Genetics Chapter 19: Mutations, DNA Repair & Homologous Recombination

What is a mutation?

A heritable change in the genetic material.

What are the two main categories of mutations?

Germ-line mutations (passed to offspring) and somatic mutations (not passed to offspring).

What is the difference between spontaneous and induced mutations?

Spontaneous mutations occur randomly, while induced mutations are caused by environmental agents.

What is a point mutation?

A change that affects a single DNA nucleotide.

What is a transition mutation?

A base substitution where a purine replaces a purine or a pyrimidine replaces a pyrimidine.

What is a transversion mutation?

A base substitution where a purine replaces a pyrimidine or vice versa.

What is an insertion mutation?

The addition of one or more nucleotides to the DNA sequence.

What is a deletion mutation?

The removal of one or more nucleotides from the DNA sequence.

What is a frameshift mutation?

An insertion or deletion that alters the reading frame of a gene.

What is a missense mutation?

A mutation that changes a sense codon into a different sense codon, resulting in a different amino acid.

What is a nonsense mutation?

A mutation that changes a sense codon into a stop codon, causing premature termination of translation.

What is a silent mutation?

A mutation that changes a sense codon into a synonymous codon, leaving the amino acid sequence unchanged.

What is a neutral mutation?

A mutation that changes the amino acid sequence without altering the protein's function.

What is a loss-of-function mutation?

A mutation that causes a complete or partial loss of function of a gene.

What is a gain-of-function mutation?

A mutation that causes the appearance of a new trait or function.

What is a lethal mutation?

A mutation that causes premature death.

What is a suppressor mutation?

A mutation that suppresses the effect of an earlier mutation at a different site.

What is the difference between intragenic and intergenic suppressor mutations?

Intragenic suppressor mutations occur within the same gene, while intergenic suppressor mutations occur in another gene.

What is the Random Mutation Theory?

The hypothesis that genetic variation occurs by chance and not because of selective pressures.

What is the role of glycosylases in DNA repair?

Glycosylases are enzymes that recognize and remove damaged bases in the base excision repair process.

What is nucleotide excision repair?

A DNA repair mechanism that removes bulky DNA lesions and replaces them with the correct nucleotides.

What is mismatch repair?

A repair process that corrects errors that occur during DNA replication.

What are mutagens?

Agents that induce mutations, which can be chemical or physical.

What is homologous recombination?

A process that repairs double-stranded breaks in DNA using a sister chromatid as a template.

What is non-homologous end joining (NHEJ)?

A DNA repair process that directly joins the ends of broken DNA without a homologous template.

What is the significance of mutations in evolution?

Mutations provide allelic variation, which is essential for adaptation and evolution.

What is a transversion in DNA mutations?

Swapping a purine for a pyrimidine (e.g., T to G).

What is the relationship between structure and function in genetics?

Changing the structure of the genome will change the function or phenotype.

What are missense mutations?

Mutations that change an amino acid in a polypeptide.

What do nonsense mutations do?

They change a codon into a stop codon, resulting in a nonfunctional protein.

What are silent mutations?

Mutations that have no effect on the amino acid produced by a codon.

What is the effect of mutations outside of coding sequences?

They can affect gene expression and phenotype.

What is a promoter mutation?

A mutation that may increase or decrease the rate of transcription.

What are enhancer/operator site mutations?

Mutations that may disrupt the ability of the gene to be properly regulated.

What is the role of 5'-UTR/3'-UTR mutations?

They may alter the ability of mRNA to be translated and affect mRNA stability.

What is a wild-type genotype?

The most prevalent genotypes in a population.

What is a mutant genotype?

A change to the wild-type genotype (a forward mutation).

What is a reversion mutation?

A reverse mutation that returns a mutant allele back to the wild-type allele.

What is a deleterious mutation?

A mutation that decreases chances of survival and reproduction.

What is a beneficial mutation?

A mutation that enhances survival or reproductive success of the organism.

What are conditional mutants?

Mutants whose phenotype is only affected under certain conditions (e.g., temperature).

What are suppressor mutations?

Mutations that reverse the phenotypic effects of the first mutation.

What are intragenic suppressors?

Suppressor mutations that occur within the same gene as the first mutation.

What are intergenic suppressors?

Suppressor mutations that occur in a different gene from the first mutation.

What is the impact of chromosomal inversions?

A change in the direction of part of the genetic material along a single chromosome.

What are translocations in genetics?

A segment of one chromosome becomes attached to a different chromosome.

How can the breakpoint of a chromosome affect gene expression?

If it breaks within a gene, it could lead to lack of expression; outside the gene, expression could change based on the new location.

What is a position effect in genetics?

A change in gene expression due to the gene's new location, which can occur when a chromosome breaks outside of the gene.

What are the two common reasons for position effects?

1. Movement to a position next to regulatory sequences. 2. Movement to a heterochromatic region.

What are spontaneous mutations?

Random changes in DNA that arise from internal cellular processes.

What are induced mutations?

Mutations caused by environmental agents that alter DNA structure, known as mutagens.

What is a mutagen?

An agent that causes changes in DNA structure, leading to mutations.

What are the four types of spontaneous mutations?

1. Depurination. 2. Deamination. 3. Tautomeric Shift. 4. Oxidative Stress.

What is depurination?

The removal of a purine base (A or G) from DNA, creating an apurinic site.

What is deamination?

The removal of an amino group from cytosine, converting it to uracil.

What is a tautomeric shift?

A temporary change in base structure that can lead to mispairing during DNA replication.

What causes oxidative stress in DNA?

Byproducts of oxygen metabolism, such as reactive oxygen species (ROS), can cause oxidative DNA damage.

What are the main types of chemical mutagens?

1. Base modifiers. 2. Intercalating agents. 3. Base analogues.

What do base modifiers do?

They covalently modify the structure of nucleotides, disrupting proper base pairing.

What are intercalating agents?

Chemicals that insert themselves between DNA bases, distorting the helical structure and causing replication errors.

What are base analogues?

Chemicals that resemble nitrogenous bases and can be incorporated into DNA, potentially leading to incorrect base pairing.

What types of radiation are considered physical mutagens?

Ionizing radiation (like X-rays and gamma rays) and non-ionizing radiation (like UV light).

What is the effect of UV light on DNA?

It can form cross-linked thymine dimers, disrupting normal DNA structure and replication.

What is the significance of mutagens in human health?

Mutagens are often involved in cancer development and can cause harmful gene mutations in future generations.

What is the chance of mutation following depurination during DNA replication?

There is a 75% chance of mutation due to random base filling at the apurinic site.

How does oxidative stress affect DNA?

It can lead to oxidative DNA damage, which may result in mutations if not repaired.

What is the role of superoxide dismutase in DNA repair?

It removes reactive oxygen species to prevent oxidative damage to DNA.

What happens during a tautomeric shift?

It causes rare forms of bases to form, leading to incorrect base pairing if replication occurs immediately after.

What are the consequences of intercalating agents during DNA replication?

They can cause single-nucleotide additions or deletions, resulting in frameshift mutations.

What is the role of chemical mutagens in cancer?

Many chemical mutagens are linked to the development of human cancers due to their ability to alter DNA.

How do base analogues affect DNA replication?

They can be incorporated into DNA and may tautomerize, leading to incorrect base pairing.

What is the outcome of a mutation caused by oxidative stress?

It can lead to permanent changes in the DNA sequence if the damage is not repaired.

Why are DNA repair systems vital for organisms?

Most mutations are harmful, so DNA repair systems are essential for survival.

What is the first step in the DNA repair process?

An irregularity in DNA structure is detected.

What happens after the abnormal DNA is removed during DNA repair?

Normal DNA is synthesized.

What does the Mismatch Repair System do?

It corrects mismatches of bases by removing and replacing incorrect bases from the daughter strand.

What type of DNA damage does Base-Excision Repair address?

It removes abnormal bases, such as Uracil, from the DNA.

What is the purpose of Nucleotide-Excision Repair?

It removes DNA damage that causes distortion to DNA structure.

What is Homologous Recombination Repair?

It repairs double-strand breaks using sister chromatids to make error-free repairs.

What is Nonhomologous End Joining?

It pieces together broken ends of DNA without using a template.

What is a base mismatch?

Bases that do not follow base pairing rules, such as G-T.

What role do DNA polymerases play in mismatch repair?

They have a 3' to 5' proofreading ability that can detect and fix base mismatches.

What proteins are involved in mismatch repair in E. coli?

MutL, MutH, and MutS.

How does MutH distinguish between parental and daughter strands?

Methylation ensures that the parental strand is methylated while the newly made daughter strand is not.

What is the role of DNA N-glycosylases in Base-Excision Repair?

They recognize abnormal bases and cleave the bond between the base and the sugar in DNA.

What happens after an abnormal base is removed in Base-Excision Repair?

AP endonuclease removes the remainder of the nucleotide, and DNA Pol I replaces it.

What types of damage does Nucleotide-Excision Repair remove?

Thymine dimers, chemically modified bases, missing bases, and some types of crosslinks.

What is the process of Nucleotide-Excision Repair?

The DNA strands are separated, a section containing distortion is removed, and DNA polymerase fills in the gap.

What causes double-strand breaks in DNA?

Ionizing radiation, chemicals, and reactive oxygen species (ROS).

What is the significance of sister chromatids in Homologous Recombination Repair?

They are genetically identical, allowing for error-free repair.

What is the process of Homologous Recombination?

Alignment of homologous chromosomes, breakage at analogous locations, and exchange of corresponding segments.

What is Sister Chromatid Exchange (SCE)?

Crossing over that occurs between sister chromatids, producing no new combinations of alleles.

What is Genetic Recombination?

Crossing over that occurs between homologous chromosomes, producing new combinations of alleles.

What is the Holliday Model?

A proposed model of genetic recombination involving alignment, breakage, strand invasion, and resolution.

What happens during branch migration in the Holliday Model?

The Holliday junction migrates laterally, causing DNA strands to be swapped between helices.

What is the role of end-binding proteins in Nonhomologous End Joining?

They bind broken ends of DNA and recruit other proteins to form a bridge for repair.

What is a potential downside of Nonhomologous End Joining?

It can lead to loss of nucleotides at the site of the break, making it less accurate.