Chapter 7: Gene mutations Flashcards
can be defined as an alteration in DNA sequence
mutation
What are the three possible forms that mutations may take regarding changes to DNA sequence?
-Single base-pair substitution,
-deletion or insertion of one or more base pairs, -major alteration in chromosome structure
Where can mutations occur within a gene, and provide examples of these locations?
Within regions coding for protein or within noncoding regions such as introns and regulatory sequences
describes the potential outcomes of a mutation with respect to its impact on observable traits?
Phenotype changes
What determines the extent to which a mutation alters the characteristics of an organism?
Type of cell affected and the degree of alteration in gene function
Differentiate between mutations in somatic cells and germ cells in terms of heritability.
Germ cell mutations are heritable, contributing to genetic diversity and diseases, while somatic cell mutations are not passed to the next generation but may affect cellular function or lead to tumors.
“Microlesion or base substitution” usually refers to a small or microscopic injury or damage, often in the context of tissues or organs.
types of gene mutation
- a change of one base pair to another in a DNA
Point mutation, or base substitution
a change of one base pair to another in a DNA which results in the
creation of a new triplet that codes for a different amino acid in the protein product
Missense mutation
the outcome is that the triplet will be changed into a stop codon,
resulting in the termination of translation of the protein
Nonsense 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)
Silent mutation
base substitution involving a pyrimidine replaces a pyrimidine or a purine
replaces a purine
Transition
- if a purine replaces a pyrimidine, or vice versa
Transversion
used to describe the type of mutation involving the insertion or deletion of one or more nucleotides within a gene?
Frameshift mutation
What consequence does the loss or addition of a single nucleotide have on the three-letter codons during translation in a gene?
It causes all subsequent three-letter codons to be changed.
What specific aspect of the triplet reading during translation is altered in frameshift mutations?
The frame of triplet reading
How would you characterize the potential severity of the results of frameshift mutations, especially when they occur early in the coding sequence?
The results can be very severe.
Explain the significance of frameshift mutations in terms of their impact on the coding sequence during translation.
Frameshift mutations alter the frame of triplet reading during translation, leading to significant consequences, particularly if they occur early in the coding sequence.
type of mutations:
Changes a single DNA nucleotide
Base substitution
Base substitution in which a purine replaces a purine or a pyrimidine replaces a pyrimidine
Transition
Base substitution in which a purine replaces a pyrimidine or a pyrimidine replaces a purine
Transversion
Addition of one or more nucleotides
Insertion
Deletion of one or more nucleotides
Deletion
Insertion or deletion that alters the reading frame of a gene
Frameshift mutation
Deletion or insertion of a multiple of three nucleotides that does not alter the reading frame
In-frame deletion or insertion
Increases the number of copies of a set of nucleotides
Expanding nucleotide repeats
Changes the wild-type phenotype to a mutant phenotype
Forward mutation
Changes a mutant phenotype back to the wild-type phenotype
Reverse mutation
Changes a sense codon into a different sense codon, resulting in the incorporation of a different
amino acid in the protein
Missense mutation
Changes a sense codon into a nonsense (stop) codon, causing premature termination of translation
Nonsense mutation
Changes a sense codon into a synonymous codon, leaving the amino acid sequence of the protein
unchanged
Silent mutation
Changes the amino acid sequence of a protein without altering its ability to function
Neutral mutation
Causes a complete or partial loss of function
Loss-of-function mutation
Causes the appearance of a new trait or function or causes the appearance of a trait in inappropriate tissue or at an inappropriate time
Gain-of-function mutation
Causes premature death
Lethal mutation
Suppresses the effect of an earlier mutation at a different site
Suppressor mutation
Suppresses the effect of an earlier mutation within the same gene
Intragenic suppressor mutation
Suppresses the effect of an earlier mutation in another gene
Intergenic suppressor mutation
4 Phenotypic Effects of Mutations
-Loss-of- function mutation
-Null mutation
-Recessive mutation
-Dominant mutation
one that reduces or eliminates the function of the gene product
Loss-of- function mutation
a mutation that results in the complete loss of function?
Null mutation
In a diploid organism, if a recessive mutation is present with the wild-type allele, what phenotype is observed?
Wild-type phenotype
What type of mutation results in a mutant phenotype in a diploid organism, even when the wild-type allele is also present?
Dominant mutation
2 Two different types of effects
Haploinsufficiency-
Dominant gain-of-function mutation
occurs when the single wild-type copy of the gene does not produce
enough gene product to bring about a wild-type phenotype (example in humans, Marfan
syndrome)
Haploinsufficiency
results in a gene product with enhanced, negative,
or new functions.
Dominant gain-of-function mutation
What are the two potential mechanisms by which dominant gain-of-function mutations can occur?
It can result from a change in the amino acid sequence of the protein, conferring a new activity. Alternatively, it may occur due to a mutation in a regulatory region of the gene, leading to higher expression levels or expression at abnormal times or places.
2 Classification based on location of mutation
somatic and germ cells mutations
this occur in any cell in the body except germ cells and are not transmitted to future generations.
Somatic mutations
are transmitted to offspring as gametes.
Germ cell mutations
What is the term for a mutation that changes a single DNA nucleotide?
Base substitution
2 Types of mutation according to its cause
- Spontaneous mutations
- Induced mutations
are changes in the nucleotide sequence of genes that appear to
occur naturally
result from the influence of extraneous factors either natural or
artificial agents, for example, radiation from cosmic and mineral sources and ultraviolet
radiation from the sun
arise from errors during DNA
replication to changes in DNA base pairing
Spontaneous Replication Errors and Chemical Change
What are the sources or causes of mutagenic changes related to DNA replication errors and chemical change?
tautomeric shifts, depurinations, deaminations, and reactive oxidant damage.
energy of radiation varies inversely with wavelength.
Ionizing Radiation
are more energetic than UV radiation.
X rays, gamma
rays, and cosmic rays
How is the energy of ionizing radiation related to its ability to penetrate tissues?
The energy of ionizing radiation varies inversely with wavelength, making X rays, gamma rays, and cosmic rays more energetic and capable of penetrating tissues deeply.
What happens as ionizing radiation penetrates cells, and what are the consequences for genetic material?
Stable molecules and atoms are transformed into free radicals, which can directly or indirectly affect genetic material by altering purines and pyrimidines in DNA, breaking phosphodiester bonds, disrupting the integrity of chromosomes,
What is the term for chemical species containing one or more unpaired electrons formed as a result of ionizing radiation?
Free radicals
What types of chromosomal aberrations can result from the impact of ionizing radiation on genetic material?
Deletions, translocations, and chromosomal fragmentation.
are agents that have the potential to damage DNA and induce mutations in cells.
Mutagens
all cells on Earth are exposed to a plethora of agents called
mutagens,
. Chemical Mutagens
Provide examples of mutagens
Fungal toxins, some industrial pollutants, medical X-rays, and chemicals within tobacco smoke.
What is the positive aspect of mutagens mentioned in the context of geneticists?
Geneticists harness some mutagens for use in analyzing genes and gene functions.
compounds that can substitute for purines or pyrimidines during nucleic
acid biosynthesis.
Base Analogs
ex: 5-bromouracil (5-BU)
a derivative of uracil, behaves as a
thymine analog but is halogenated at the number 5 position of the pyrimidine ring.
5-bromouracil (5-BU)
Explain the role of alkylating agents in causing mutations
Alkylating agents, such as sulfur-containing mustard gases, donate an alkyl group, such as CH3 or CH3CH2, to amino or keto groups in nucleotides, causing alterations in the structure of DNA and inducing mutations.
are chemicals that have dimensions and shapes that
allow them to wedge between the base pairs , causing distortion and unwinding of DNA strands
Intercalating agents
How do intercalating agents affect the functions of transcription, replication, and repair in DNA?
by causing distortion and unwinding of DNA strands.
How do alterations in DNA structure caused by intercalating agents contribute to mutations during DNA replication and repair?
Alterations in DNA structure caused by intercalating agents lead to deletions and insertions during DNA replication and repair, ultimately causing frameshift mutations.
waves of shorter length than visible light, being inherently more
energetic, have the potential to disrupt organic molecules
Ultraviolet Light
At what wavelength do purines and pyrimidines most intensely absorb UV radiation?
at a wavelength of about 260 nm
Despite the ozone layer’s protection, what is the potential consequence of sufficient UV radiation exposure to cells?
Sufficient UV radiation can induce thousands of DNA lesions per hour in any exposed cell.
What is one major effect of UV radiation on DNA?
creation of pyrimidine dimers in DNA.
also known as transposons or “jumping genes,” since it can move or
transpose within and between chromosomes, inserting themselves into various locations
within the genome.
Transposable Elements
What is the primary consequence of the insertion of a transposon into the coding region of a gene?
The insertion of a transposon into the coding region of a gene may disrupt the gene’s normal translation reading frame or induce premature termination of translation of the mRNA transcribed from the gene.
Besides creating mutations, what other impact can transposable elements have on a genome?
Transposable elements can alter gene expression and contribute to evolution by moving within a genome.
How do geneticists utilize transposons as research tools?
to create mutations, clone genes, and introduce genes into model organisms.
7.3.DNA Repair System to Detect and Correct Mutations
How do organisms counteract mutations?
Organisms counteract mutations using various DNA repair systems.
What are the main types of DNA repair systems
proofreading,
mismatch repair,
photoreactivation repair,
SOS repair,
base excision repair,
nucleotide excision repair,
and double-strand break repair.
What is a common source of mutations during DNA replication?
Mutations often arise when an incorrect nucleotide is inserted by DNA polymerase.
How frequently does DNA polymerase III, the major DNA synthesizing enzyme in bacteria, make errors during insertion?
DNA polymerase III makes an error approximately once every 100,000 insertions, resulting in an error rate of 10^-5.
What is the efficiency of DNA polymerase proofreading in catching errors during replication?
DNA polymerase proofreads each step, catching 99 percent of errors.
What action does DNA polymerase take if an incorrect nucleotide is inserted during polymerization?
If an incorrect nucleotide is inserted, the enzyme recognizes the error and “reverses” its direction.
What enzymatic activity does DNA polymerase exhibit when acting as a 3ʹ to 5ʹ exonuclease during proofreading?
DNA polymerase acts as a 3ʹ to 5ʹ exonuclease, cutting out the incorrect nucleotide.
How does DNA polymerase improve replication efficiency when proofreading?
By cutting out the incorrect nucleotide and replacing it with the correct one, DNA polymerase improves replication efficiency 100-fold, creating only 1 mismatch in every 10^7 insertions.
What is the final error rate after DNA polymerase proofreading during replication?
The final error rate is 10^-7, creating only 1 mismatch in every 10^7 insertions.
How does mismatch repair address mismatches in replicated DNA?
the incorrect nucleotide is removed,
and the correct nucleotide is inserted in its place.
Post replication Repair and the SOS Repair System
When does post-replication repair respond,
post-replication repair responds after damaged DNA has escaped repair and failed to be completely replicated.
purpose or post-replication repair
carry out repair synthesis as replication proceeds.
How does the SOS repair system response to the damaged DNA?
by inducing the expression of specific genes, including lexA, recA, and uvr. This system is activated in the presence of a large number of unrepaired DNA mismatches and gaps.
Why is the SOS repair system considered a last resort
to minimize DNA
damage
what happens during SOS repair that makes it potentially mutagenic?
During SOS repair, DNA synthesis becomes error-prone, inserting random and possibly incorrect nucleotides, making it mutagenic. This allows the cell to survive DNA damage that might otherwise be lethal.
What is the dependent factor for the process of photoreactivation repair
photoreactivation enzyme (PRE).
this enzyme’s mode of action is to cleave the bonds between thymine dimers, thus directly
reversing the effect of UV radiation on DNA.
photoreactivation enzyme (PRE).
7.4.Evolutionary Significance of Mutations
What role do mutations play in terms of genetic variation and natural selection?
- Mutations serve as a source of genetic variation and provide the raw material for natural selection.
- Mutations are also the source of genetic damage that contributes to cell death, genetic diseases, and cancer.
