AP BIOLOGY

Detailed explanation-1: -After a primer is synthesized on a strand of DNA and the DNA strands unwind, synthesis and elongation can proceed in only one direction. As previously mentioned, DNA polymerase can only add to the 3’ end, so the 5’ end of the primer remains unaltered.

Detailed explanation-2: -The need for accuracy probably explains why DNA replication occurs only in the 5′-to-3′ direction. If there were a DNA polymerase that added deoxyribonucleoside triphosphates in the 3′-to-5′ direction, the growing 5′-chain end, rather than the incoming mononucleotide, would carry the activating triphosphate.

Detailed explanation-3: -The template DNA strand and RNA strand are antiparallel. RNA polymerase always builds a new RNA strand in the 5’ to 3’ direction. That is, it can only add RNA nucleotides (A, U, C, or G) to the 3’ end of the strand.

Detailed explanation-4: -DNA is always synthesized in the 5’ to 3’ direction because DNA polymerases can only catalyze phosphodiester bond formation (a condensation reaction) between the 3’ OH of the sugar of the last nucleotide of an existing strand and the 5’ phosphate group of the incoming nucleotide.