MUTATIONS

The changes in the genetic makeup of a cell are Called mutations. The mutations alter the structure of chromosomes. Sometimes, a point mutation occurs in a gamete, or in a cell. This cell gives rise to gametes. Thus the mutation is transmitted to offsprihg and to future generations. Sometimes, the mutation has an adverse effect on the phenotype. In this case the mutant condition is taken as a genetic disorder, or hereditary disease. For example, sickle-cell anemia affects a single nucleotide in the gene. This gene codes for oie of the polypeptides of haemoglobin.

Types of Mutations

Point mutations within a gene can be divided in to two general categories: base­oair substitutions and base pair insertions or deletions.

(a) A basepair substitution

The replacement of one nucleotide and its partner from the complementary DNA s rand with another pair of nucleotides is called a base-pair substitution. -There May be following types of base pair substitution:

(‘ )        Missense mutation:

In this case altered codons still code for amino acids and thus make sense. But these sense may not be the right sense. There are two types of missense mutations:

(a)    Harmless missense mutation: There is redundancy of the genetic code. So some substitution mutations have no effect on the protein coded. In other words, a change in a base pair may transform one codon into another. The new codon is translated into the same amino acid.

(b)    Lethal missense mutations: The alteration of a single amino acid in important area of a protein is called lethal missense mutation. This alteration significantly alters protein. Such mutations are dangerous. They create useless or less active proteins that impair cellular function.

(2)     Nonsense mutation

Alterations in change of codon of an amino acid to a stop signal are called nonsense mutation. In this case a point mutation changes a code for an amino acid into a codon that signals termination.

(b) Base pair Insertions and Deletions or frame shift mutations Insertions and deletions are additions or losses of one or more nucleotide pair in a gene. These mutations have a more dangerous effect. The mRNA is read as a series of nucleotide triple during translation. Therefore, the insertion or deletion of nucleotides may alter the reading frame of the genetic message. Such mutations are called frames shift mutation. These mutation’s occur whenever the number of nucleotides inserted or deleted is not a multiple of 3.


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Mutagenesis

The creation of mutations is called mutagenesis. In the 1920s, Hermann Muller discovered X-rays causes genetic changes in the drosophila. Muller obtained mutant Drosophila. He used this drosophila in his genetic studies. But he also found dangerous aspects of his discovery. X-rays and other forms of radiation cause many lethal effects.

Mutagenesis can occur in a number of ways. Errors during DNA replication, repair, or recombination can lead to base-pair substitutions, insertions, or deletions. Mutations resulting from such errors are called spontaneous mutations.

Mutagen is a chemical or physical agent that causes mutation. Mutagens interact with DNA to cause mutations. There are two categories of mutagens:

  1. X rays and ultraviolet (UV): X rays and ultraviolet (UV) light are examples of physical mutagens. The UV of sunlight can produce mutations in DNA.
  2. Chemical mutagens: Chemical mutagens have several categories. Some of these are base analogues. Base analogues are chemicals that are similar to normal DNA bases. But they pair incorrectly.

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