Question

1. Why does introducing mutations in bacteria help us understand mutation frequency?

2.) how is studying UV radiation in bacteria helpful in genetics?

Answer 1

1) Mutant frequency is defined as the proportion of mutant cells in a population and is readily estimated. It should be distinguished from mutation rate, which relates to the rate at which mutation events arise, and is generally expressed as events per cell division. Since one mutation event may give rise to one or many mutant cells, depending on the generation in which it has arisen, the relationship of mutant frequency to the underlying mutation rate is complex. A large number of estimates of mutant frequency at the hprt locus in human lymphocytes are available, from our two laboratories among others. From our two extensive data sets, we have determined median hprt mutant frequencies of different age groups and used the method of Lea and Coulson (J. Genet., 49, 1949, 264-285) to attempt to estimate the underlying mutation rate at this locus. It is in principle possible to obtain estimates of mutation rate from the mutant frequency in newborns, from the increase in mutant frequency with age, and from the difference between the upper and lowers quartile mutant frequencies. We discuss reasons for the discrepancies between these estimates and argue that the best estimate can probably be obtained from the increase in mutant frequency with age. We arrive at an estimate of mutation rate to 6-thioguanine resistance at the hprt locus of about 5 x 10(-7) mutation events per nominal cell division. In gene mutation, one allele of a gene changes into a different allele. Because such a change takes place within a single gene and maps to one chromosomal locus, a gene mutation is sometimes called a point mutation. This terminology originated before the advent of DNA sequencing and therefore before it was routinely possible to discover the molecular basis for a mutational event. Nowadays, point mutations typically refer to alterations of single base pairs of DNA or of a small number of adjacent base pairs. In this chapter, we focus on such simple point mutations. The constellation of possible ways in which point mutations could change a wild-type gene is very large and varies according to the particular structure and sequence of the gene. However, it is always true that mutations that reduce or eliminate gene function (loss-of-function mutations) are the most abundant class. The reason is simple: it is much easier to break a machine than to alter the way that it works by randomly changing or removing one of its components. For the same reason, mutations that increase or alter the type of activity of the gene or where it is expressed (gain-of-function mutations) are much rarer.

2) Mutations are a heritable change in the base sequence of DNA. We learned previously that some mutations can be neutral or beneficial to an organism, but most are actually harmful because the mutation will often result in the loss of an important cellular function. Mutations occur naturally in bacteria at a rate 10-7 – 10-8 per base pair during one round of replication. In the presence of a mutagen, however, this rate can increase dramatically. Mutagens can be in the form of a chemical, such as nicotine, or in the form of electromagnetic radiation. There are two forms of electromagnetic radiation that are mutagenic; ionizing radiation and non-ionizing radiation. Ionizing radiation, such as x-rays or gamma radiation carries enough energy to remove electrons from molecules in a cell. When electrons are removed from molecules, ions called free radicals are formed. Free radicals can damage most other molecules in a cell, such as DNA or RNA, by oxidizing them. Non-ionizing radiation, such as ultraviolet (UV) light, exerts its mutagenic effect by exciting electrons in molecules. The excitation of electrons in DNA molecules often results in the formation of extra bonds between adjacent pyrimidines (specifical thymine) in DNA. When two pyrimidines are bound together in this way, it is called a pyrimidine dimer. These dimers often change the shape of the DNA in the cell and can cause problems during replication. The cell often tries to repair pyrimidine dimers before replication, but the repair mechanism can also lead to mutations as well. Electromagnetic radiation can be differentiated according to its wavelength, frequency and/or energy. Conventionally, as the wavelength of the radiation decreases the energy emitted by the radiation increases. For instance, ionizing radiation has a wavelength that is typically less than 1 nanometer and energy that is greater than 100 eV (electron Volts). Nonionizing radiation has a wavelength that is between 100 and 390 nm and energy that is only a few to 100 eV. Because of this, ionizing radiation is considered more dangerous than nonionizing radiation. For instance, clothing can block UV light, but it takes a lead vest to block x-rays. Both ionizing and nonionizing radiation are used to control the growth of microorganisms in clinical settings, the food industry and in laboratories. Since ionizing radiation has more energy, it can penetrate cells and endospores very easily and quickly. It is used to sterilize medical supplies and some food products you eat. Only some forms of nonionizing radiation is useful for controlling microbial growth. There are 3 general types of UV light; UVa, UVb, and UVc. Each of these has a different wavelength. UVa and UVb have a longer wavelength than UVc; therefore, they have less energy and cannot penetrate cells as well. While they are still dangerous, they are not considered germicidal because they produce only a small effect on most microbes. If cells are exposed to UVc long enough then it can penetrate and kill them.