The process of DNA replication involves the accumulation of a nick in one of the two strands of the DNA helix. This nick results in the unwinding of the double helical structure and the decrease in the torsion of the DNA structure. The nicked strand consequently results in the generation of the two single stranded DNA structures, namely the leading and the lagging strands.
The leading strand is characterized by the 5’to 3’ growth of a complementary strand which is produced by the enzyme DNA polymerase. On the other hand, the lagging strand is stabilized by single-stranded binding (SSB) proteins, as short DNA segments known as Okazaki fragments adhere to this strand. The fragments serve as primers for the elongation of the newly synthesized DNA strand. DNA polymerase performs the addition of nucleotides to the growing strand. One cycle of DNA replication thus results in two copies of the original DNA strand.
A mutation is a change in the nucleotide sequence of a DNA strand (Reece and Campbell, 2001). A substitution is a type of mutation that involves the modification of a single nucleotide, which in turn results in a change in the protein product. An insertion is another type of mutation that involves in inclusion of at least one nucleotide into a particular DNA sequence. A deletion is a type of mutation that involves the loss of at least one nucleotide in a stretch of DNA.
Data Analysis Exercise on page 227
- Tumors 1, 2, 4 and 5 were missing the marker D17S250. This marker is located outside the NFI gene.
- The four tumors, namely 1, 2, 4 and 5, show a deletion of the entire long arm of chromosome 17.
- Reece, J.B. and Campbell, N.A. (2001). Biology, 6th ed. San Francisco: Benjamin Cummings. 1175 pages.