Biology Dept
Kenyon College
Chapter 6 A.
Fall Section Spring Section 1 Spring Section 2
So far  we have discussed reassortment and recombination of alleles.
Now we discuss “allele conversion” by mutation. 

Mutation is change in DNA sequence that is inherited by offspring.
A mutation event is how the allele sequence changes.  Two things must happen:

  • A change in the molecular structure of DNA
  • Failure of editing enzymes to correct the change; copying into new DNA
Mutant strain: A population of descendents of the individual in which the original mutation event occurred.  The “mutation” is now inherited by the regular reassortment and recombination mechansisms, as are other alleles.
Rate of mutation: How often a given map position mutates.  In practice, this is hard to measure.
Frequency of mutation: What percent of alleles contain a sequence defined to be “mutant,” in a given population at a given point in time.  This is easy to measure. 

Note: In nature there is no such thing as “wild type.”  All existing alleles are the result of the past 4 billion years of mutation events.

Mutation events are rare.  How to detect them?

  • Observation of large numbers of progeny.  (Patience.)
  • Positive selection.  For traits which confer survival advantage: Subject them to the selective environment.  Example: plate bacteria on agar containing an antibiotic.
  • Negative selection.  For traits which prevent survival, under a given condition.  Example: Replica plate bacteria colonies on agar lacking a nutrient which the “wild type” strain can make with its own enzymes.
Problem (1): Explain how and why each of the above approaches reveals mutant strains.  Compare the advantages and disadvantages of each.

Classifying mutations

  • Phenotype.  Appearance; behavior; auxotrophy; drug resistance; conditional on environmental factors, etc.
  • DNA structure.  Gene mutations vs. Chromosomal mutations.  Point mutations: transitions, transversions
  • Deletion; Insertion; Inversion
  • Information effect.  Silent; nonsense; missense; frameshift
Problems--"Spontaneous" mutations:
(2) Chromosome mutations:  How do they occur?  Review the process of Recombination.  Think of mistakes that can happen, especially with the Holliday structure, and with supposedly homologous base pairing.
(3) Point mutations: How do they occur?  Review DNA replication.  Where can "errors" creep in?

Note that many different kinds of mutation can prevent mRNA transcription, resulting in the same phenotype.

Successive mutations play a major role in the appearance and progression of malignant tumors.  From the Cornell University Medical College:

     A major factor in progression appears to be that most tumor cells are genetically less stable than normal cells and this instability produces variant clones. Chromosomal abnormalities in number and structure are often seen in tumor cells. These abnormalities include: a gain or loss of chromosomes (aneuploidy); deletion (loss of a segment of a chromosome); inversion ("flip-flop" of two segments of a chromosome); translocation (rearrangement of segments between two chromosomes); and mutation (heritable change in the structure or expression of a gene) ranging from chromosomal change to single base-pair substitution (point mutation).  Molecular genetic mechanisms implicated in tumor progression include: chromosomal rearrangements or mutations that "activate" cell oncogenes (proto-oncogenes); and loss of putative "tumor suppressor" genes.
Note: The phenotypic result of a mutation is hard to predict; it depends on the physiology of the particular case.   The magnitude of the mutation may have no correlation with the magnitude of the phenotype.  For example:
  • A major chromosome inversion may result in completely normal phenotype, so long as no genes are lost.
  • A single base pair replacement (point mutation) may decrease or eliminate function of a gene, resulting in lethality.  Example, Sickle-cell anemia.
Problem (4): Transcribe the following DNA sequence into RNA: 

5' A A T G G G C T A C T T A G C C A C T A G G C T T T A G C C 3'
3' T T A C C C G A T G A A T C G G T G A T C C G A A A T C G G 5'

You should find two ways to do it.
Which way could be mRNA to be translated into a short protein?  Why?
Write the protein, using the genetic code in your text.
(Note however: Real coding sequences would have hundreds of base pairs.)
Why cannot the other RNA encode an entire protein?

Perform the following types of  "mutation" in your DNA sequence:

Frame shift
Base pair substitution
Silent mutation
Missense mutation
Nonsense mutation

Show the protein that would result from each mutation.

Certain chemicals and environmental factors may increase the rate of mutation. These are called mutagens.  The Ames test provides a rough measure of the effect of a chemical mutagen.

Mutagens include:

  • Base analogs which incorporate into DNA and pair incorrectly.  An example is 5-bromouracil.
  • Chemicals which modify existing bases of DNA and cause incorrect pairing.
  • Intercalating agents are base pair analogs which intercalate between bases, resulting in addition or loss of a base pair.  Lead to frame shift mutations.  An example is the acridine derivative proflavin:
    • High-energy electromagnetic radiation (UV or gamma rays) cause breakage of the backbone, or cross-linkage of bases, such as thymine dimers, TT or TC.
    Biological mutagenesis is caused by:
    • Mutator genes.  Mutations within genes responsible for editing lead to high frequency of mutations throughout the genome.

    • Transposition.  Transposable elements can insert into a chromosome, excise themselves out of the chromosome, or copy themselves into new locations.   Transposition is mediated by enzymes, such as a transposase.
    Solutions to problems 

    Finding Mutants

    Positive selection for Gain-of-function mutations:  Spread organisms on petri plate of media containing antibiotic.  Only those with antibiotic-resistance mutation will produce colonies. 

    Negative selection for Loss-of-functionmutations:  Grow large number of colonies.  Replica plate or pick colonies onto non-permissive growth medium.  Whichever colonies fail to grow, go back to original source to obtain mutant strain.

    Are gain-of-function alleles dominant or recessive?  What about loss-of-function?

    Case Example: Stickler's Disease
    Phenotype class
    Dominant Recessive
    New function covers up "normal" version.

    Huntington's disease
    Drug resistance pump

    One copy of the new function is not enough.

    Sickle cell anemia

    Haploinsufficiency ofwild-type allele. 
    Usually co-dominant.

    Stickler's disease.
    Homeotic fly gene, makes leg instead of antenna.

    Loss of function is covered up by wild-type allele producing enough functional protein.

    Cystic fibrosis

    Find your own disease in OMIM.

    to problems