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.
During the Cell Cycle DNA Replication happens during Interphase .
Interphase is the longest cycle in the Cell Cycle accounting for 90% of the cycle. The remaining 10% accounts for cell division which is Mitosis (occurs on somatic cells only, somatic cells are all cells except sex cells) and Meiosis (which involves sex cells, for reproduction) During the interphase, the chromosomes are very elongated, they appear in mass called chromatin. In the early part of interphase the cell is active producing cell organelles such as mitochondria and chloroplasts.
In later part of the interphase, the chromosomes produce identical copies of themselves. The duplication of a chromosome produces a chromosome with two strands. The two strands have identical genes. DNA REPLICATION HAPPENS IN DURING THE INTERPHASE OF THE CELL CYCLE..
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The cell replicates in the interphase cycle of the cell