Mutation (cont.)
Bypass of replication
Chemical change or damage to bases is so severe that the bases
can no longer base pair at all, and cannot serve as a template
Example: formation of thymine dimers by UV irradiation; these
cannot hydrogen bond at all
Normally this would be lethal, but there are repair mechanisms
that can bypass the damaged template, although many of these
repair mechanisms are error-prone so generate a high
frequency of mutations
CIS- AND TRANS-ACTING
REGULATORY ELEMENTS
 Not to be confused with cis and trans linkage
 Cis-regulatory elements (CREs) are regions of non-coding DNA which regulate the
transcription of nearby genes. The Latin prefix cis translates to “on this side”. CREs are
found in the vicinity of the gene, or genes, they regulate. CREs typically regulate gene
transcription by functioning as binding sites for transcription factors. Example: the lac o
gene (lac operator)
 A trans-acting element is usually a DNA sequence that contains a gene. This gene codes
for a protein (or microRNA or other diffusible molecule) that will be used in the
regulation of another target gene.[1] The trans-acting gene may be on the same
chromosome as the target gene, but the activity is via the intermediary protein or RNA
that it encodes. Cis-acting elements, on the other hand, do not code for protein or RNA.
Both the trans-acting gene, and the protein/RNA that it encodes are said to "act in
trans" on the target gene. Example: lac i gene (lac repressor gene)
THE HISTONE CODE
The histone code is a hypothesis that the transcription of genetic information encoded in DNA is in part regulated by
chemical modifications to histone proteins, primarily on their unstructured ends.
Together with similar modifications such as DNA methylation it is part of the epigenetic code.
Histones associate with DNA to form nucleosomes, which themselves bundle to form chromatin fibers, which in turn make
up the more familiar chromosome.
Histones are globular proteins with a flexible N-terminus (taken to be the tail) that protrudes from the nucleosome.
Many of the histone tail modifications correlate very well to chromatin structure and both histone modification state and
chromatin structure correlate well to gene expression levels.
The critical concept of the histone code hypothesis is that the histone modifications serve to recruit other proteins by
specific recognition of the modified histone via protein domains specialized for such purposes, rather than through
simply stabilizing or destabilizing the interaction between histone and the underlying DNA.
These recruited proteins then act to alter chromatin structure actively or to promote transcription.
Modifications to histones include methylation, acetylation, phosphorylation, and ubiquination