Cracking the AP Biology Exam


Cell Reproduction


Every cell has a life cycle—the period from the beginning of one division to the beginning of the next. The cell’s life cycle is known as the cell cycle. The cell cycle is divided into two periods: interphase and mitosis. Take a look at the cell cycle of a typical cell on the next page.

Notice that most of the life of a cell is spent in interphase.


Interphase is the time span from one cell division to another. We call this stage interphase (inter- means between) because the cell has not yet started to divide. Although biologists sometimes refer to interphase as the “resting stage,” the cell is definitely not inactive. This phase is when the cell carries out its regular activities. All the proteins and enzymes it needs to grow are produced during interphase.

The Three Stages of Interphase

Interphase can be divided into three stages: G1, G2, and S phase.

The most important phase is the S phase. That’s when the cell replicates its genetic material. The first thing a cell has to do before undergoing mitosis is to duplicate all of its chromosomes, which contain the organism’s DNA “blueprint.” During interphase, every single chromosome in the nucleus is duplicated.

You’ll notice that the original chromosome and its duplicate are still linked, like Siamese twins. These identical chromosomes are now called sister chromatids (each individual structure is called a chromatid). The chromatids are held together by a structure called the centromere. Although the chromosomes have been duplicated, they are still considered a single unit. Once duplication has been done, we’re ready for the big breakup: mitosis.

We’ve already said that replication occurs during the S phase of interphase, so what happens during G1 and G2? During these stages, the cell produces proteins and enzymes. For example, during G1 the cell produces all of the enzymes required for DNA replication (as we saw inChapter 6, that means DNA helicase, DNA polymerase, and DNA ligase). By the way, “G” stands for “gap,” but we can also associate it with “growth.”

Let’s recap:

  • The cell cycle consists of two things: interphase and mitosis.
  • During the S phase of interphase, the chromosomes replicate.
  • Growth and preparation for mitosis occur during the G1 and G2 stages of interphase.