Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

3. 5' - 3' direction

Explanation:

DNA replication occurs in 5'to 3' direction whereas the parent or template DNA is read in 3'to 5' DNA. Normal DNA polymerases are 5'-to-3' polymerases. DNA polymerases extend the 3' tail of the DNA molecule but it synthesises 5'-to-3'. 3' to 5' polymerases would never work because the energy required would be way too high.

When DNA is replicating, there are two strands necessary to do this. One is called a template strand which serves as the strand who's complementary strand will be synthesised. The other is called primer strand which will be extended in the direction of synthesis.

When polymerization occurs, the DNA polymerase attaches to the 3' -OH of the pentose sugar of the previous nucleotide of the primer strand. Unless it finds that -OH, synthesis is halted.