At most, a point mutation will cause a single amino acid in a protein to change. While this usually is not a deadly mutation, it may cause issues with that protein's folding pattern and the tertiary and quaternary structures of the protein. One example of a point mutation that is not harmless is the incurable blood disorder sickle cell anemia.
This happens when a point mutation causes a single nitrogen base in a codon for one amino acid in the protein glutamic acid to code for the amino acid valine instead. This single small change causes a normally round red blood cell to instead be sickle-shaped. Frameshift mutations are generally much more serious and often more deadly than point mutations. Even though only a single nitrogen base is affected, as with point mutations, in this instance, the single base is either completely deleted or an extra one is inserted into the middle of the DNA sequence.
This change in sequence causes the reading frame to shift—hence the name "frameshift" mutation. A reading frame shift changes the three-letter codon sequence for messenger RNA to transcribe and translate.
That not only changes the original amino acid but all subsequent amino acids as well. This significantly alters the protein and can cause severe problems, even possibly leading to death. One type of frameshift mutation is called insertion. As the name implies, an insertion occurs when a single nitrogen base is accidentally added in the middle of a sequence.
This throws off the reading frame of the DNA and the wrong amino acid is translated. It also pushes the entire sequence down by one letter, changing all codons that come after the insertion, completely altering the protein. Even though inserting a nitrogen base makes the overall sequence longer, that does not necessarily mean the amino acid chain length will increase. In fact, quite the opposite may be true. If the insertion causes a shift in the codons to create a stop signal, a protein may never be produced.
If not, an incorrect protein will be made. If the altered protein is essential to sustain life, then most likely, the organism will die. Deletion is one last type of frameshift mutation and occurs when a nitrogen base is taken out of the sequence. Any chemical or physical change that alters the nucleotide sequence in DNA is called a mutation.
When a mutation occurs in an egg or sperm cell that then produces a living organism, it will be inherited by all the offspring of that organism. Common types of mutations include substitution a different nucleotide is substituted , insertion the addition of a new nucleotide , and deletion the loss of a nucleotide.
Because an insertion or deletion results in a frame-shift that changes the reading of subsequent codons and, therefore, alters the entire amino acid sequence that follows the mutation, insertions and deletions are usually more harmful than a substitution in which only a single amino acid is altered. The chemical or physical agents that cause mutations are called mutagens.
Examples of physical mutagens are ultraviolet UV and gamma radiation. Radiation exerts its mutagenic effect either directly or by creating free radicals that in turn have mutagenic effects.
Radiation and free radicals can lead to the formation of bonds between nitrogenous bases in DNA. If not repaired, the dimer prevents the formation of the double helix at the point where it occurs.
The genetic disease xeroderma pigmentosum is caused by a lack of the enzyme that cuts out the thymine dimers in damaged DNA. Individuals affected by this condition are abnormally sensitive to light and are more prone to skin cancer than normal individuals.
Sometimes gene mutations are beneficial, but most of them are detrimental. For example, if a point mutation occurs at a crucial position in a DNA sequence, the affected protein will lack biological activity, perhaps resulting in the death of a cell.
In such cases the altered DNA sequence is lost and will not be copied into daughter cells. Nonlethal mutations in an egg or sperm cell may lead to metabolic abnormalities or hereditary diseases. Such diseases are called inborn errors of metabolism or genetic diseases. In most cases, the defective gene results in a failure to synthesize a particular enzyme. PKU results from the absence of the enzyme phenylalanine hydroxylase. Without this enzyme, a person cannot convert phenylalanine to tyrosine, which is the precursor of the neurotransmitters dopamine and norepinephrine as well as the skin pigment melanin.
When this reaction cannot occur, phenylalanine accumulates and is then converted to higher than normal quantities of phenylpyruvate. All rights reserved. Mutations are changes in the genetic sequence, and they are a main cause of diversity among organisms. These changes occur at many different levels, and they can have widely differing consequences.
In biological systems that are capable of reproduction , we must first focus on whether they are heritable; specifically, some mutations affect only the individual that carries them, while others affect all of the carrier organism 's offspring , and further descendants. For mutations to affect an organism's descendants, they must: 1 occur in cells that produce the next generation, and 2 affect the hereditary material. Ultimately, the interplay between inherited mutations and environmental pressures generates diversity among species.
Although various types of molecular changes exist, the word " mutation " typically refers to a change that affects the nucleic acids. One way to think of DNA and RNA is that they are substances that carry the long-term memory of the information required for an organism 's reproduction.
This article focuses on mutations in DNA, although we should keep in mind that RNA is subject to essentially the same mutation forces. If mutations occur in non-germline cells, then these changes can be categorized as somatic mutations. The word somatic comes from the Greek word soma which means "body", and somatic mutations only affect the present organism's body. From an evolutionary perspective, somatic mutations are uninteresting, unless they occur systematically and change some fundamental property of an individual--such as the capacity for survival.
For example, cancer is a potent somatic mutation that will affect a single organism's survival. 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. Otherwise, the mutation is non-conservative, and can lead to severe destabilization in the chain of codons. Silent and nonsense mutations can also occur, but these are more specific and, therefore, less common types of substitution mutations.
They lead to either a stop codon nonsense mutation or a near identical codon to the original being formed. If an extra base pair is added to a sequence of base pairs, then the mutation that occurs is an insertion mutation. Deletion mutations, on the other hand, are opposite types of point mutations.
They involve the removal of a base pair.
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