Molecular Basis of Mutations

Question 1

How do mutations alter nucleotide sequences?

Answer 1

Mutations alter the nucleotide sequences of genes in several ways:

• Substitution of one base pair for another
• Deletion or addition or one or a few base pairs

Question 2

What is the mechanism used for mutation?

Answer 2

Watson & Crick pointed out that the structure of bases in DNA are not static:

• H atoms can move from one position in the purine or pyrimidine ring to another position (called Tautomeric shifts)
• Stable keto forms of T & G -> enol form
• Stable amino forms of C & A -> imino form

Question 3

Study the diagrams of the Keto and Enol forms of bases.

Answer 3

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 4

What is a Tautomeric Shift?

Answer 4

• The spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding form, possibly resulting in a mutation

Question 5

How does a Tautomeric shift link to mutations?

Answer 5

A Tautomeric base form, present at the moment of replication or incorporation into a nascent DNA chain can lead to a mutation

• Rare imino -> A:C base pairing
• Rare enol -> G:T base pairing

Question 6

What is the effect of a mutation caused by a Tautomeric Shift?

Answer 6

Effect of mismatched base pairs following replication is:

• Rare imino -> A:C base pairing -> C:G base-pair substitution
• Rare enol -> G:T base pairing -> T:A base-pair substitution

Question 7

What are the types of Base Substitutions?

Answer 7

• A “Transition” replaces a pyrimidine with another pyrimidine or a purine for another purine
• A “Transversion” replaces a pyrimidine with a purine or a purine with a pyrimidine
• 3 substitutions/base pair (1 transition & 2 transversions), 12 in total -> Point mutations

Question 8

What is a Frameshift Mutation?

Answer 8

• A mutation caused by the addition or deletion of a base pair or base pairs in the DNA of a gene resulting in the translation of the genetic code in an unnatural reading frame from the position of the mutation to the end of the gene.
• Worse to have the frameshift mutation at the beginning of a gene

Question 9

What is a point mutation?

Answer 9

• A mutation affecting only one or very few nucleotides in a gene sequence.

Question 10

What are Induced mutations?

Answer 10

• Induced mutations occur upon exposure to physical or chemical mutagens

Question 11

What experiment was done for Induced Mutations?

Answer 11

• Muller and Alternburg measured the frequency of X linked recessive lethal mutations in Drosophila
• Muller demonstrated that exposing Drosophila sperm to X-rays increased the mutation frequency
• However, better understanding of mutations at the molecular level was provided by the discovery of chemical mutagens that have specific effects on DNA

Question 12

What are the different types of chemical mutagens?

Answer 12

• Chemicals that are mutagenic only to replicating DNA (e.g., base analogs and acridine dyes)
• Chemicals that are mutagenic to both replicating and non-replicating DNA (e.g., alkylating agents and nitrous acid)

Question 13

What does Intercalation mean?

Answer 13

• The insertion of molecules between the planar bases of deoxyribonucleic acid (DNA). This process is used as a method for analyzing DNA and it is also the basis of certain kinds of poisoning

Question 14

What does the Intercalation of an Acridine Dye cause?

Answer 14

• Causes Frameshift
  Mutations
• Intercalate or sandwich between stacked base pairs in DNA
• Increases rigidity and alters conformation of double helix
• Subsequent DNA replication-> additions & deletions of one to a few base pairs results

Question 15

What are Alkylating Agents?

Answer 15

• Chemicals that donate alkyl (CnH2n+1) groups to other molecules
• Induce transitions, transversions, frameshifts, and chromosome aberrations

Question 16

What do Alkylating Agents do?

Answer 16

• Alkylating of bases can change base-pairing properties
• EMS -> ethylation (+ C2H5) of bases at 7-N & 6-O.
• Eg. 7-ethylguanine base pairs with thymine: G:C -> A:T transition
• Can also activate error prone DNA repair processes
• Less specific mutagenic effects

Question 17

What mutagenic affects does smoking have?

Answer 17

• 60 different mutagenic chemicals identified
• All bind DNA, many intercalate or chemically modify DNA
• Most common cancer in humans with the highest morbidity (150 people died in the past hour)
• 23,000 mutations induced by smoking (15 cigarettes = 1 mutation)
• Genome sequencing of small-cell lung cancer
• 15% of all lung cancer cases, 2 year survival <15%
• Several anti-cancer genes mutated

Question 18

What is the Ames test?

Answer 18

• The Ames test provides a simple and inexpensive method for detecting the mutagenicity of chemicals with histidine auxotrophic mutants of Salmonella

Question 19

What types of radiation cause mutation?

Answer 19

• X-rays induce mutations through ionization: breaks chromosomes and can cause deletions, duplications, inversions, and translocations
• Ultraviolet light induces mutations through excitation (nonionizing radiation)
• The increased reactivity of atoms present in DNA molecules is responsible for most of the mutagenicity

Question 20

What is significant about Mutagenesis by Ultraviolet Irradiation

Answer 20

• UV is potent mutagen in unicellular organisms
• Two main products of UV absorption
  1. Pyrimidine hydrates
  2. Pyrimidine dimers

Question 21

What results form UV absorbtion?

Answer 21

• Hydrolysis of cytosine to a hydrate may cause mispairing during replication
• Cross-linking of adjacent thymine forms thymidine dimers, which block DNA replication and activate error prone DNA repair mechanisms.

Question 22

What does it mean when something is Transposable?

Answer 22

• A transposable element is a DNA sequence that can change its position within a genome, sometimes creating or reversing mutations and altering the cell’s genetic identity and genome size. Transposition often results in duplication of the same genetic material.

Question 23

How is a mutation induced by a Transposon?

Answer 23

• The insertion of a transposon into a gene will often render the gene nonfunctional
• Mendel’s wrinkled allele in the pea and the first mutation causing white eyes in Drosophila both resulted from the insertion of transposable elements

Question 24

What are Trinucleotide Repeats?

Answer 24

• Simple tandem repeats are repeated sequence of one to six nucleotide pairs
• Trinucleotide repeats (repeats of 3 nucleotide pairs) can increase in copy number and cause inherited diseases

Question 25

What are some examples of Trinucleotide Repeats and what effect do they have?

Answer 25

• Fragile X Syndrome, Huntington disease, spinocerebellar ataxia
• These diseases are characterized by anticipation, the increased severity of disease or earlier age of onset in successive generations as the trinucleotide copy number increases