HW: Mutations: Helpful or Harmful?

1. What is a mutation? At first it may sound like a bad thing, and sometimes it is. However, life as we know it today could not exist without mutations. They are the reason that organisms have changed over the course of time. What does this mean? Mutations generate the raw material for natural selection. They are the main source for genetic variation in the gene pool. Mutations can be harmful, beneficial, or neutral. Neutral means they have no effect on the organism in which the mutation occurs. Let us look at mutations, how they occur, and how they contribute to genetic variation.

2. First, a quick review of how genetic information is carried and passed along in individual organisms. All living organisms have DNA, or deoxyribonucleic acid. The DNA molecule is made of a long strand of compounds called nucleotides attached to a sugar-phosphate backbone. It is the sequence of nucleotides along the strand that makes individual organisms unique. Why? Along each DNA strand are segments that code for traits. The segments of DNA that code for traits are called “genes”. Traits include such things as skin color, height, fruit size, or amount of fur. Each organism has its own sequence of nucleotides along its DNA. It is this sequence that makes every organism unique.

3. Every time a cell is replicated, the sequence of the nucleotides found on the DNA strand must be copied exactly. If the copies are exact, the organism can function and reproduce properly. However, the cellular machinery that copies DNA sometimes makes mistakes. There may be changes in the sequence of nucleotides that is copied. An extra base may be inserted or left out. In its most basic sense, a mutation is any change in the sequence of DNA in a cell. In other words, mutations are random changes in the sequence of DNA that occur to individual organisms. Many mutations occur in sections of the DNA strand that do not code for traits. Therefore they have no effect on an organism’s traits. If the mutations do occur in a gene sequence, the mutation may be either harmful or helpful. Mutations Variation of Traits 1 Mutations: Helpful or Harmful?

4. Mutations may happen during the replication process in several ways. A point mutation happens when one “letter” of the genetic code is changed to another. In another type of mutation, sections of DNA can be added to or removed from genes. In a third type of mutation, genes, or parts of genes, can become reversed or duplicated. But these “mistakes” do not occur to all of the DNA within an organism at the same time. Most mutations only occur within single cells. It is the type of cells in which these mutations occur that is important.

5. In single-celled organisms, all mutations can be passed to the next generation. Things are more complicated in multicellular organisms. Mutations that occur in the somatic (nonreproductive) cells usually only affect the individual organism with the mutation. If the mutation occurs within these types of cells, the individual organism may change. Those changes will not be passed to future generations. It is the mutations that occur within the reproductive cells that become important for evolutionary change. Mutations that occur in reproductive cells (gametes) can be passed on to offspring. Mutations in any particular gene sequence gene are rare. They are the only way, however, that new traits can be generated within a population.

6. All of the mutations that occur to DNA sequences are random. They do not occur as the result of a “need”. For example, imagine that an organism’s environment suddenly turns much colder than it was before. Its DNA will not simply generate a mutation that results in thicker fur. A mutation that results in thicker fur would need to arise naturally. An organism cannot just generate a mutation for a specific purpose. To follow this example, consider an organism in a colder environment which has a mutation, by chance, that creates thicker fur. This individual organism will be more likely to survive to reproduce. It might be possible for the organism to pass this new trait of thicker fur, caused by the mutation, to its offspring. If it can pass on the trait, its offspring will also be more likely to survive. If the individuals with less fur become less likely to survive, then more and more individuals with the thick fur mutation will live. Each new generation is able to pass on this beneficial mutation to the next generation. Since individuals with thick fur are more likely to survive, they are more likely to reproduce successfully. Therefore it will not take long before the entire population will have the mutated gene for thicker fur. The population will have evolved.

7. New traits that come from a mutation start out at a very low frequency within the population. In fact, a mutation will usually start with one individual out of the total population. If this individual is born with a beneficial mutation, that individual will be more likely to survive than others without the mutation. This principle is called “survival of the fittest”. The beneficial mutation will likely be passed on to the next generation. This is why mutations are the raw materials for evolutionary change. Natural selection favors beneficial mutations, increasing their frequencies, and eliminates harmful ones. If a particular mutation happens to work well with the environment, that mutation could be favored by natural selection. The frequency of that gene would increase within that population. On the other hand, it could have a negative effect on the organism and cause it to be less successful or even die. Therefore, the effect of a mutation often depends on the environment and on the other traits within the organism and the population.