MD. SHAKIL SARKER

                                          Mutation Introduction Mutation refers to any change in the base sequence of DNA i.e., heritable change in the genome. The most common change is a substitution, addition, rearrangement, or a deletion of one or more bases. A mutation need not give rise to a mutant phenotype.  Individuals showing these changes are known as mutants. An individual showing an altered phenotype due to mutation are known as variant. And a mutagen is a physical agent or a chemical reagent that causes mutation. For example, Nitrous acid reacts with some DNA bases, changing their identity and hydrogen-bonding properties. So, it is a mutagen. Mutation which causes changes in base sequence of a gene are known as gene mutation or point mutation. Generally mutant alleles...

REPAIR OF DNA Damage to DNA caused by replication errors or mutations may have serious consequences. The cell possesses an inbuilt system to repair the damaged DNA. This may be achieved by four distinct mechanisms (Table 24.2).  Base excision-repair  Nucleotide excision-repair  Mismatch repair Double-strand break repair Base excision-repair   The bases cytosine, adenine and guanine can undergo spontaneous depurination to respectively form uracil, hypoxanthine and xanthine. These altered bases do not exist in the normal DNA, and therefore need to be removed. This is carried out by base excision repair (Fig. 1). A defective DNA in which cytosine is deaminated to uracil is acted upon by the enzyme uracil DNA glycosylase. This results in the removal of the defective base uracil. An endonuclease cuts the backbone of DNA strand near the defect and removes a few bases. The gap so created is filled up by the action of repair DNA polymerase and DNA ligase. Fig. 1 : A diagramm...