Correct Option: B

DNA sequencing is the determination of the precise sequence of nucleotides nucleotides in a sample of DNA. The most popular method for doing this is called the dideoxy method or Sanger method. DNA is synthesized from four deoxynucleotide triphosphates. The dideoxy method gets its name from the critical role played by synthetic nucleotides that lack the -OH at the 3′  carbon atom. A dideoxynucleotide can be added to the growing DNA strand but when it is, chain elongation stops because there is no 3′  -OH for the next nucleotide to be attached to. For this reason, the dideoxy method is also called the chain termination method. The DNA sample is divided into four separate sequencing reactions, containing the four standard deoxynucleotides (dATP, dGTP, dCTP and dTTP) and the DNA polymerase. To each reaction is added only one of the four dideoxynucleotides (ddATP, ddGTP, ddCTP, or ddTTP). These dideoxynucleotides are the chain-terminating nucleotides, lacking a 3′ -OH group required for the formation of a phosphodiester bond between two nucleotides during DNA strand elongation. Incorporation of a dideoxynucleotide into the nascent (elongating) DNA strand therefore terminates DNA strand extension, resulting in various DNA fragments of varying length. The dideoxynucleotides are added at lower concentration than the standard deoxynucleotides to allow strand elongation sufficient for sequence
analysis.