Gene Mutation

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Gene Mutation: Change in genes caused by change in structure of the DNA.

Mutations are heritable changes in genetic material.

A silent mutation is a change in a nucleotide that does not result in a changed the amino acid sequence, i.e., the new codon just happens to code for the same amino acid.

Macroalterations are large changes, such as duplications, deletions, inversions or rearrangements of a large number of bases.

Microalterations involve single base pairs.

Click the above image to see a transition occur for the forth base. Notice that just one of the strands gets an error. After replication of a double-stranded DNA molecule that has a base change in one site, one of the daughter strands will be correct and the other mutant. Prior to replication, opportunity exists to repair the mistake.

Another classification of mutations has to due with multicellular organisms where cells might belong to the germ-line, if they contribute genetic information to future generations; or, if part of the body that will never contribute genetic information to future generations, the somatic cell line.

Another classification system for mutations has to do with using the "normal" or most common state as a reference point. The normal state is called "wild-type".

Another classification of mutations is based on the functional effects of the change.

FROM THE ABOVE YOU CAN SEE THAT ANY ONE MUTATION CAN BE DESCRIBED AS BEING MANY DIFFERENT KINDS OF MUTATIONS. FOR EXAMPLE, ONE MUTATION COULD BE A FORWARD MUTATION THAT IS CONDITIONALLY LETHAL, A POINT MUTATION, A GERMINAL MUTATION, A BIOCHEMICAL MUTATION AND A LOSS-OF-FUNCTION MUTATION.

DETECTION SYSTEMS

How can we observe mutations when they occur?

Mutation Methods for Prokaryotes

First, some background:

Nutritional Mutants in Prokaryotes

Resistance and Sensitivity

Penicillin can be used to isolate auxotrophic mutants. For example, to get met- mutants from met+ prototrophs:

  1. Grow large number of bacteria; transfer to medium lacking methione and containing penicillin.
  2. Let stand for a predetermined length of time.
  3. The met+ bacteria that don't need met, grow, and growing kills them. Those that need methione can't grow and live, if you get them out of there before they starve to death.
  4. Wash the bacteria out of the Penicillin containing medium and transfer all the bacteria to a medium that contains methione.
  5. Those that survived are those that had mutated to met- before the exposure to penicillin.
  6. What you did is select for the mutants, making it so that they were the only survivors.

This procedure is extremely useful.

The Genetic Basis for Bacterial Antibiotic Tolerance

Replica Plating is a method for screening bacteria after selection. It involves transferring bacteria from discreet colonies on a plate to another plate while maintaining their relative positions to each other. The plate transferred to usually contains another medium useful for selection or screening.

Before 1952, it was thought that bacteria adapted to living in the presence of substances like antibiotics by developing a biochemical tolerance. In fact, some scientists actually failed to come to grips with bacteria being like eukaryotes: organisms that depend on the use of hereditary information.

The Lederbergs' experiment verified that antibiotic resistance in bacteria was do to the presence of genetic variants in the population of bacteria prior to exposure to the specific antibiotic. They also demonstrated that the resistance was heretically transmitted. See Figure 13.3.

Replica Plating is still a widely use and useful technique. We will speak to it further in a future chapter.

Mutation Rate and Mutagens

What are some of the reasons for spontaneous, naturally occurring mutations? One cause is built into the very structures of the DNA monomers.

 

Mutagens or Mutagenic Agents

Some mutagens' mode of action fall into distinct classes shared by other mutagens.

 

Screening for Mutagens: The Ames Assay

So how can we test a new or suspect substance to determine if it is a mutagen?

It was discovered that some substances that cause cancer in humans are not mutagenic to bacteria. This was a puzzle. Ames discovered that if liver microsomes (fragments of smooth ER) were added to the suspected compounds, and then the combination was added to bacterial strains, the reversion rate increased.

What does this mean?

 

DNA Repair

There are many different repairs systems used by organisms.

Earlier, we discussed the damage UV light causes to DNA, pyrimidine dimers. These occur when there are two pyrimidines next to each other on the same strand of DNA. Actual covalent bonds form, due to the excitation of the electrons of the benzene rings just after electron/photon coupling.

Cancer: Mutations can cause cancer. Proto-oncogenes are genes that help regulate cell proliferation. Mutations can make things go a little crazy. Cells dividing out of control leads to tumors.

Mutations can be useful to mankind's evolution as well. But mutation induction for generation of useful mutations works best for organisms with high reproductive rates, and certainly not for humans.

Homework Set for Mutations Section

Dr. Herr's Biology 171 lecture notes on mutations

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