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What is a Mutation?

Back in 1987, I submitted a paper to The National Association of Biology Teachers to be included in their refereed journal “American Biology Teacher”. I had been inspired to write this piece based on my student’s feedback that they really liked the way I explained a certain topic. I was crushed when the referees initially rejected the paper as being too simple. They said that its simplicity was potentially leading to students being mislead. As I was contemplating whether I would proceed, the word heuristic came to mind. I added this one word to my paper, and it was published the next semester (A Handy Model for Mitosis, American Biology Teacher 50:170 -172, March 1988). This paper has been cited several times subsequently as an example in teaching/learning from the right brain. I don’t really know about sides of the brain. What I do know is that students need to be given rather simplified models to begin with, and once the understanding is in place the exceptions and anomalies can be introduced. All too often, the editors of text books in their quest for shortening the amount of written material have as their example something that does not even behave according to the model presented. For example, some text books also operationally define chromosomal abnormalities as a type of mutation. In this blog entry, I am not going to include for discussion the type of mutation which affects large segments of the chromosome such as non-disjunction. I am only going to discuss the type of change which takes place more or less at the molecular level. If you have had a chance to see the curricular disaggregation in the previous blog, you will have noted that I broke the topic down to gene mutations and chromosomal abnormalities as shown in the following:

What happens when genetic information is changed?

• Mutations

-Point mutations, also known as base substitution, wobble, missense, nonsense - Base deletion – frame shift leading to nonsense

-3 base deletions

– frame shift usually with no extensive difference

- Single nucleotide polymorphisms - SNP’s 40% may cause change in amino acid.

- SNP’s evolutionarily stable, personalized versus traditional medicine

A strand of DNA which is transcribed and translated into a functioning protein makes sense. There can be a change in the sequence of bases in the strand of DNA, transmitted to the offspring and future generations. It may still make sense. If the change ultimately ends up in more than 1% of the population, it is called a single nucleotide polymorphism (SNP). Only 40% of SNP’s result in a change in the amino acid sequence of the protein being produced. This is because of what is called wobble. Wobble describes the fact that more than one triplet codons translates into the same amino acid. Changes in the third base of the triplet often cause no change in the protein at all. If there is a change in the amino acid, and if that results in an adverse functioning of the protein, then it may be called a genetic disorder. When the DNA leads to a change in an amino acid it is said to be missense. If the change in code leads to no functioning protein, it is called nonsense. Nonsense, for example, can result when the sequence inserts a ‘stop’ too early so that no functioning protein is made. These types of mutations are called point mutations. A point mutation is when only one base in the sequence is incorrect. This happens by chance when the DNA polymerase makes a mistake about 1 in 100,000,000 times. There is some evidence that this actually can happen more frequently but is corrected by another enzyme which corrects the mistakes. The risk of a point mutation also increases when environmental agents chemically alter guanine (G) so that it pairs with thymine (T). Consequently the sequence is changed in future replication. Base deletions and/or insertions are more serious mutations since the resulting frameshift leads to immediate nonsense. Interestingly, a deletion of a triplet leads to a frameshift which may still result a functioning protein. Potentially more serious than a frameshift, are translocations of entire pieces of DNA which are caused by environmental agents which break down the sugar-phosphate backbone. The repair enzymes then put the DNA back together, but not necessarily in the correct position. The concept of mutation was also challenged with the discovery in 1993 of the gene for Huntington Disease. It was found that the gene has a repeating sequence of CAG (cytosine/adenine/guanine). Furthermore, if the number of repeats was more than 36, then HD will be the result.

What is a Mutation?

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