Point mutations occur when a single base pair of a gene is changed. These changes are classified as transitions or transversions. A transversion results when a purine is converted to a pyrimidine or a pyrimidine is converted to a purine. A transversion example is adenine being converte d to a cytosine. You can determine other examples.
Two major classes of chemical mutagens are routinely used. These are alkylating agents and base analogs. Each has a specific effect on DNA. By contrast, a base analog 5-bromouracil and 2-aminopurine only mutate DNA when the analog is incorporated into replicating DNA.
Each class of chemical mutagen has specific effects that can lead to transitions, transversions or deletions. Three different types of common mutagens are observed in nature- physical and chemical mutagens agents and biological agents. Physical Agents: Heat and radiation. Chemical Agents: Base analogs. Biological Agents: Viruses, Bacteria, Transposons.
Physical Mutagenic Agents. Radiations are the primary mutagenic agent reported in UV rays, X-rays, alpha rays, neutrons, and other ionizing and non-ionizing radiations are mutagenic. Usually, radiation directly damages the DNA or nucleotide structure, which could be either lethal or sub-lethal. The electromagnetic wave is additionally one in every of the known mutagens that cause harmful or sub-lethal mutations. Not even the DNA but also proteins and lipids present in an exceeding cell.
The rapidly dividing cells are a chief target for ionizing radiations like X-rays. Heat is another mutagen that provokes mutations in our DNA. Also, extreme heat damages DNA and breaks the phosphodiester bonds too. Why is it more likely that insertions or deletions will be more detrimental to a cell than point mutations? Why do you think the Ames test is preferable to the use of animal models to screen chemical compounds for mutagenicity?
Envision that each is a section of a DNA molecule that has separated in preparation for transcription, so you are only seeing the template strand. What type of mutation is each? Skip to content Mechanisms of Microbial Genetics.
Learning Objectives Compare point mutations and frameshift mutations Describe the differences between missense, nonsense, and silent mutations Describe the differences between light and dark repair Explain how different mutagens act Explain why the Ames test can be used to detect carcinogens Analyze sequences of DNA and identify examples of types of mutations. Think about It What type of a mutation occurs when a gene has two fewer nucleotides in its sequence?
Think about It What are the reasons a nucleotide change in a gene for a protein might not have any effect on the phenotype of that gene? Is it possible for an insertion of three nucleotides together after the fifth nucleotide in a protein-coding gene to produce a protein that is shorter than normal? How or how not? A Beneficial Mutation Since the first case of infection with human immunodeficiency virus HIV was reported in , nearly 40 million people have died from HIV infection, [1] the virus that causes acquired immune deficiency syndrome AIDS.
Figure 2. HIV is highly prevalent in sub-Saharan Africa, but its prevalence is quite low in some other parts of the world. Think about It How does a base analog introduce a mutation? How does an intercalating agent introduce a mutation? What type of mutagen causes thymine dimers? Think about It During mismatch repair, how does the enzyme recognize which is the new and which is the old strand? What type of mutation does photolyase repair?
Think about It Why are cells plated on a nutritionally complete plate in addition to nutrient-deficient plates when looking for a mutant? Think about It What mutation is used as an indicator of mutation rate in the Ames test? Why can the Ames test work as a test for carcinogenicity?
A mutation may lead to a change in the amino-acid sequence of a protein, possibly affecting its function. A point mutation affects a single base pair. A point mutation may cause a silent mutation if the mRNA codon codes for the same amino acid, a missense mutation if the mRNA codon codes for a different amino acid, or a nonsense mutation if the mRNA codon becomes a stop codon. Missense mutations may retain function, depending on the chemistry of the new amino acid and its location in the protein.
Nonsense mutations produce truncated and frequently nonfunctional proteins. A frameshift mutation results from an insertion or deletion of a number of nucleotides that is not a multiple of three. The change in reading frame alters every amino acid after the point of the mutation and results in a nonfunctional protein.
Spontaneous mutations occur through DNA replication errors, whereas induced mutations occur through exposure to a mutagen. Mutagenic agents are frequently carcinogenic but not always. However, nearly all carcinogens are mutagenic.
Chemical mutagens include base analogs and chemicals that modify existing bases. In both cases, mutations are introduced after several rounds of DNA replication. Nonionizing radiation like ultraviolet light may introduce pyrimidine thymine dimers, which, during DNA replication and transcription, may introduce frameshift or point mutations. Cells have mechanisms to repair naturally occurring mutations. DNA polymerase has proofreading activity. Mismatch repair is a process to repair incorrectly incorporated bases after DNA replication has been completed.
Pyrimidine dimers can also be repaired. In nucleotide excision repair dark repair , enzymes recognize the distortion introduced by the pyrimidine dimer and replace the damaged strand with the correct bases, using the undamaged DNA strand as a template.
Bacteria and other organisms may also use direct repair , in which the photolyase enzyme, in the presence of visible light, breaks apart the pyrimidines. Through comparison of growth on the complete plate and lack of growth on media lacking specific nutrients, specific loss-of-function mutants called auxotrophs can be identified.
Abstract Important methods to artificially induce mutations are the use of chemical and physical agents. Substances Mutagens Ethyl Methanesulfonate.
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