Genetic Mutations Essay, Research Paper
A mutation is a change in the genetic material that controls heredity. The genetic material is contained in chromosomes, which are divided into genes. Point mutations involve single genes and occur by substitution and insertion or deletion of a single base in DNA. Insertions or deletions, referred to as frameshift mutations, shift the reading of the code by one or more bases so that all amino acids produced after the addition or deletion are affected. Translation is a series of complex biochemical processes through which the genetic information in mRNA is converted into protein. Mutations do not always yield identical results. In the majority of instances, they are either harmful or have no effect. Although much less common, mutations with little effect can also occur.
Point mutations are divided into two general categories: base-pair substitutions and base-pair insertions or deletions. A base-pair substitution is the replacement of one nucleotide and its partner from the complementary DNA strand with another pair of nucleotides. Certain base-pair substitutions are referred to as silent mutations because they have no effect on the protein coded for. Missense mutations, the most common type of substitution mutation, occur when the altered codons are still able to code for amino acids. Alterations that change an amino acid codon to a stop signal are called nonsense mutations, which almost always lead to nonfunctional proteins.
Insertions and deletions are additions or losses of one or more nucleotide pairs in a gene. These mutations usually have a more disastrous effect than substitutions on the resulting protein. A frameshift mutation occurs whenever the number of nucleotides inserted or deleted is not a multiple of three. All the nucleotides following the deletion or insertion will be improperly grouped into codons. Hence, the decoding of the entire gene sequence will be radically altered and the amino acid sequence of the protein produced will also be very different. Synthesis of the polypeptide chain may even be stopped too early if the shift in the reading of the codons has created a spurious termination signal, or too late if the stop signal is misread. Unless the frameshift is very near the end of the gene, it will produce a protein that is almost certain to be nonfunctional.
Translation is the process through which a cell interprets a genetic message and builds a protein accordingly. The message is contained in a series of codons along the transcribed mRNA, which is identical to the complementary DNA strand with the exception that thymine is replaced by uracil. The fact that each type of tRNA molecule associates a particular mRNA codon with a particular amino acid makes this entire process possible. As a tRNA molecule arrives at a ribosome, it bears a specific amino acid at one of its end. A triplet called the anticodon, which binds to a complementary codon on mRNA, is at the other end. The ribosome, with the aid of ribosomal enzymes, will then join the amino acids into a chain. Codon by codon, the genetic message is translated, as tRNAs deposit amino acids in the order prescribed.
Most of the time mutations either have no perceptible effect at all, or are harmful. Coding genes map into proteins using the genetic code. The genetic code is redundant (the technical term is degenerate), i.e., different triplets of nucleotides will produce the same amino acid. Because of the redundancy a point mutation may have no effect at all on the protein being coded for; these are known as silent mutations. For example, if CCG mutated to CCA, the mRNA codon that used to be GGC would become GGU, and a glycine would still be inserted at the proper location in the protein. However, if the sequence is altered in a crucial area such as the active site, protein activity will be significantly altered. In almost all cases, these mutations are harmful, creating a useless or less active protein that impairs cellular function. Proteins also become nonfunctional when frameshift mutations occur, because all following nucleotides are affected.
Almost all mutations that are neither harmful nor silent tend to have relatively minimal effects. This can occur if the mutation is located in a region of the protein where the exact sequence of amino acids is not essential to the protein s function. A second way this can occur is if the new amino acid has properties similar to those of the amino acid it replaces. In most cases, the new protein works in a manner very similar to that of the old protein, i.e., it catalyzes the same reactions. However, in some rare instances, the functional capability of the protein changes; it now catalyzes a different reaction. Whether these mutations are helpful or harmful depends on the environment.
Mutations do not always yield identical results. Under normal circumstances, they are either harmful or have no effect. It is less common for mutations to have little effect. The series of complex biochemical processes through which the genetic information within mRNA is converted into protein is called translation. Point mutations involve single genes and occur by substitution, and insertion or deletion of a single base in DNA. The latter, referred to as frame-shift mutations, shift the reading of the code by one or more bases so that all amino acids produced after the insertion or deletion are affected.