DNA helicase unwinds the double helix, separating the two strands so they may be replicated by DNA polymerase.

Primase adds an RNA primer to help initiate DNA replication. DNA ligase is responsible for joining Okazaki fragments on the lagging strand during replication.

DNA replication is the process of copying the parent DNA helix into two identical daughter helices. The process is semi-conservative, which means that one parent strand is passed down to each daughter strand. The process begins when helicase unwinds the double helix and separates the two strands to create the replication fork. Topoisomerase helps this process by relieving rotational strain on the helix when it is being unwound. DNA polymerase adds new nucleotides to the daughter strand, synthesizing the new DNA strand.

During replication there is a leading strand, which occurs when replication occurs from 5' to 3' and moves towards the replication fork, and a lagging strand, when replication occurs away from the replication fork. Replication occurs in short segments on the lagging strand, known as Okazaki fragments. The protein DNA ligase is responsible for finally fusing these fragments together after they are made by DNA polymerase.

The question states that DNA helicase "unzips" the two strands of DNA; therefore, this enzyme must be breaking down the bonds between base pairs.

The bonds between base pairs are called hydrogen bonds, which is a noncovalent bond. This means that the DNA helicase is breaking down the hydrogen bonds between base pairs in order to separate the two strands. In DNA, there are two kinds of base pairs: purines and pyrimidines. Recall that adenine and guanine are classified as purines whereas thymine and cytosine are classified as pyrimidines; therefore, a base pairing in DNA always occurs between a purine and a pyrimidine. This means that the DNA helicase is breaking down the hydrogen bonds between purines and pyrimidines.

DNA ligase is the protein responsible for linking, or ligating, Okazaki fragments together in order to form a single complete DNA strand. This action only necessary on the lagging strand; the leading strand can be made continuously by DNA polymerase since it is able to read away from the replication fork in the 3'-to5' direction. Since the DNA polymerase on the lagging strand must read toward the replication form, it cannot by synthesized continuously.

Helicase is the protein resposible for unwinding the DNA double-helix. Single-strand binding proteins attach to the freshly unwound strands of DNA and ensure that the strands do not re-anneal. Helicase creates the replication fork opening, allowing replication proteins to enter and bind; single-strand binding proteins keep the replication fork open as proteins enter.

A DNA fragment will be formed in the 5' to 3' direction because of the polarity of the DNA molecule. Adding nucleotides to the 3' end allows DNA polymerase to use the phosphate molecules as "fuel," and add a new nucleotide to the DNA strand.

DNA Polymerase I removes the primer from the 5' end of a lagging strand, and replaces it with DNA nucleotides. This allows DNA synthesis to begin on the lagging strand. 

Messenger RNA, or mRNA, is the RNA strand that is transcribed from the gene found on DNA. It is responsible for being read by a ribosome in order to create a protein.

Ribosomal RNA (rRNA) forms a structural component of the ribosomes. Transfer RNA (tRNA) carries amino acid residues and provides an anticodon to add the amino acids to the growing protein at the ribosome. Small nuclear RNA (snRNA) are found in the nucleus and help regulate transcription and maintain telomere length.

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.