What is the difference between missense and point mutation

The new reading frame may also include a stop codon before the end of the coding sequence. Consequently, proteins made from genes containing frameshift mutations are nearly always nonfunctional. Figure 1. Click for a larger image. Mutations can lead to changes in the protein sequence encoded by the DNA.

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.

There is no cure for HIV infection, but many drugs have been developed to slow or block the progression of the virus. Unfortunately, this is also a part of the world where prevention strategies and drugs to treat the infection are the most lacking.

Figure 2. HIV is highly prevalent in sub-Saharan Africa, but its prevalence is quite low in some other parts of the world. In recent years, scientific interest has been piqued by the discovery of a few individuals from northern Europe who are resistant to HIV infection. In , American geneticist Stephen J. CCR5 is a coreceptor found on the surface of T cells that is necessary for many strains of the virus to enter the host cell.

The mutation leads to the production of a receptor to which HIV cannot effectively bind and thus blocks viral entry. People homozygous for this mutation have greatly reduced susceptibility to HIV infection, and those who are heterozygous have some protection from infection as well.

It is not clear why people of northern European descent, specifically, carry this mutation, but its prevalence seems to be highest in northern Europe and steadily decreases in populations as one moves south. Research indicates that the mutation has been present since before HIV appeared and may have been selected for in European populations as a result of exposure to the plague or smallpox.

This mutation may protect individuals from plague caused by the bacterium Yersinia pestis and smallpox caused by the variola virus because this receptor may also be involved in these diseases. The age of this mutation is a matter of debate, but estimates suggest it appeared between years to years ago, and may have been spread from Northern Europe through Viking invasions.

Although DNA testing to determine which individuals carry the CCR5-delta 32 mutation is possible, there are documented cases of individuals homozygous for the mutation contracting HIV. For this reason, DNA testing for the mutation is not widely recommended by public health officials so as not to encourage risky behavior in those who carry the mutation.

Mistakes in the process of DNA replication can cause spontaneous mutations to occur. The error rate of DNA polymerase is one incorrect base per billion base pairs replicated. Exposure to mutagens can cause induced mutations , which are various types of chemical agents or radiation Table 1. Exposure to a mutagen can increase the rate of mutation more than fold.

Mutagens are often also carcinogens , agents that cause cancer. However, whereas nearly all carcinogens are mutagenic, not all mutagens are necessarily carcinogens. Various types of chemical mutagens interact directly with DNA either by acting as nucleoside analogs or by modifying nucleotide bases. Chemicals called nucleoside analogs are structurally similar to normal nucleotide bases and can be incorporated into DNA during replication Figure 3.

These base analogs induce mutations because they often have different base-pairing rules than the bases they replace. Other chemical mutagens can modify normal DNA bases, resulting in different base-pairing rules.

For example, nitrous acid deaminates cytosine, converting it to uracil. Uracil then pairs with adenine in a subsequent round of replication, resulting in the conversion of a GC base pair to an AT base pair. Nitrous acid also deaminates adenine to hypoxanthine, which base pairs with cytosine instead of thymine, resulting in the conversion of a TA base pair to a CG base pair.

Figure 3. This chemical modification, as shown here, results in converting a CG base pair to a TA base pair. Chemical mutagens known as intercalating agents work differently. These molecules slide between the stacked nitrogenous bases of the DNA double helix, distorting the molecule and creating atypical spacing between nucleotide base pairs Figure 4.

As a result, during DNA replication, DNA polymerase may either skip replicating several nucleotides creating a deletion or insert extra nucleotides creating an insertion. Either outcome may lead to a frameshift mutation. Combustion products like polycyclic aromatic hydrocarbons are particularly dangerous intercalating agents that can lead to mutation-caused cancers.

The intercalating agents ethidium bromide and acridine orange are commonly used in the laboratory to stain DNA for visualization and are potential mutagens. Figure 4. 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. Sometimes missense mutations cause amino acids to be incorporated, which make the protein more effective in doing its job. More frequently, it causes the protein to be less effective in doing its job. But this is really the grist of evolution, when missense mutations happen, and therefore small changes, frequently small changes in proteins, happen, and it happens to be that it improves the function of a protein.

There are different types of mutations such as point mutations, frameshift mutations , missense mutation, silent mutations and nonsense mutations.

Missense mutation is a point mutation which results in the substitution of a different amino acid in the amino acid sequence due to the change of a single nucleotide in the mRNA sequence. Nonsense mutation is a point mutation which results in a truncated, incomplete, nonfunctional protein product due to the introduction of a premature stop codon in the transcribed mRNA sequence.

The key difference between missense and nonsense mutation is that missense mutation substitutes a different amino acid in the amino acid sequence while nonsense mutation introduces a stop codon to the mRNA sequence. Overview and Key Difference 2. What is a Missense Mutation 3. What is a Nonsense Mutation 4.

Similarities Between Missense and Nonsense Mutation 5. Missense mutation is a point mutation where a single nucleotide is changed to cause a substitution of a different amino acid.

In missense mutation, stop codon is not generated to terminate amino acid sequence synthesis similar to nonsense mutation. When one nucleotide changes in the DNA sequence, it changes the genetic code of the gene.

When it transcribes, the resultant mRNA will have a different codon nucleotide tripled which results in an amino acid. The changed codon results in a different amino acid. The resultant amino acid sequence will differ from the unique amino acid sequence due to the substitution of a different amino acid by the mutation. From Genetics Home Reference. Variant types include the following: Substitution This type of variant replaces one DNA building block nucleotide with another. Missense : A missense variant is a type of substitution in which the nucleotide change results in the replacement of one protein building block amino acid with another in the protein made from the gene.

The amino acid change may alter the function of the protein. Nonsense : A nonsense variant is another type of substitution. Instead of causing a change in one amino acid, however, the altered DNA sequence results in a stop signal that prematurely signals the cell to stop building a protein.

This type of variant results in a shortened protein that may function improperly, be nonfunctional, or get broken down. Insertion An insertion changes the DNA sequence by adding one or more nucleotides to the gene. Deletion A deletion changes the DNA sequence by removing at least one nucleotide in a gene. Deletion-Insertion This variant occurs when a deletion and insertion happen at the same time in the same location in the gene.

Duplication A duplication occurs when a stretch of one or more nucleotides in a gene is copied and repeated next to the original DNA sequence. Inversion An inversion changes more than one nucleotide in a gene by replacing the original sequence with the same sequence in reverse order.

Frameshift A reading frame consists of groups of three nucleotides that each code for one amino acid. Repeat expansion Some regions of DNA contain short sequences of nucleotides that are repeated a number of times in a row.