The G₁ Checkpoint is the correct answer, because if a cell gets a signal at this checkpoint then the cell goes on to complete the S, G₂, and M phases and will end up dividing. If this signal is not received at the G₁ checkpoint then the cell enters the non-dividing G₀ phase.

Once the cell passes the G1 checkpoint, the cell becomes committed to the cell cycle and enters the S phase where DNA is replicated. The checkpoint is to ensure the cell has grown enough and has enough resources to begin DNA replication. The next checkpoint is the G2 checkpoint, where the cell checks and makes sure the DNA replicated correctly before beginning mitosis. If the cell does not pass this checkpoint, it commences apoptosis and dies. 

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.