07-18-2010, 08:35 PM
(This post was last modified: 04-09-2015, 10:18 PM by SunilNagpal.)
A mutation is a permanent change in the DNA sequence of a gene. Sometimes mutations can be useful but mostly they are harmful as changes in DNA can change the way a cell behaves. As genes are a set of hereditary materials that contain instructions necessary for a cell to work so if some of these instructions go wrong the cell may not know how to function.
Mutations can be inherited which means the mutated genetic code can be passed on to the next generations. For example heart disease, diabetes, stroke or high blood pressure, run in the family. If parents suffered from them, their children may also develop them. Ten million men in the U.S. are colour blind but less than 600,000 American women have the same disability. That's because this mutation is located on the X-chromosome. Men only have one X-chromosome, so that one is enough to induce the condition, but women have two X chromosomes, and they require the mutation in double set to experience the condition.
Mutations can also be acquired depending on what sort of environment a person lives in as some environmental agents can damage the DNA or when mistakes occur during cell division. For example radiations released during the nuclear disasters in Hiroshima and Nagasaki and Chernobyl are still affecting and causing mutations in the genetic makeup of the people living in those areas. Moreover, different types of cancers are also caused by mutations.
Types of Mutation:
A substitution is a mutation in which there is an exchange between two bases (i.e. a change in a single "chemical letter" such as switching a T to a C). Such a substitution could change a codon to one that encodes a different amino acid and cause a change in the protein produced. Sometimes substitutions may not effects the protein structure, such mutations are called silent mutations and sometimes they may change an amino-acid-coding codon to a single "stop" codon and cause an incomplete protein. This can seriously affect the protein structure which may completely change the organism.
Example of Substitution Mutation: Sickle Cell Anaemia is caused by substitution mutation, where in codon (GAG mutates to --> GTG) and leads to (Glu --> Val) change.
Insertions are mutations in which extra base pairs are inserted into a new place in the DNA. The number of base pairs inserted can range from one to thousands!
Example of Insertion Mutation: Huntington's disease and the fragile X syndrome are examples of insertion mutation wherein trinucleotide repeats are inserted into the DNA sequence leading to these diseases.
Deletions are mutations in which a section of DNA is lost, or deleted. The number of base pairs deleted can again range from one to thousands!
Insertions and Deletion mutations are often together dubbed as INDELS.
Example of Deletion Mutation: 22q11.2 deletion syndrome is caused by the deletion of some bases of chromosome 22. This disease is characterized by cleft palate, heart defects, autoimmune disorders etc.
Protein-coding DNA is divided into codons which are three bases long, insertions and deletions in these codons can completely change a gene so its message cannot be decoded correctly. Such mutations are called frameshift mutations. For example, consider the sentence, "The cat ate her rat." Each word represents a codon. If we delete the first letter and read the sentence in the same way, it doesn't make sense. Similarly if the codons become jumbled up, they would no longer make any sense, in such frameshifts, a similar error occurs at the DNA level, where the codons cannot be parsed correctly. This usually gives rise to truncated proteins that are as useless as "rca tet hce tee" is uninformative.
Examples of Frameshif Mutation: Tay-Sachs Disease, Cancers of many types, Crohn's Disease, cystic fibrosis have been associated with Frameshift Mutation.
Following Video describes the various types of mutations in a very nice way:
A very good description about the types of mutations. A person who is doing an assignment or project about the mutations, this article will be of great help. Keep it up Natasha!
10-04-2012, 12:08 AM
(This post was last modified: 10-04-2012, 12:17 AM by Administrator.)
Mutations, is the change in the blue print of life and that blue print is our DNA in which all phenotypic expressions are coded. The Biotechnologist and researches in this field had today discovered that mutation does not only change the appearance but are involved in behavior of men and women’s!
Recent studies in this field had shown that with the mutation of BRCA gene (BRCA1 and BRCA2), lead young women’s to take complex decisions about life plans and feel differently about their view for treatment, relationship, childbearing as well as career! Thus it is very interesting fact that mutations are also involved in psychosocial consequences. Mutations had lead to human evolutions and may be to the evolution of their behavior which are being proved today with the studies of BRCA gene mutations and many other which are in progress! Also the type of mutations leads to chances of different types of diseases like women with inherit BRCA 1 mutation have increased risk of developing cancers related to breast & ovarian. While the mutation in BRCA2 gene leads to increase in risk of cancer of pancreas, stomach and gall bladder. Therefore research in mutation are helping to better understand its impact well in advance and thus helping scientists to cure and prevent related diseases.
Even though Mutation occurs but Survival of the species demands genetic stability! In nature, Mutations occurs but very rarely it remains uncorrected, because DNA, RNA replication and sequence are maintained with a very high fidelity. Even a small change in sequence gets corrected by the DNA repair mechanism. Most of the mutations are deleterious and are eliminated by natural selection process and mutation rate. Changes in hereditary instruction had shown the way of evolution on earth. Mutation is unpredictable. If mutation occur in somatic cells then that will not get pass on to next generation but if occurs in germile cells then it will fatal, as this may pass on to next generation.
Mutations are broadly categorized in six ways as Point mutation-change will occur in one gene sequence. Frame shift mutation -change occur as addition or deletion of one or more base sequence in gene, Deletion mutation-this mutation is more or less related to frame shift mutation but in this more bases deleted which may affect large number of genes in chromosome, Insertion mutation- is the addition of additional bases in gene, Inversion mutation- Entire DNA strand get inverted or say all base sequence get reversed, DNA expression Mutation- causes the variation in DNA expression process , so less or more protein get synthesized !
This is a very informative post. I would just like to make a few clarifications and additions to help students coming to get information.
There are three types of base-substitution mutations. As the original poster noted, when a base substitution mutation does not change the amino acid inserted into the gene, it is called a “silent mutation.” When the base substitution does change the amino acid, this is called a “missense mutation.” When the substitution results in a stop codon being inserted, this is called a “nonsense mutation.” I remember this by thinking that “nonsense” is similar to saying “no more”, so the protein is ended.
Insertion and deletion mutations can result in frameshift mutations, when the reading frame of the gene is changed. This frameshift only happens when the insertion or mutation is not a multiple of 3 nucleotides. For example, look at the sentence:
The big red pig ate the ham.
If we insert 2 letters, the frame is shifted.
The big rec adp iga tet heh am.
If we insert 3 letters, the frame is not shifted; however, an extra amino acid is added, which changes the sentence.
The big red cat pig ate the ham.
Insertion and deletion mutations can be as small as one nucleotide, to thousands of nucleotides long. These long insertion or deletion mutations normally occur when one part of a chromosome crosses over and changes genetic information with a different chromosome.
All mutations have the potential to be very damaging, but most are benign. Mutations are an important part of evolution, allowing us to develop adaptations and diversity.
Mutations in a broad sense include all those heritable changes which alter the phenotype of an individual. The term mutation was first used by Hugo de Vries; thus characterizing phenotypic changes as separate from environmental variations. However, the term mutation, as it is used now refers to only those changes that alter the chemical structure of the gene at the molecular level. These mutations are usually called point mutations. Theoretically it may be easy to differentiate structure changes and mutations, but it is not so easy at the molecular level. This is because some structural changes may have the same phenotypic effects as gene mutations. Hence, scientists use cytology to differentiate between the two. Also, gene mutations are able to give reverse mutations.
The earliest record of point mutation was in 1791 when Seth Wright noticed a lamb with unusually short legs in his flock of sheep. He bred an entire flock of these short legged sheep and realized that individuals with short legs were homozygous recessive. He confirmed his results by perform a back cross.
Mutations can be artificially induces with the help of mutagenic agents which are of two types- physical mutagens and chemical mutagens. Physical mutagens are radiations, change in pH etc. Ionizing radiations like X rays and gamma rays are the main physical mutagens. There use in radiation therapy, nuclear reactors has made us prone to radiation exposure. Acute or chronic exposure to highly penetrable irradiation like gamma rays can be lethal if not treated immediately. Chemical mutagens like Mustard Gas, MMS, EMS, and DMN have been used since the beginning of world war II for their delayed mutagenic effect. Scientists like C. Auerbach are credited to be the pioneers in this field.
The molecular mechanism for mutation depends greatly on the type of mutagen. Physical mutagens like ionizing radiations are primarily known to cause thymine dimerisation. They may also be responsible for backbone breakage, depurination etc. Chemical mutagens cause a wider variety of chemical changes in the DNA molecule- incorporation of base analogues, inhibition of nucleic acid precursors, changing the nucleic acid resting state structure. All these chemical reactions alter the reactivity of the purine and pyrimidines molecule and therefore the DNA.
Due to the seriousness of the effects of mutagens and the threat of lethal mutations, scientists all over the world are working on radiation counter measure agents while focusing on compounds with antioxidant properties.
GOOD TOPIC AND IT IS EXPLAINED WELL
Mutation is a permanent change in the sequence of DNA that makes up a gene is known as mutation. Mutations may vary in size from single DNA base to a large fragment of a chromosome. Let us focus on different types of mutation, starting with substitution.
A type of mutation wherein a single nucleotide is exchanged for or substituted with a different nucleotide that alters the amino acid sequence in translation rendering ineffective newly synthesized protein is known as substitution. One example of is substitution of purine with another purine (A → G) or a pyrimidine with another pyrimidine (C → T). This type of substitution is also known as transition. Another kind is transversion in which a purine with pyrimidine or a pyrimidine with a purine is substituted. Mutations due to substitution replaces one base with another. Due to redundancy in genetic code some substitutions may not pose any effect at all. As, for example a uracil substitution for a cytosine in the CCU codon does not impart any effect on the protein synthesised as both CCC and CCU code for proline. On the other hand, depending on the region of substitution in the amino acid chain a substitution replacing the amino acid with another may vary widely in the effect that might occur.
For example a substitution producing AUG (the stop codon) is the most serious one as it will prematurely end an amino acid chain. Changes that cause a lesser effect on the configuration of protein does not dramatically affect the protein function. Mutation in an individual that occurs due to single base substitution may have significant consequences. As, for example, sickle cell anemia, which is a serious disease occurs due to mutation from single base substitution. A point mutation of the β- globin gene in codon 6 results in the substitution of glutamic acid by valinethereby causing sickle cell anemia. β-globin is an important component of haemoglobin (HbA). Amino-acid substitution results in HbS type of haemoglobin, with different properties from the normal HbA. In certain conditions like low oxygen tension following exercise or lesser oxygen content of the atmosphere, the following changes occur:
(1) Agglutination of the hemoglobin, forming insoluble rod-shaped polymers;
(2) Distorted sickle –shaped red blood cells
(3) Rupture of the sickle-shaped cells, causing haemolytic anemia;
(4) Blockage of capillaries due to sickle shaped cells, causing interference with the blood flow to the organs.
Another disease caused by substitution mutation is thalassemia. In this case the in codon 39 a single base substitution of C by U form of a stop signal UAG in place of glutamate and a shortened globin chain having only 39 amino acids in the β-globin chain of protein instead of the normal 146. This protein is not functionally active getting equivalent to absence of β-globin thereby causing medical symptoms of thalassemia.
After discussing substitution in detail, let us consider an insertion type of mutation in detail. Insertion mutation is the insertion or addition of nucleotide base pairs into a sequence of DNA, thereby making it longer than the usual length. For example, if a DNA sequence reads CAGC and a T is inserted between G and C during the copying of the sequence, it would then read CAGTC. This is a mutation due to insertion. This mutation could be detrimental if it occurs in a gene, or the region of a DNA sequence coding for a protein, resulting in the formation of a nonfunctional protein.
Certain inherited human disorders occur from insertion of several copies of the same nucleotide triplets. Some of the diseases caused from trinucleotide repeat are Huntington's disease and the fragile X syndrome. These disorders are caused by the genetic inheritance wherein insertions of 3 to 4 nucleotides repeated over and over. In human a locus on X chromosome consists of such stretch of nucleotide in which CGG triplet is repeated (CGGCGGCGGCGG, etc.). If these repeats are in a noncoding region of the gene then it may not cause any harmful phenotype. Even 100 or more repeats cause no harm. But, these long stretches of nucleotide repeat may grow longer from one generation to the other may be up to 4000 repeats, which causes a constriction region on the X chromosome making it quite fragile. Males inheriting such X chromosome from their mothers possess quite a number of harmful phenotypic effects such as mental retardation. Males with the syndrome rarely become fathers. However, females inheriting such a fragile X chromosome also from their mothers only mildly get affected.
In another disorder Polyglutamine Diseases, the trinucleotide repeat CAG adds a string of glutamines to the protein encoded. These types of mutation are implicated in several disorders related to central nervous system that including:
• Huntington's disease: Here the protein known as Huntingtin carries the extra string of glutamines. The abnormal protein resulted from insertion increases the p53 protein level in the brain cells, thereby causing death due to apoptosis.
• Certain cases of Parkinson's disease wherein the extra string of glutamines is inserted to the encoded protein ataxin-2.
• Certain forms of muscular dystrophy appearing in adults are due to the formation of trinucleotide or tetranucleotide repeats, e.g. (CTG)n and (CCTG)n, where n may be up to thousands. Due to the formation of the large RNA transcript interference of the alternative splicing of other transcripts occurs.[/align]
OK, I am confused! Are insertions and deletions considered point mutations or frameshift mutations? Different sources are labeling them as different things. I actually have a textbook that says they are both point AND frameshift! They cannot be both, can they? I would assume they would be frameshift mutations since they are affecting more than one "point" in the gene. Could anyone help me clear this up and put the issue to rest? Thanks!
(03-29-2017, 10:14 AM)forester7 Wrote: OK, I am confused! Are insertions and deletions considered point mutations or frameshift mutations? Different sources are labeling them as different things. I actually have a textbook that says they are both point AND frameshift! They cannot be both, can they? I would assume they would be frameshift mutations since they are affecting more than one "point" in the gene. Could anyone help me clear this up and put the issue to rest? Thanks!
Frameshift is a consequence and Insertions/ Deletions are the causes.
Let's say I have this reading frame:
And now, an insertion of 1 base takes place at the third spot:
AAA-CAT-CCA-GCC-ATT-C.... (this single base insertion is a 'Point Mutation' in this case as it involved/ affected only 1 nucleotide [mutations affecting/ involving only 1-2 nucleotides are termed point]. But this point insertion led to the change of entire reading frame! Hence the consequence of this Insertion type point mutation is 'Frameshift mutation'].
If there was an insertion of 3 bases
, then reading frame won't have changed. So it would have been an Insertion type mutation only.
Same goes for deletion process.
Lesson to be taken is:
It is not obligatory that 'One event leads to One Mutation only'. A single event can have numerous consequences. Single or double point mutations are most dangerous ones as they lead to frameshifts (a consequence).
Hope it clears your doubt to some extent..