During the cell replication cycle there are two major "phases" that the cell goes through. The beginning and majority of the time is known as Interphase. Interphase is 80% of the total cycle and is where the cell does most of the work in replicating organelles, chromosomes, and preparing to undergo Mitosis which is the actual splitting process where one cell becomes two. 

During Interphase there are 3 major steps, G1, S, and the G2 phase. In the G1 phase the cell begins creating all of the proteins required to build chromosomes and DNA. During the S phase the cell replicates the chromosomes by taking one chromatid and effectively doubling it (these are later ripped apart during Anaphase in Mitosis). 

So during the S phase of the Interphase cycle, chromosomes are replicated into two sister chromatids. Normally after this phase there is a check off to make sure nothing went wrong with the replication however should there be any mutations that will create some new DNA form this is where it would be made. 

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During what phase of the cell cycle is DNA replicated?

There are two main phases of the cell cycle in eukaryotes: Mitosis and interphase. Since the time a cell spends in interphase takes approximately 90% of the entire cell cycle, it is helpful to further break interphase down into three sub-phases: G1 (first period of growth), S (synthesis), and G2 (second period of growth).

DNA is replicated during the S phase of the cell cycle. After passing the G1 checkpoint, which evaluates whether a cell has the size, nutrients, and DNA integrity to continue with replication, the cell becomes committed to division.

Upon entering the S phase, the enzyme helicase will "unzip" the double helix structure, breaking apart the bonds between adenine and thymine and between cytosine and guanine. The structure which resembled a ladder will now look more like a Y as the prongs of the replication fork become templates for the new strands of DNA.

A leading strand and a lagging strand will undergo different processes as new DNA is created using the "unzipped" DNA on each side. When the process is complete, an enzyme called DNA ligase will seal up the new sequence of DNA into two separate double strands of DNA. Each new strand of DNA winds back up into the recognizable double helix structure and prepares to enter the G2 phase of cellular division.

During what stage of the cell cycle does replication occur?

Cell cycle consists of events that ultimately result in generation of two daughter cells from a single cell. These events include Interphase, mitotic phase and cytokinesis. The interphase is divided into three parts: G1, S and G2. It is during the S-phase (or synthesis phase), that replication takes place. During the preceding G1 phase (or growth phase), the cell starts gaining mass and grows in size, builds organelles and protein. In the S-phase, DNA replication takes place and the cell ends up with exactly double the amount of chromosomes, with each chromosome consisting of two chromatids. This is a relatively fast step, especially compared to the G1 phase and proceeds at a fast pace to protect the exposed base pairs.

hope this helps.