Gene Mutation and DNA Repair Quiz

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Questions and Answers

Gene mutations can only be caused by inserting additional nucleotides.

False (B)

Chromosomal mutations generally affect more than one gene.

True (A)

Point mutations occur at multiple points in the DNA sequence.

False (B)

All mutations are harmless to organisms.

<p>False (B)</p> Signup and view all the answers

Mutations can be heritable if they alter the DNA in a germ cell.

<p>True (A)</p> Signup and view all the answers

A chromosomal mutation involving deletion results in the loss of part of a chromosome and can affect gene expression.

<p>True (A)</p> Signup and view all the answers

The term 'position effect' refers to a situation where a gene is permanently inactivated during cell division.

<p>False (B)</p> Signup and view all the answers

Germ-line cells are responsible for giving rise to somatic cells such as muscle and skin cells.

<p>False (B)</p> Signup and view all the answers

Induced mutations are caused solely by spontaneous errors during DNA replication.

<p>False (B)</p> Signup and view all the answers

Chromosomal rearrangements have no effect on the genes they contain.

<p>False (B)</p> Signup and view all the answers

The temperature range for E. coli with a ts mutation is 33-38°C.

<p>True (A)</p> Signup and view all the answers

Spontaneous mutations can occur due to errors in DNA replication.

<p>True (A)</p> Signup and view all the answers

Heterochromatic regions are often associated with increased gene expression.

<p>False (B)</p> Signup and view all the answers

A point mutation involves the substitution of only a single base pair.

<p>True (A)</p> Signup and view all the answers

Transversions are changes of a purine to a pyrimidine or a pyrimidine to a purine.

<p>True (A)</p> Signup and view all the answers

Silent mutations alter the amino acid sequence of a polypeptide.

<p>False (B)</p> Signup and view all the answers

Mutations that involve the addition or deletion of three nucleotides do not cause a frameshift.

<p>True (A)</p> Signup and view all the answers

A beneficial mutation reduces an organism's chance of survival and reproduction.

<p>False (B)</p> Signup and view all the answers

Down promoter mutations decrease the rate of transcription.

<p>True (A)</p> Signup and view all the answers

A neutral mutation typically lowers the chance of an organism's survival.

<p>False (B)</p> Signup and view all the answers

Conditional mutations express their effects only under specific environmental conditions.

<p>True (A)</p> Signup and view all the answers

Nonsense mutations result in a polypeptide that is complete and functional.

<p>False (B)</p> Signup and view all the answers

Frameshift mutations involve addition or deletion of nucleotides in multiples of one or two.

<p>True (A)</p> Signup and view all the answers

Ionizing radiation includes X-rays and gamma rays and has short wavelength.

<p>False (B)</p> Signup and view all the answers

Nonionizing radiation can penetrate deeply into biological materials.

<p>False (B)</p> Signup and view all the answers

Thymine dimers caused by nonionizing radiation may lead to mutations during DNA replication.

<p>True (A)</p> Signup and view all the answers

The first step in DNA repair is the synthesis of normal DNA.

<p>False (B)</p> Signup and view all the answers

Mismatch repair involves correcting a base pair mismatch in DNA.

<p>True (A)</p> Signup and view all the answers

Direct repair involves the removal of both the abnormal DNA and the normal DNA segment.

<p>False (B)</p> Signup and view all the answers

Homologous recombination repair is used at single-strand breaks in DNA.

<p>False (B)</p> Signup and view all the answers

Base excision repair removes an abnormal base or nucleotide from DNA.

<p>True (A)</p> Signup and view all the answers

Deamination of cytosine produces uracil.

<p>True (A)</p> Signup and view all the answers

5-methylcytosine deamination results in the formation of uracil.

<p>False (B)</p> Signup and view all the answers

Tautomeric shifts can lead to AC and GT base pairs.

<p>True (A)</p> Signup and view all the answers

X-rays are a type of physical mutagen that can cause base deletions.

<p>True (A)</p> Signup and view all the answers

Alkylating agents add methyl or ethyl groups to bases and disrupt nucleotide pairing.

<p>True (A)</p> Signup and view all the answers

Intercalating agents distort the helical structure of DNA.

<p>True (A)</p> Signup and view all the answers

Base analogs such as 5-bromouracil can be incorporated into DNA during replication.

<p>True (A)</p> Signup and view all the answers

Adenine and cytosine predominantly exist in their enol forms.

<p>False (B)</p> Signup and view all the answers

Chemical mutagens are classified only as physical mutagens.

<p>False (B)</p> Signup and view all the answers

Deamination leads to mutation only if repair enzymes fail to correct the problem.

<p>True (A)</p> Signup and view all the answers

Proteins associated with DNA repair include UvrA, UvrB, UvrC, and UvrE.

<p>False (B)</p> Signup and view all the answers

Mismatch repair systems are found in all species.

<p>True (A)</p> Signup and view all the answers

Thymine dimers can be repaired by photolyase through a process called photoreactivation.

<p>True (A)</p> Signup and view all the answers

Xeroderma pigmentosum is caused by defects in genes involved in nucleotide excision repair.

<p>True (A)</p> Signup and view all the answers

DNA polymerase has a 5’ to 3’ proofreading ability that can correct base mismatches.

<p>False (B)</p> Signup and view all the answers

Mismatch repair mutations in humans are connected to certain types of cancer.

<p>True (A)</p> Signup and view all the answers

Nucleotide excision repair can only fix thymine dimers.

<p>False (B)</p> Signup and view all the answers

The molecular mechanism of NER is better understood in eukaryotes than in prokaryotes.

<p>False (B)</p> Signup and view all the answers

Flashcards

Mutation

A change in the DNA nucleotide sequence, causing heritable changes in genetic information.

Gene Mutation

Mutations affecting a single gene.

Chromosomal Mutation

Mutations affecting whole chromosomes.

Point Mutation

Changes in one or a few nucleotides.

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DNA Replication Error

The process where mutations often occur.

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Position Effect

A gene's expression is altered due to its new location on a chromosome.

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Deletion Mutation

A chromosome segment is lost.

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Germ-Line Cell

Cells that create gametes (eggs & sperm).

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Somatic Cell

Any cell in the body other than germ cells.

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Spontaneous Mutation

Mutations due to errors in biological processes.

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Induced Mutation

Mutations caused by environmental factors (mutagens).

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Mutability in Temperature

Some bacteria (e.g., coli with a ts mutation) have growth limits based on temperature.

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Transition Mutation

A point mutation where a purine replaces a purine, or a pyrimidine replaces a pyrimidine.

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Transversion Mutation

A point mutation where a purine replaces a pyrimidine, or vice versa.

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Frameshift Mutation

A mutation caused by insertion or deletion of a number of nucleotides that is not a multiple of three. This changes the reading frame of the genetic code downstream of the mutation.

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Silent Mutation

A mutation that does not change the amino acid sequence of the protein.

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Missense Mutation

A mutation that changes one amino acid in the resulting protein.

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Nonsense Mutation

A mutation that changes a codon to a stop codon, resulting in a truncated protein.

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Neutral Mutation

A mutation that has no observable effect on the organism's phenotype.

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Deleterious Mutation

A mutation that reduces the organism's chances of survival or reproduction.

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Beneficial Mutation

A mutation that increases the organism's chances of survival or reproduction.

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Deamination

The removal of an amino group (-NH2) from a cytosine base, converting it to uracil.

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Deamination of 5-methylcytosine

Deamination of 5-methylcytosine produces thymine, a normal DNA base, making it difficult for repair enzymes to identify the error.

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Tautomeric Shift

A temporary change in the structure of a base, such as thymine or guanine, due to a shift in a hydrogen atom. This can cause incorrect base pairing during DNA replication.

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Types of Chemical Mutagens

Chemical mutagens come in three primary categories: base modifiers, intercalating agents, and base analogs.

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Base Modifiers

Chemical mutagens that directly alter the structure of a nucleotide base, preventing it from pairing correctly.

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Intercalating Agents

Chemical mutagens that insert themselves between DNA base pairs, distorting the helix and leading to insertions or deletions during replication.

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Base Analogs

Chemical mutagens that mimic normal DNA bases, but have altered pairing properties, causing errors during replication.

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Ionizing Radiation

High-energy radiation (like X-rays and gamma rays) that can penetrate deep into biological materials, damaging DNA by creating reactive molecules called free radicals.

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Nonionizing Radiation

Lower-energy radiation (like UV light) that cannot penetrate deeply, but can still cause DNA damage by creating cross-linked thymine dimers.

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Free Radicals

Chemically reactive molecules created by ionizing radiation, which can damage DNA by causing base deletions, single/double-strand breaks, and cross-linking.

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Thymine Dimers

Cross-linked thymine bases in DNA caused by UV radiation, which can lead to mutations during replication.

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DNA Repair

Cellular mechanisms that fix various types of DNA damage, involving detection, removal, and replacement of the damaged DNA segment.

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Direct Repair

A DNA repair mechanism where an enzyme directly corrects a specific DNA damage, restoring the correct structure.

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Excision Repair

A DNA repair mechanism where a damaged DNA segment is removed and replaced with a new, correct segment, using the undamaged strand as a template.

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Mismatch Repair

A DNA repair mechanism that corrects mismatched base pairs in DNA, ensuring accuracy during replication.

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Photolyase

An enzyme involved in direct repair that uses light to split thymine dimers (a type of DNA damage caused by UV radiation) restoring the DNA to its original state.

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Nucleotide Excision Repair (NER)

A major DNA repair pathway that removes a short segment of DNA containing damaged nucleotides, including thymine dimers, chemically modified bases, missing bases, and certain cross-links. This system is present in all eukaryotes and prokaryotes.

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UvrA, UvrB, UvrC, UvrD

Key proteins involved in E. coli's nucleotide excision repair system, named after their role in ultraviolet light repair. They recognize, remove, and replace damaged DNA segments.

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Xeroderma Pigmentosum (XP)

A genetic disease caused by defects in NER genes resulting in increased sensitivity to sunlight and increased risk of skin cancer.

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Cockayne Syndrome (CS)

A genetic disease caused by defects in NER genes, similar to XP, but also affecting growth and development.

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Study Notes

Gene Mutation and DNA Repair

  • Mutations are variations in DNA, arising from errors during DNA copying, such as inserting the wrong base or skipping a base.
  • Mutations originate from the Latin word mutare, meaning "to change."
  • Mutations in DNA can cause heritable changes in genetic information.
  • Organisms have developed repair mechanisms to fix damaged DNA.
  • Mutations can occur at either the chromosomal or gene level.
  • Gene mutations affect single genes.
  • Chromosomal mutations affect whole chromosomes.
  • Point mutations are changes in one or a few nucleotides, occurring at a specific point in the DNA sequence.
  • Point mutations typically arise during DNA replication.
  • A point mutation involves a base substitution, resulting in either a transition or a transversion.
  • A transition is a change from one pyrimidine to another, or one purine to another.
  • A transversion is a change from a pyrimidine to a purine, or vice versa.
  • Mutations can also involve additions or deletions of short sequences of DNA.
  • Mutations in the coding sequence may lead to different types of mutations.
  • Silent mutations do not alter the amino acid sequence due to the degeneracy of the genetic code.
  • Missense mutations alter the amino acid sequence, some may not affect function.
  • Nonsense mutations change a codon to a stop codon, resulting in a truncated polypeptide.
  • Frameshift mutations involve the addition or deletion of nucleotides in multiples of one or two, but not three (the size of a codon), altering the reading frame downstream from the mutation.
  • New mutations are more likely to lead to reduced function than enhanced function, except for silent mutations.
  • Occasionally, a mutation can lead to an enhanced ability of a polypeptide to function, raising the likelihood of survival and reproduction.
  • Mutations can occur in germ-line cells (producing gametes) or somatic cells (all other cells).
  • The earlier the mutation, the larger the affected area of the body.
  • Mutations can be spontaneous or induced.
  • Spontaneous mutations arise due to abnormalities in cellular processes, such as during DNA replication.
  • Induced mutations are caused by environmental factors, such as chemical or physical mutagens.
  • Types of spontaneous mutations include depurination, deamination, and tautomeric shifts.
  • Depurination is the removal of a purine (guanine or adenine) from the DNA.
  • Deamination is the removal of an amino group from a cytosine base, changing it to uracil.
  • Tautomeric shifts are temporary changes in base structure which can lead to mismatches.
  • Chemical mutagens, such as nitrous acid, alkylating agents, intercalating agents, and base analogs, can alter the structure of DNA, contributing to mutations.
  • Physical mutagens, such as ionizing radiation (X-rays, gamma rays) and non-ionizing radiation (UV light) can cause DNA damage potentially leading to mutations.
  • DNA repair mechanisms, such as direct repair, nucleotide excision repair, and mismatch repair, are essential for fixing various types of DNA damage.
  • Several human diseases are associated with defects in genes involved in DNA repair.
  • Diseases like xeroderma pigmentosum, and Cockayne syndrome are examples of such diseases arising from defects in DNA repair.
  • The correct recognition and removal of abnormal DNA is critical in DNA repair processes.

DNA Repair

  • Living cells contain several mechanisms for DNA repair.
  • DNA repair is mostly a multi-step process.
  • DNA repair starts with detecting an irregularity in DNA structure.
  • The abnormal DNA is then removed.
  • Normal DNA is synthesized to repair the damage.
  • Types of DNA repair mechanisms include direct repair, base excision repair, nucleotide excision repair, mismatch repair, homologous recombination repair, and non-homologous end joining.
  • Photolyase repairs thymine dimers, restoring the DNA.
  • Nucleotide excision repair fixes a wide range of DNA damages including thymine dimers and chemically modified bases.

Mismatch Repair

  • DNA replication can occasionally lead to base pair mismatches.
  • DNA polymerases have 3' to 5' proofreading abilities that correct these errors.
  • A mismatch repair system is activated if proofreading fails, coming to the rescue, by directing the removal of the error in the DNA strand
  • These systems are found in all species.
  • In humans, defects can lead to specific types of cancer.
  • MutL, MutH, and MutS proteins are involved in mismatch repair in E. coli and recognize/remove the error in the newly synthesized DNA strand.

Recombination Repair

  • DNA double-strand breaks are very dangerous and can lead to chromosomal rearrangements and deletions.
  • Homologous recombination repair (HRR) and non-homologous end joining (NHEJ) repair these breaks in DNA structure by pairing with a homologous chromosome.

Position Effects

  • Chromosome rearrangements may affect the expression of a gene due to the breakpoint in the gene itself or its new location.
  • This is called position effect; gene expression can be affected because the gene is moved to a position near regulatory sequences for a different gene or to a heterochromatic region.
  • The location of regulatory sequences is often bidirectional.

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