Which mutation is least harmful

The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels. The individual in which the mutation first appeared has even been identified. Harmful Mutations Imagine making a random change in a complicated machine such as a car engine.

A genetic disorder is a disease caused by a mutation in one or a few genes. A human example is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs. Cancer is a disease in which cells grow out of control and form abnormal masses of cells.

It is generally caused by mutations in genes that regulate the cell cycle. Because of the mutations, cells with damaged DNA are allowed to divide without limits. Cancer genes can be inherited. Albino Redwoods, Ghosts of the Forest What happens if a plant does not have chlorophyll? Summary Mutations are essential for evolution to occur because they increase genetic variation and the potential for individuals to differ.

The majority of mutations are neutral in their effects on the organisms in which they occur. Beneficial mutations may become more common through natural selection. Making Connections. As a different focus, evolutionary theory is mostly interested in DNA changes in the cells that produce the next generation. The statement that mutations are random is both profoundly true and profoundly untrue at the same time.

The true aspect of this statement stems from the fact that, to the best of our knowledge, the consequences of a mutation have no influence whatsoever on the probability that this mutation will or will not occur. In other words, mutations occur randomly with respect to whether their effects are useful.

Thus, beneficial DNA changes do not happen more often simply because an organism could benefit from them. Moreover, even if an organism has acquired a beneficial mutation during its lifetime, the corresponding information will not flow back into the DNA in the organism's germline.

However, the idea that mutations are random can be regarded as untrue if one considers the fact that not all types of mutations occur with equal probability. Rather, some occur more frequently than others because they are favored by low-level biochemical reactions.

These reactions are also the main reason why mutations are an inescapable property of any system that is capable of reproduction in the real world. Mutation rates are usually very low, and biological systems go to extraordinary lengths to keep them as low as possible, mostly because many mutational effects are harmful.

Nonetheless, mutation rates never reach zero, even despite both low-level protective mechanisms, like DNA repair or proofreading during DNA replication , and high-level mechanisms, like melanin deposition in skin cells to reduce radiation damage.

Beyond a certain point, avoiding mutation simply becomes too costly to cells. Thus, mutation will always be present as a powerful force in evolution. So, how do mutations occur? The answer to this question is closely linked to the molecular details of how both DNA and the entire genome are organized.

The smallest mutations are point mutations, in which only a single base pair is changed into another base pair. Yet another type of mutation is the nonsynonymous mutation, in which an amino acid sequence is changed. Such mutations lead to either the production of a different protein or the premature termination of a protein.

As opposed to nonsynonymous mutations, synonymous mutations do not change an amino acid sequence, although they occur, by definition, only in sequences that code for amino acids. Synonymous mutations exist because many amino acids are encoded by multiple codons. Base pairs can also have diverse regulating properties if they are located in introns , intergenic regions, or even within the coding sequence of genes.

For some historic reasons, all of these groups are often subsumed with synonymous mutations under the label "silent" mutations. Depending on their function, such silent mutations can be anything from truly silent to extraordinarily important, the latter implying that working sequences are kept constant by purifying selection.

This is the most likely explanation for the existence of ultraconserved noncoding elements that have survived for more than million years without substantial change, as found by comparing the genomes of several vertebrates Sandelin et al. Mutations may also take the form of insertions or deletions, which are together known as indels. Indels can have a wide variety of lengths. At the short end of the spectrum, indels of one or two base pairs within coding sequences have the greatest effect, because they will inevitably cause a frameshift only the addition of one or more three-base-pair codons will keep a protein approximately intact.

At the intermediate level, indels can affect parts of a gene or whole groups of genes. At the largest level, whole chromosomes or even whole copies of the genome can be affected by insertions or deletions, although such mutations are usually no longer subsumed under the label indel.

At this high level, it is also possible to invert or translocate entire sections of a chromosome, and chromosomes can even fuse or break apart. If a large number of genes are lost as a result of one of these processes, then the consequences are usually very harmful. Of course, different genetic systems react differently to such events. Finally, still other sources of mutations are the many different types of transposable elements, which are small entities of DNA that possess a mechanism that permits them to move around within the genome.

Some of these elements copy and paste themselves into new locations, while others use a cut-and-paste method. Such movements can disrupt existing gene functions by insertion in the middle of another gene , activate dormant gene functions by perfect excision from a gene that was switched off by an earlier insertion , or occasionally lead to the production of new genes by pasting material from different genes together.

Figure 1: The overwhelming majority of mutations have very small effects. This example of a possible distribution of deleterious mutational effects was obtained from DNA sequence polymorphism data from natural populations of two Drosophila species. The spike at includes all smaller effects, whereas effects are not shown if they induce a structural damage that is equivalent to selection coefficients that are 'super-lethal' see Loewe and Charlesworth for more details. A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations with small effects.

Mutational effects can be beneficial, harmful, or neutral, depending on their context or location. Most non-neutral mutations are deleterious. In general, the more base pairs that are affected by a mutation, the larger the effect of the mutation, and the larger the mutation's probability of being deleterious.

Mutations can be induced in a variety of ways, such as by exposure to ultraviolet or ionizing radiation or chemical mutagens. Since the s, over 2, crop varieties have been developed by inducing mutations to randomly alter genetic traits and then selecting for improved types among the progeny.

Quick Points. Mutations are changes in the DNA of an organism. Mutations can be beneficial, benign, or malignant, depending on where in the genetic code they are located. Examples of beneficial mutations include HIV resistance, lactose tolerance, and trichromatic vision. But the mutations we hear about most often are the ones that cause disease. Some well-known inherited genetic disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, phenylketonuria and color-blindness, among many others.

All of these disorders are caused by the mutation of a single gene. The deadliest disease in the world is coronary artery disease CAD. Also called ischemic heart disease, CAD occurs when the blood vessels that supply blood to the heart become narrowed. Untreated CAD can lead to chest pain, heart failure, and arrhythmias. Stoneman Syndrome. Frequency: one in two million people. Intercalating agents, such as acridine, introduce atypical spacing between base pairs, resulting in DNA polymerase introducing either a deletion or an insertion, leading to a potential frameshift mutation.

Exposure to either ionizing or nonionizing radiation can each induce mutations in DNA, although by different mechanisms. Strong ionizing radiation like X-rays and gamma rays can cause single- and double-stranded breaks in the DNA backbone through the formation of hydroxyl radicals on radiation exposure Figure 5. Ionizing radiation can also modify bases; for example, the deamination of cytosine to uracil, analogous to the action of nitrous acid.

Nonionizing radiation, like ultraviolet light, is not energetic enough to initiate these types of chemical changes. However, nonionizing radiation can induce dimer formation between two adjacent pyrimidine bases, commonly two thymines, within a nucleotide strand.

During thymine dimer formation, the two adjacent thymines become covalently linked and, if left unrepaired, both DNA replication and transcription are stalled at this point. DNA polymerase may proceed and replicate the dimer incorrectly, potentially leading to frameshift or point mutations. Figure 5. The process of DNA replication is highly accurate, but mistakes can occur spontaneously or be induced by mutagens.

Uncorrected mistakes can lead to serious consequences for the phenotype. Cells have developed several repair mechanisms to minimize the number of mutations that persist. Most of the mistakes introduced during DNA replication are promptly corrected by most DNA polymerases through a function called proofreading. In proofreading , the DNA polymerase reads the newly added base, ensuring that it is complementary to the corresponding base in the template strand before adding the next one.

If an incorrect base has been added, the enzyme makes a cut to release the wrong nucleotide and a new base is added.

Some errors introduced during replication are corrected shortly after the replication machinery has moved. This mechanism is called mismatch repair. The enzymes involved in this mechanism recognize the incorrectly added nucleotide, excise it, and replace it with the correct base. One example is the methyl-directed mismatch repair in E. The DNA is hemimethylated. This means that the parental strand is methylated while the newly synthesized daughter strand is not.

It takes several minutes before the new strand is methylated. MutH cuts the nonmethylated strand the new strand. An exonuclease removes a portion of the strand including the incorrect nucleotide. Because the production of thymine dimers is common many organisms cannot avoid ultraviolet light , mechanisms have evolved to repair these lesions.

In nucleotide excision repair also called dark repair , enzymes remove the pyrimidine dimer and replace it with the correct nucleotides Figure 6. If a distortion in the double helix is found that was introduced by the pyrimidine dimer, the enzyme complex cuts the sugar-phosphate backbone several bases upstream and downstream of the dimer, and the segment of DNA between these two cuts is then enzymatically removed. DNA pol I replaces the missing nucleotides with the correct ones and DNA ligase seals the gap in the sugar-phosphate backbone.

The direct repair also called light repair of thymine dimers occurs through the process of photoreactivation in the presence of visible light. An enzyme called photolyase recognizes the distortion in the DNA helix caused by the thymine dimer and binds to the dimer.

Then, in the presence of visible light, the photolyase enzyme changes conformation and breaks apart the thymine dimer, allowing the thymines to again correctly base pair with the adenines on the complementary strand.

Photoreactivation appears to be present in all organisms, with the exception of placental mammals, including humans. Photoreactivation is particularly important for organisms chronically exposed to ultraviolet radiation , like plants, photosynthetic bacteria, algae, and corals, to prevent the accumulation of mutations caused by thymine dimer formation.

Figure 6. Bacteria have two mechanisms for repairing thymine dimers. One common technique used to identify bacterial mutants is called replica plating. This technique is used to detect nutritional mutants, called auxotrophs , which have a mutation in a gene encoding an enzyme in the biosynthesis pathway of a specific nutrient, such as an amino acid.

As a result, whereas wild-type cells retain the ability to grow normally on a medium lacking the specific nutrient, auxotrophs are unable to grow on such a medium.

During replica plating Figure 7 , a population of bacterial cells is mutagenized and then plated as individual cells on a complex nutritionally complete plate and allowed to grow into colonies. Cells from these colonies are removed from this master plate, often using sterile velvet. This velvet, containing cells, is then pressed in the same orientation onto plates of various media.

At least one plate should also be nutritionally complete to ensure that cells are being properly transferred between the plates. The other plates lack specific nutrients, allowing the researcher to discover various auxotrophic mutants unable to produce specific nutrients. Cells from the corresponding colony on the nutritionally complete plate can be used to recover the mutant for further study.

Figure 7. Identification of auxotrophic mutants, like histidine auxotrophs, is done using replica plating.