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Genetics 2021Chapter 15
gene mutations = A mutation can be defined as an alteration in the nucleotide sequence of an
organism’s genome.
• A mutation may comprise a single base-pair substitution, a deletion or insertion of one or more
base pairs, or a major alteration in the structure of a chromosome
• Mutations can occur in somatic cells or within germ cells.
▪ germ cells are heritable and are the basis for the transmission of genetic diversity and
evolution, as well as genetic diseases.
▪ somatic cells are not transmitted to the next generation but may lead to altered cellular
function or tumors.
Classification Based on Type of Molecular Change
▪ A change of one base pair to another in a DNA-molecule is known as a point mutation, or base
substitution
You will often see two other terms used to describe base substitutions.
1. If a pyrimidine replaces a pyrimidine or a purine replaces a purine, a transition has occurred.
2. If a purine replaces a pyrimidine, or vice versa, a transversion has occurred.
▪ missense mutation = change of one nucleotide of a triplet within a protein- coding portion of a
gene may result in the creation of a new triplet that codes for a different amino acid in the
protein product.
▪ nonsense mutation = the triplet will be changed into a stop codon, resulting in the termination
of translation of the protein.
▪ silent mutation = If the point mutation alters a codon but does not result in a change in the
amino acid at that position in the protein (due to degeneracy of the genetic code),
▪ neutral mutations if they do not affect gene products or gene expression.
▪ The loss or addition of a single nucleotide causes all of the subsequent three-letter codons to be
changed. These are called frameshift mutations because the frame of triplet reading during
translation is altered. A frameshift mutation will occur when any number of bases are added or
deleted, except multiples of three, which would reestablish the initial frame of reading
Classification Based on Effect on Function
▪ Loss-of-function mutation is one that reduces or eliminates the function of the gene product.
Mutations that result in complete loss of function are known as null mutations
▪ Recessive mutation results in a wild-type phenotype when present in a diploid organism and
the other allele is wild type
▪ A dominant mutation results in a mutant phenotype in a diploid organism, even when the wild-
type allele is also present
▪ A dominant negative mutation in one allele may encode a gene product that is inactive and
directly interferes with the function of the product of the wild-type allele
▪ A dominant negative mutation can also result from haploinsufficiency, which occurs when one
allele is inactivated by mutation, leaving the individual with only one functional copy of a gene.
▪ gain-of-function mutation codes for a gene product with enhanced, negative, or new functions.
, ▪ A suppressor mutation is a second mutation that either reverts or relieves the effects of a
previous mutation. A suppressor mutation can occur within the same gene that suffered the first
mutation (intragenic mutation) or else- where in the genome (intergenic mutation).
Classification Based on Location of Mutation
▪ Somatic mutations are those occurring in any cell in the body except germ cells, whereas germ-
line mutations occur only in germ cells
▪ Autosomal mutations are mutations within genes located on the autosomes, whereas X-linked
and Y-linked mutations are those within genes located on the X or Y chromosome, respectively
▪ X-linked recessive mutations arising in the gametes of a female (the homogametic sex; having
two X chromosomes) may be expressed in male offspring, who are by definition hemizygous for
the gene mutation because they have one X and one Y chromosome
Spontaneous and Induced Mutations
▪ Spontaneous mutations are changes in the nucle- otide sequence of genes that appear to occur
naturally.
▪ The mutation rate is defined as the likelihood that a gene will undergo a mutation in a single
generation or in forming a single gamete.
1. the rate of spontaneous mutation is exceedingly low for all organisms.
2. the rate varies between different organisms.
3. even within the same species, the spontaneous mutation rate varies from gene to gene.
▪ Some DNA sequences appear to be highly sus- ceptible to mutation and are known as mutation
hot spots
▪ mutations that result from the influence of exogenous factors are induced mutations
Spontaneous Germ-Line Mutation Rates in Humans
Single-nucleotide polymorphisms (SNPs) (chapter 5/lecture)
Rate is higher when the parents are older. Mostly the father.
Spontaneous Somatic Mutation Rates in Humans
▪ Somatic mutations can be responsible for other diseases besides cancer and can even become a
source of new germ-line mutations.
▪ If a somatic mutation occurs in one cell very early in development, when the zygote contains
only a few cells, that mutation may ulti- mately contribute to a large portion of the adult
organism—a condition known as somatic mosaicism
agents called mutagens, which have the potential to damage DNA and cause induced mutations.
▪ Some of these agents, such as some fungal toxins, cosmic rays, and UV light, are natural
components of our environment.
▪ some industrial pollutants, medical X rays, and chemicals within tobacco smoke, can be
considered as unnatural or human- made additions
Base Analogs
▪ base analogs, compounds that can substitute for purines or pyrimidines during nucleic acid
biosynthesis.
, ▪ 5-bromouracil (5-BU), a derivative of uracil, behaves as a thymine analog but with a bromine
atom substituted at the number 5 position of the pyrimidine ring. (If 5-BU is chemically linked to
deoxyribose, the nucleoside analog bromodeoxyuridine (BrdU) is formed.
▪ 2-amino purine (2-AP) can act as an analog of adenine.
Alkylating, Intercalating, and Adduct-Forming Agents
▪ Alkylating agents—that is, they donate an alkyl group, such as CH3 or CH2CH3, to amino or keto
groups in nucleotides.
▪ Intercalating agents are chemicals that have dimen- sions and shapes that allow them to wedge
between the base pairs of DNA.
• Wedged intercalating agents cause base pairs to distort and DNA strands to unwind.
These changes in DNA structure affect many functions including transcription, rep-
lication, and repair.
▪ Group of chemicals that cause mutations are known as adduct-forming agents.
• DNA adduct is a substance that covalently binds to DNA, altering its conformation and
interfering with replication and repair.
Ultraviolet Light
▪ Full range of wavelengths is referred to as the electromagnetic spectrum, and the energy of any
radiation in the spectrum varies inversely with its wave- length.
▪ Purines and pyrimidines absorb ultraviolet (UV) radiation most intensely at a wavelength of
about 260 nm
▪ One major effect of UV radiation on DNA is the creation of pyrimidine dimers—chemical species
consisting of two identical pyrimidines—particularly ones consisting of two thymidine residues
• The dimers distort the DNA conformation and inhibit normal replication.
Ionizing Radiation
▪ X rays, gamma rays, and cosmic rays are more energetic than UV radiation they penetrate
deeply into tissues, causing ionization of the molecules encountered along the way. Hence, this
type of radiation is called ionizing radiation.
▪ As ionizing radiation penetrates cells, stable molecules and atoms are transformed into free
radicals—chemical species containing one or more unpaired electrons.
▪ Free radicals can directly or indirectly affect the genetic material, altering purines and
pyrimidines in DNA, breaking phosphodiester bonds, disrupting the integrity of chromosomes,
and producing a variety of chromosomal aberrations, such as deletions, translocations, and
chromosomal fragmentation.
most human genetic diseases are polygenic that is, caused by variations in several genes even a single
base-pair change in one of the approximately 20,000 human genes can lead to a serious inherited
disorder. These monogenic diseases can be caused by many different types of single-gene mutations
Single-Gene Mutations and b@Thalassemia
▪ B@thalassemia is an inherited autosomal recessive blood disorder resulting from a reduction or
absence of hemoglobin
, ▪ People with b@thalassemia have varying degrees of anemia—from severe to mild—with
symptoms including weakness, delayed development, jaundice, enlarged organs, and often a
need for frequent blood transfusions.
▪ Mutations in the b@globin gene (HBB gene) cause b@thalassemia. The HBB gene encodes the
146-amino-acid b@globin polypeptide. Two b@globin polypeptides associate with two
a@globin polypeptides to form the adult hemoglobin tetramer.
▪ Most mutations change a single nucleotide within or surrounding the HBB gene or create small
insertions and deletions.
▪ The types of mutations that cause b@thalassemia not only affect the b@globin amino acid
sequence (missense, nonsense, and frameshift mutations), but also alter HBB transcription
efficiency, mRNA splicing and stability, translation, and protein stability.
Mutations Caused by Expandable DNA Repeats
▪ Mutant genes contain an expansion of trinucleotide repeat sequences—specific short DNA
sequences repeated many times
▪ Diseases associated with these trinucleotide repeat expansions are fragile-X syndrome myotonic
dystrophy, and Huntington disease
DNA repair systems are absolutely essential to the maintenance of the genetic integrity of organisms
and, as such, to the survival of organisms
Proofreading and Mismatch Repair
▪ to cope with errors such as base–base mismatches, small insertions, and deletions that remain
after proof- reading, another mechanism, called mismatch repair MMR), may be activated
▪ During MMR, the mismatches are detected, the incorrect nucleotide is removed, and the correct
nucleotide is inserted in its place.
• But how does the repair system recognize which nucleotide is correct (on the template
strand) and which nucleotide is incorrect (on the newly synthesized strand)?
If a mismatch is recognized but no such discrimination occurs, the excision will be ran-
dom, and the strand bearing the correct base will be clipped out 50 percent of the time.
DNA methylation.
▪ bacteria contain an enzyme, DNA adenine methylase, which recognizes the DNA sequence as a
substrate, adding a methyl group to each of the adenine residues during DNA replication.
▪ An endonuclease enzyme creates a nick in the backbone of the unmethylated DNA strand,
either 5′ or 3′ to the mismatch.
▪ An exonuclease unwinds and degrades the nicked DNA strand, until the region of the mismatch
is reached.
▪ DNA polymerase fills in the gap created by the exonuclease, using the correct DNA strand as a
template. DNA ligase then seals the gap.
Postreplication Repair and the SOS Repair System
▪ Postreplication repair, responds after damaged DNA has escaped repair and has failed to be
completely replicated
▪ To correct the gap, RecA protein directs a recombinational exchange with the corresponding
region on the undamaged parental strand of the same polarity (the “donor” strand). When the
undamaged segment of the donor strand DNA replaces the gapped segment, a gap is created on
the donor strand. The gap can be filled by repair synthesis as replication proceeds.
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