DNA is replicated every time a cel l divides. DNA polymerse, the enzyme responsible for the synthesis of DNA, "copies" the DNA sequence by adding nucleotides according to the A-T and C-G pairing. Sometimes it makes mistakes, most of the times it can correct these mistakes but sometimes it cannot and the sequence of the copied DNA results different from the original. Changes in the DNA can also occur due to damage of the DNA
To understand the consenquence of DNA mutation one has to consider two major factors: WHAT kind of mutation has occurred and WHERE it has occurred.
If the mutation occurs in the junk DNA then it may have no consequence. If it occurs in the promotor region of a gene it may alter the regulation of the expression of that gene. If it occurs in the coding region of a gene it may affect the protein sequence.
The main type of mutations are deletions, insertions (these can be due to translocation of one segment of DNA from one location to another) and point mutations (i.e. substitutions of one nucleotide for another)
Point mutations in the coding region of DNA can have one of three consequences:
1) No effect if the mutated codon still codes for the same amino acid
2) It can result in the protein having one different amino acid. The effect on protein function can vary from no effect to devastating diseases such as cycstic fibrosis.
3) It can troncate the protein if the mutation results in a formation of a Stop codon
An insertion or deletion of bases has a much bigger consequence on protein sequences. This is because the DNA sequence will still be translated by reading the nucleotide 3 by 3 but the reading frame will have shifted resulting in a completely different protein. This is not the case when 3 or a multiple of 3 nucleotides are deleted/inserted, which result in the removal/addition of one or more amino acids
A mutation in the DNA by definition alters the genotype of the individual, however it may not affect the phenotype (ie it may or may not have a functional effect)
Mutations can either be recessive or dominant.
If a mutation simply inactivates a protein it generally results in recessive trait. Since we have a pair of chromosomes each carrying a copy of the genes, the non mutated gene can compensate for the mutated gene. Only when both chromosomes contain the mutation will the phenotype be affected.
If the mutation results in a protein that is toxic than the other gene on the other chromosome cannot counteract the mutation, such mutation is said to be dominant.
Dominat and recessive mutations result in different patterns of inheritance. In dominant mutation every individual with the mutated gene will have a mutated phenotype. In a recessive mutation , individuals with only one mutated gene will have a normal phenotype but are said to be carrier of the mutation. When two carrier individuals have children those that will have inherited mutated genes from both the mother and father will display the mutated phenotype.
