DNA replication involves the separation of the two original DNA strands. Both of these strands are then replicated using DNA polymerase. This results in two DNA double helices, each with a new strand and an original strand.

Consider this example, in which the parent strands are represented by "P" and the daughter strands are represented by "D."

Before replication there are two parent strands: PP

The parent strands are split: P   P

Daughter strands are made for each parent strand: PDDP

The fully-replicated double strands separate: PD   DP

Each final strand has one parent strand (old DNA) and one daughter strand (new DNA).

O=Original strand, N=New strand

Before any replications: OO

After one round of replication, both new double-helices contain one strand from the original double-helix and one newly synthesized strand.

1 replication: ON₁, N₁O

After the second replication, there are now four double-helix molecules. Two contain original strands in combination with new strands, and two contain only new strands.

2 replications: ON₂, N₂N₁, N₁N₂, N₂O

After the thrid replication event there will be eight total molecules. Of these, six will contain only new strands and two will contain a combination of original and new strands.

3 replications: ON₃, N₃N₂, N₂N₃, N₃N₁, N₁N₃, N₃N₂, N₂N₃, N₃O

There must always be two double-helices that contain original strands, as there are always only two original strands and they do not disappear.

RNA primer is the correct answer. A protein called RNA primase reads the existing DNA strand and adds a short sequence of RNA nucleotides. DNA polymerase then builds onto the 3' end of the RNA primer. After replication, the RNA primer is removed and replaced with DNA nucleotides.

The correct name of the fragments is Okazaki fragments. The lagging strand aligns in the 5'-to-3' direction (away from the replication fork), but must be read in the 3'-to-5' direction (toward the replication fork) by DNA ploymerase. The result is non-continuous synthesis of the strand in small fragments, called Okazaki fragments. DNA ligase fuses these fragments together later in the replication process.

The enzyme helicase divides the two strands of the double helix; henceforth, single stranded binding (SSB) proteins stabilize the newly single strands, and prevent reannealing. The enzyme DNA gyrase ensures the double stranded areas beyond the replication bubbles do not supercoil, relieving the newly-added tension. Primase is a type of RNA polymerase that adds an RNA primer to the DNA to begin replication. DNA Polymerase III cannot begin replication without this primer. Ligase joins ends of Okazaki fragments that were produced on the lagging strand.

The enzyme primase adds sequences of RNA to the DNA strand to begin replication. Primase is a type of RNA polymerase, and thus, it does not need a free 3' hydroxyl group as a substrate. The nucleotides it lays down act as a substrate for DNA polymerase. Okazaki fragments from the lagging strand are joined by ligase, and helicase is responsible for unzipping the DNA to prepare for replication.

During DNA replication, the parent strand is used as a template that the new strand uses to add the correct nucleotides (via complementary base pairing). The entire parent strand (template) is conserved, while the daughter strand is completely synthetic, meaning the nucleotides came from free nucleoside triphosphates (ATP, TTP, GTP, and CTP). Thus, DNA replication is said to be semi-conservative. The Meselson-Stahl experiment illustrated this principle through the use of different isotopes of nitrogen.

Primase, like all RNA polymerases, can lay down nucleotides with their only substrate being the template strand. Even though the nucleotides are RNA, not DNA, they still have the substrate that DNA polymerase needs in order to add nucleotides—a free 3' . Since the difference between deoxyribose and ribose occurs at the 2' position, DNA polymerase can use either RNA or DNA as a substrate. Helicase and gyrase help with formation and maintenance of the replication bubble, however they indirectly help DNA polymerase add new nucleotides.

DNA ligase catalyzes the formation of bonds between the Okazaki fragments and the DNA that has replaced the RNA primers on the lagging strand.

DNA polymerase I is the only polymerase that has 5'  3' exonuclease activity. This means that it can remove nucleotides in the 5'  3' direction. It also has 3'  5' exonuclease activity, as does DNA polymerase III; this is like a "backspace" for nucleotides that have just been added and need to be removed. DNA polymerase II's functions are largely unknown, DNA polymerase V plays a complex role in DNA repair, not replication.