Genetics Chapter on Mutations

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Questions and Answers

Which type of mutation replaces a pyrimidine with a purine?

Tautomeric Shift

Transversion (correct)

Simple Tandem Repeat

Transition

Tautomeric shifts have no effect on base-pairing.

False

What are simple tandem repeats?

Repeated sequences of one to six nucleotide pairs.

Exposure to _____ mutagens can cause induced mutations.

physical or chemical Signup and view all the answers

Match the following conditions with their effect on mutation rates:

Accuracy of DNA replication machinery = Reduces mutation rates Efficiency of DNA repair mechanisms = Reduces mutation rates Exposure to mutagenic agents = Increases mutation rates Trinucleotide repeat increase = Increases severity of diseases Signup and view all the answers

What is a consequence of increasing trinucleotide copy numbers?

Inherited genetic diseases Signup and view all the answers

Hermann J. Muller is known for his work with induced mutations in fruit flies.

True Signup and view all the answers

What genetic condition is associated with CCG repeats on the X-chromosome?

Fragile X Syndrome Signup and view all the answers

What type of mutation is caused by ultraviolet irradiation that blocks DNA replication?

Thymidine dimers Signup and view all the answers

Alkylating agents can only affect replicating DNA.

False Signup and view all the answers

What is the mutagenic effect of ethyl methane sulfonate on guanine?

It adds an ethyl group to guanine, converting it to 7-ethylguanine. Signup and view all the answers

Nitrous acid converts adenine to ______, causing an AT to GC transition.

hypoxanthine Signup and view all the answers

Which of the following chemicals is classified as a base analog?

5-Bromouracil Signup and view all the answers

What type of mutation do acridine dyes primarily induce?

Frameshift mutations Signup and view all the answers

Match the following chemical mutagens with their effects:

Ethyl methane sulfonate = Transition mutation G:C ➔ A:T Nitrous acid = Deamination of bases 5-Bromouracil = Base analog mimicking T or C Acridine dyes = Frameshift mutation Signup and view all the answers

Ionizing radiation can cause alterations in chromosome structure, including ______, duplications, and translocations.

deletions Signup and view all the answers

Study Notes

Mutation

Mutations alter the nucleotide sequences of genes.
Examples include substitution of one base pair for another or deletion or addition of one or more base pairs.

Base Substitutions

A transition replaces a pyrimidine with another pyrimidine or a purine with another purine.
A transversion replaces a pyrimidine with a purine or a purine with a pyrimidine.
There are 12 different possible base substitutions in DNA.

Tautomeric Shifts

Movement of hydrogen atoms within a base can occur.
This shifts the base to different tautomeric forms (rare form).
The shift in position causes changes in base form.
Rare forms cause incorrect base pairing during DNA replication.

Tautomeric Shifts Affect Base Pairing

Hydrogen-bonded A:C and G:T base pairs form when cytosine and guanine are in their rare imino and enol tautomeric forms.
These pairings lead to permanent changes in the DNA sequence, causing mutations during replication.

Mutation Caused by Tautomeric Shifts

Rare enol tautomeric form of guanine (G*) pair to Thymine (T) during DNA replication
Results in a mutation of the DNA sequence.
This change is carried over to the daughter cells.

Simple Tandem Repeats

These are sequences of nucleotides that repeat consecutively (e.g. CCG, GAGCC).
Trinucleotide repeats have increased copy numbers, causing inherited diseases.
Examples include CCG repeats (6 to 50 copies) on the X chromosome causing Fragile X Syndrome.
Other repeats (CAG and CTG) lead to Huntington's disease and spinocerebellar ataxia.
Disease severity increases as the trinucleotide repeat copy number increases.

Mutations by Transposons

Transposable elements (jumping genes) move within the genome.
Their movement can cause gene disruptions and mutations.
Results in the addition or deletion of genes, altering the original DNA sequence.
This might result in an active gene product or truncated/inactive one.

Factors Influencing Rate of Spontaneous Mutations

Accuracy of DNA replication machinery.
Efficiency of DNA repair mechanisms for damaged DNA.
Degree of exposure to environmental mutagens.

Induced Mutations

Mutations occur due to exposure to physical or chemical mutagens.

Physical Mutagen

Hermann J. Muller and Edgar Alternburg measured the frequency of X-linked recessive lethal mutations in Drosophila.
Muller demonstrated that exposing Drosophila sperm to X-rays increased mutation frequency.
Irradiation dosage correlates with mutation frequency.

Electromagnetic Spectrum

Electromagnetic radiation with higher energy levels have more potential to cause mutation.

Mutagenesis by Ultraviolet Irradiation

Hydrolysis of cytosine to a hydrate may lead to mispairing during replication.
Cross-linking adjacent thymine forms thymidine dimers, blocking DNA replication.

Ionizing Radiation Causes Changes in Chromosome Structure

Ionizing radiation breaks chromosomes and causes deletions, duplications, inversions, and translocations.

Types of Chemical Mutagens

Chemicals mutagenic to both replicating and non-replicating DNA include alkylating agents and nitrous acid.
Chemicals mutagenic only to replicating DNA include base analogs and acridine dyes.

Alkylating Agents

Chemical mutagens that donate alkyl groups to DNA molecules.
Induce transitions, transversions, frameshifts, and chromosome aberrations.
Change base-pairing properties of both replicating and non-replicating DNA.
Can also activate DNA repair mechanisms.

Chemical Mutagens - Alkylating Agents (Examples)

Di-(2-chloroethyl) sulfide (Mustard gas)
Ethyl methane sulfonate (EMS)
Ethyl ethane sulfonate (EES)

Ethyl Methane Sulfonate (EMS)

Add ethyl group to Guanine, becoming 7-ethylguanine
7-ethylguanine base pairs with Thymine instead of Cytosine.
Resulting in a transition from G:C to A:T.
When DNA replicates, GC pairs converts to AT pairs during DNA replication.

Chemical Mutagens - Deamination Agent (Nitrous Acid)

Acts on either replicating and non-replicating DNA.
Converts amino groups to keto groups.
Converts adenine to hypoxanthine, causing AT to GC transition.
Converts cytosine to uracil, resulting in GC to AT transition.
Oxidative deamination of bases causes changes in their chemical structure.

Base Analogs

Structures similar to normal DNA bases.
Incorporated into DNA during replication.
Examples include 5-Bromouracil and 2-Aminopurine.

A Base Analog: 5-Bromouracil

5-Bromouracil mimics thymine.
Can pair with either adenine or guanine.
In its keto form, 5-Bromouracil pairs with adenine.
In its enol form, 5-bromouracil pairs with guanine.
This change in pairing during replication leads to mutations.

Mutagenic Effects of 5-Bromouracil (5-BU)

5-BU can mimic T or C bases in DNA.
Depending on its form (enol or keto), 5-BU base pairs with inappropriate bases during replication.
Lead to A:T to G:C changes.

Chemical Mutagens - Acridines Dyes

Induce frameshift mutations.
Intercalate between stacked base pairs.
Acridine dyes become inserted between base pairs.
During replication, they can cause insertions or deletions of one or more base pairs.
The frameshift mutations cause changes in the reading frame of the genetic code resulting in changes in the amino acid sequence of the protein.

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Description

Explore the fascinating world of mutations and their impact on genetic sequences. This quiz covers base substitutions, tautomeric shifts, and their roles in causing permanent changes in DNA during replication. Test your understanding of these critical concepts in genetics.