Does Changing The Sequence Of Nucleotides Always Result In A Different Amino Acid Sequence? Explain.

Mutations aren't just grouped co-ordinate to where they occur — frequently, they are as well categorized by the length of the nucleotide sequences they affect. Changes to brusque stretches of nucleotides are called gene-level mutations, considering these mutations affect the specific genes that provide instructions for various functional molecules, including proteins. Changes in these molecules tin have an impact on whatever number of an organism's concrete characteristics. As opposed to gene-level mutations, mutations that alter longer stretches of Dna (ranging from multiple genes up to entire chromosomes) are chosen chromosomal mutations. These mutations oftentimes have serious consequences for afflicted organisms. Because gene-level mutations are more than mutual than chromosomal mutations, the following sections focus on these smaller alterations to the normal genetic sequence.

Base of operations substitution

Base substitutions are the simplest type of gene-level mutation, and they involve the swapping of ane nucleotide for another during Dna replication. For case, 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 just 1 codon is affected (Figure 1).

Figure 1: Only a single codon in the gene sequence is changed in base substitution mutation.

Although a base commutation alters just a single codon in a gene, it tin can still have a pregnant touch on on protein production. In fact, depending on the nature of the codon alter, base substitutions can pb 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 wrong amino acrid into a protein molecule during translation; the 2d consists of nonsense mutations, in which the contradistinct codon prematurely terminates synthesis of a protein molecule; and the third consists of silent mutations, in which the contradistinct codon codes for the same amino acrid every bit the unaltered codon.

Insertions and deletions

Figure 2: During an insertion mutation, the replicating strand \"slips\" or forms a wrinkle, which causes the extra nucleotide to be incorporated.

Insertions and deletions are two other types of mutations that can touch cells at the gene level. An insertion mutation occurs when an actress nucleotide is added to the Dna strand during replication. This can happen when the replicating strand "slips," or wrinkles, which allows the extra nucleotide to exist incorporated (Figure 2). Strand slippage tin also atomic number 82 to deletion mutations. A deletion mutation occurs when a wrinkle forms on the DNA template strand and subsequently causes a nucleotide to be omitted from the replicated strand (Figure three).

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.

Frameshift mutations

Figure iv: If the number of bases removed or inserted from a segment of DNA is non a multiple of three (a), a dissimilar sequence with a unlike prepare of reading frames is transcribed to mRNA (b).

Insertion or deletion of one or more nucleotides during replication can also lead to another type of mutation known as a frameshift mutation. The issue of a frameshift mutation is complete amending of the amino acid sequence of a protein. This alteration occurs during translation because ribosomes read the mRNA strand in terms of codons, or groups of three nucleotides. These groups are chosen the reading frame. Thus, if the number of bases removed from or inserted into a segment of DNA is not a multiple of 3 (Figure 4a), the reading frame transcribed to the mRNA will be completely changed (Effigy 4b). Consequently, once it encounters the mutation, the ribosome will read the mRNA sequence differently, which can result in the production of an entirely different sequence of amino acids in the growing polypeptide chain.

To better sympathise frameshift mutations, let's consider the analogy of words as codons, and messages inside those words equally nucleotides. Each discussion itself has a separate meaning, as each codons represents one amino acrid. The following sentence is equanimous entirely of three-letter words, each representing a three-letter codon:

THE Big BAD Wing HAD One Carmine EYE AND ONE BLU EYE.

Now, suppose that a mutation eliminates the 6th nucleotide, in this example the letter "G". This deletion means that the letters shift, and the residue of the sentence contains entirely new "words":

THE BIB ADF LYH ADO NER EDE YEA NDO NEB LUE YE.

This error changes the relationship of all nucleotides to each codon, and finer changes every single codon in the sequence. Consequently, at that place is a widespread change in the amino acid sequence of the protein. Lets consider an case with an RNA sequence that codes for a sequence of amino acids:

AUG AAA CUU CGC AGG AUG AUG AUG

With the triplet code, the sequence shown in figure 5 corresponds to a protein fabricated of the post-obit amino acids:

Methionine-Lysine-Leucine-Arginine-Arginine-Methionine-Methionine-Methionin

Figure 5: This sequence of mRNA codes for the amino acids methionine-lysine-leucine-arginine-arginine-methionine-methionine-methionine.

Now, suppose that a mutation occurs during replication, and information technology results in deletion of the fourth nucleotide in the sequence. When separated into triplet codons, the nucleotide sequence would at present read every bit follows (Figure 6):

AUG AAC UUC GCA GGA UGA UGA UG

This series of codons would encode the following sequence of amino acids:

Methionine-Asparagine-Phenylalanine-Alanine-Glycine-STOP-STOP

A schematic shows 23 nucleotides arranged horizontally as a single strand of MRNA. Each nucleotide is labeled with a G (representing guanine), U (representing uracil), A (representing adenine), or C (representing cytosine). Three-nucleotide units, or codons, are enclosed in brackets from left to right. An arrow points from the codon to a colored sphere representing the corresponding amino acid.

Effigy 6: If the quaternary nucleotide in the sequence is deleted, the reading frame shifts and the amino acid sequence changes to methionine-asparagine-phenylalanine-alanine-glycine-STOP-Cease.

Each of the stop codons tells the ribosome to terminate protein synthesis at that point. Consequently, the mutant poly peptide is entirely dissimilar due to the deletion of the fourth nucleotide, and information technology is also shorter due to the appearance of a premature stop codon. This mutant protein will be unable to perform its necessary function in the cell.