The short answer is before they divide. But it’s not that simple. Some cells don’t divide much, or at all, after we’re born. Our muscle and nerve cells are like that. Other cells, like our skin cells, divide fast and keep dividing all our lives. They do slow down as we age.

A new cell (which was just created by a cell dividing) starts off by growing. It has to take on nutrients for the building blocks of new cell parts, and the energy to do the work of building them. That is the first growth phase (G1). Then the DNA replicates. This is called the synthesis (S) phase because new DNA is being synthesized (made) using the original DNA as a pattern, or template. Then there’s anothergrowth phase (G2). The cell is making things it needs in order to divide.

Cells duplicate their DNA when they decide they need to divide. Cells divide for three main reasons: growth, repair, and reproduction. This could be during mitosis or meiosis. Mitosis is the dividing of your non-reproductive cells, and meiosis is the dividing of your reproductive cells. In mitosis, there is a phase that happens before the cell divides called interphase. Interphase can be further broken down into the G1, S and G2 phase. G1 phase causes the cell to grow in size and produce new organelles, which prepare for S phase, when the DNA replicates.

Cells will duplicate or copy their DNA right before they divide. The process of cell division is called mitosis. Since the cell is dividing it needs two copies of its DNA - one is kept by the parent cell and the other is passed to the daughter cell. If cells don't replicate their DNA or don't do it completely, the daughter cell will end up with no DNA or only part of the DNA. This cell will likely die. So this process of duplicating DNA is very important. Cells also copy their DNA right before a special cell division event called meiosis, which results in special cells called gametes (also known as eggs and sperm.)

Are you familiar with the cell cycle? It describes the process by which cells grow and divide to make new cells. Since every cell needs a copy of DNA then cells need to replicate their DNA at some point before they divide.

There are four main phases in the cell cycle that describe the times the cell is growing and then making new cells. These stages are G₁,S, G₂,and M. I know this already sounds a little complicated, but that’s ok, once you have all the information this idea should be a little clearer.

G₁ is the first growth phase where the cell just grows bigger. This phase also serves as a checkpoint where the cell can make sure it’s ready for the next step which is S phase. The S is for synthesis which is where the DNA replicates itself so the cell has two copies of DNA. The next phase is G₂ which is another phase of growth. At this point the cell checks itself again, makes sure it is big enough and ready to go into the M phase. M phase stands for mitosis which is where the cell goes through a lot of complex steps in order to split into two daughter cells.

Since the cell cycle is a circle, both daughter cells can then move straight into G₁ phase to start the cycle all over again. Phases G₁, S, and G₂ are all collectively called “interphase” which is just a fancy word for “not mitosis”. The cell cycle can be a really difficult idea to wrap your head around but asking questions like these is the first step toward understanding science better. Thanks for the question!

Cells duplicate DNA during interphase, usually shortly before entering either mitosis or meiosis (eukaryotic cells) or fission (prokaryotes).