Such mutation can occur either spontaneously or due to environmental stimuli. An interesting fact is that frameshift mutations generally occur in the Adenine-Thymine AT -rich regions of the nucleic acid. Frameshift mutations can occur either by deleting or inserting the nucleotide in the nucleic acid Figure 3.
Deletion frameshift mutation , wherein one or more nucleotides are deleted in a nucleic acid, resulting in the alteration of the reading frame, i. Deletion is a more common mechanism for inducing the frameshift mutation that results in an altered reading frame.
Insertion frameshift mutation , wherein one or more nucleotides are added to the base sequence of the nucleic acid, which results in the change in the reading frame.
The severity of this type of frameshift mutation is dependent on the number of nucleotides and the position of insertion of nucleotides. This mutation is also referred to as - 1 frameshift mutation. This genetic code is present as a three nucleotides sequence. Each triplet of the nucleotide is eventually translated to form specific proteins required for various life processes. The conversion of this genetic code to protein occurs in two essential steps Figure 4. The transmission of genetic traits in initial genetic experiments by Gregor Mendel indicated that genetic information is carried from one generation to another in some discrete physical and chemical entity.
Later, amino acids were thought to be the carriers of genetic information. Marshall Nirenberg, Heinrich J. Matthaei, and Har Gobind Khorana revealed the nature of a codon and deciphered the codons. The whole-genome sequence is divided into consecutive, non-overlapping sequences of three nucleotides. The triplet codon that initiates the translation process defines the reading frame.
Each triplet of the nucleotide encodes a specific amino acid or a stop signal known as a codon. There are 64 codon combinations that encode 20 amino acids. However, out of these 64 codons, three are the stop codons; thus, 61 codons code for amino acids and three codons for the termination of the translation process i.
Each codon is translated from an mRNA to an amino acid. These amino acids are then joined together by the ribosomes in a process known as ribosome translocation. Synthesis of protein is a cyclic process wherein, after joining one amino acid to the growing chain of the polypeptide, the ribosome moves forward by three bases i.
The movement of ribosomes has disproportionate effects on protein or polypeptide function. In case mutation occurs in the above sequence and an A nucleotide is added or inserted after the start codon AUG. This will completely change the reading frame to:. Thus, we can see, the addition of only a single nucleotide in the RNA sequence completely altered the base sequence that resulted in the formation of completely different amino acids during the translation process.
The reading frame of any mRNA is the coding sequence for a given polypeptide and is read continuously from the start codon AUG to one of the three stop codons. In translation, the ribosome moves down the mRNA three bases at a time and reads whatever codons follow the start codon.
Adding or subtracting one or two bases or any other number that is not a multiple of 3 can disrupt the normal reading frame and lead to the production of a completely nonfunctional protein. Frame shifts may also accidentally introduce an early stop codon. Original coding sequence: atggtgc at ctgactcctgaggagaagtct. Frameshift remove underlined at : atggtgcctgactccTGAggagaagtct. Mutations are a source of variation; however certain mutations can be deleterious and results in a disease condition.
Some of the known diseases that are caused due to frameshift mutations are-. Let us compare and understand the difference between point mutation and frameshift mutation. In point mutation , one base is replaced by another base in the nucleotide sequence. Thus, the sequence of the nucleotide or the reading frame of the nucleic acid remains unchanged. Due to this reason, point mutation is also known as single base substitution. The point mutation can be — transition and transversion.
DNA is made up of purines and pyrimidines. And keep in mind your body has a lot of proteins; everything from the material that makes up your skin, to the material that makes up your hair, to the digestive juices that help you digest that yummy lunch you just had.
If a mutation disrupts one of those reading frames, so that the wrong amino acid is put in place, then the entire DNA sequence following the mutation will be disrupted or read incorrectly. Very often, what we see is a premature termination. Instead of the encoded protein being of a certain particular size, it'll end up being much shorter, and it won't be able to accomplish the role that's been set out for it.
Elaine A. Thus, it is safe to say that the ultimate effects of mutations are as widely varied as the types of mutations themselves. This page appears in the following eBook.
Aa Aa Aa. Where do mutations occur? Germ-line mutations occur in gametes or in cells that eventually produce gametes. In contrast with somatic mutations, germ-line mutations are passed on to an organism's progeny.
As a result, future generations of organisms will carry the mutation in all of their cells both somatic and germ-line. What kinds of mutations exist? Base substitution. Base substitutions are the simplest type of gene-level mutation, and they involve the swapping of one nucleotide for another during DNA replication. For example, during replication, a thymine nucleotide might be inserted in place of a guanine nucleotide.
With base substitution mutations, only a single nucleotide within a gene sequence is changed, so only one codon is affected Figure 1. Figure 1: Only a single codon in the gene sequence is changed in base substitution mutation. The nitrogenous bases are paired so that blue and orange nucleotides are complementary and red and green nucleotides are complementary.
However, the 5 th nucleotide from the right on both the bottom and top strand form a mismatched pair: an orange nucleotide pairs with a red nucleotide. This mismatched pair is highlighted in cyan. The sugar molecules of each individual nucleotide in the chain are connected to adjacent sugar molecules, which are represented by gray horizontal cylinders. The nitrogenous bases hang down from the sugar molecules and look like vertical bars that are twice as long and half as wide as the gray cylinders; the bases are either blue, red, green, or orange.
A second chain of 12 nucleotides forms the second DNA strand below the upper template strand; this strand is labeled the replicating strand in the lower right. Here, the nitrogenous bases point upward from the sugar-phosphate chain, nearly meeting the ends of the nitrogenous bases from the upper strand.
Because there are only 12 nucleotides in the lower strand and 16 nucleotides in the upper strand, four nucleotides on the left side of the upper strand are not bound to a complementary nucleotide on the lower strand. A 13 th nucleotide is shown joining the left end of the lower replicating strand. Although a base substitution alters only a single codon in a gene, it can still have a significant impact on protein production.
In fact, depending on the nature of the codon change, base substitutions can lead to three different subcategories of mutations. The first of these subcategories consists of missense mutations , in which the altered codon leads to insertion of an incorrect amino acid into a protein molecule during translation; the second consists of nonsense mutations , in which the altered codon prematurely terminates synthesis of a protein molecule; and the third consists of silent mutations , in which the altered codon codes for the same amino acid as the unaltered codon.
Insertions and deletions. A second chain of 13 nucleotides forms the second DNA strand below the upper template strand; this strand is labeled the replicating strand in the lower right. The sixth nucleotide from right to left has slipped out of place, causing a bulge in the DNA strand.
The presence of this bulge causes a misalignment of nucleotide pairs; therefore, an extra nucleotide has been added to complete the remaining DNA strand with correct base pairs. This extra nucleotide in position 8 from the right has a cyan aura around it.
A 14 th nucleotide is shown joining the left end of the lower replicating strand. Figure 3: In a deletion mutation, a wrinkle forms on the DNA template strand, which causes a nucleotide to be omitted from the replicated strand. A second chain of eight nucleotides forms the second DNA strand below the first strand. The nucleotide that should have paired with nucleotide 7 on the upper strand has been left out of the replicating bottom strand, causing a bulge in the upper strand.
As a result, upper nucleotides 6, 7, and 8 do not align with a complementary nucleotide on the lower strand. The nucleotides in the bottom strand that would have aligned with upper nucleotides 6 and 8 have a cyan aura around them. Because there are only eight nucleotides in the lower strand and 13 nucleotides in the upper strand, several nucleotides on the left side of the upper strand are not bound to a complementary nucleotide on the lower strand.
One additional free-floating nucleotide is about to be added to the growing bottom strand. Frameshift mutations. Figure 4: If the number of bases removed or inserted from a segment of DNA is not a multiple of three a , a different sequence with a different set of reading frames is transcribed to mRNA b.
The nitrogenous bases hang down from the sugar molecules and look like vertical bars that are twice as long and half as wide as the gray cylinders; the bases are blue, red, green, or orange. A second chain of 12 nucleotides forms the lower DNA strand, labeled the replicating strand in the lower right.
The first, fifth, tenth, and fifteenth nucleotides from left to right are absent in the replicating strand. The effect of the missing nucleotides is illustrated in panel B. The intended sequence of the replicating strand is shown color-coded below the template strand sequence. The actual nucleotide sequence of the replicating strand with the missing nucleotides is shown below the intended sequence.
The sugar molecules of each of the 12 nucleotides in the replicating strand have been connected to adjacent sugar molecules, so that gaps in the strand formed by the four missing nucleotides from panel A are no longer present. The 12 nucleotides are labeled with a letter that represents the chemical identity of the nitrogenous base in each molecule. Three-letter-long units, from left to right, are highlighted with a cyan aura. The absence of the four nucleotides has caused a frameshift mutation, which has altered the DNA code.
Now, suppose that a mutation occurs during replication, and it results in deletion of the fourth nucleotide in the sequence. Compared to Figure 5, this mRNA sequence is missing the fourth nucleotide, causing a frameshift mutation. Because there are 23 nucleotides, there are seven codons, each containing three nucleotides, with two nucleotides left over.
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