Final answer:
Dideoxynucleotides cause chain termination in Sanger sequencing because they lack a free 3' OH group, preventing the addition of the next nucleotide. A mixture of deoxynucleotides and dideoxynucleotides in a specific ratio is used to create DNA fragments of different sizes.
Step-by-step explanation:
The dideoxynucleotides (ddNTPs) cause chain termination in Sanger sequencing because they lack a free 3' OH group on the sugar molecule, which is required for the addition of the next nucleotide in the DNA chain. When a ddNTP is incorporated into the growing DNA strand, it terminates the chain since it cannot form a bond with the next nucleotide. By using a mixture of regular deoxynucleotides (dNTPs) and ddNTPs in a predetermined ratio, DNA fragments of different sizes can be generated.