Cell Cycle

The cell cycle is composed of the mitotic (M) phase and interphase (Figure 1). The mitotic phase, which includes both mitosis and cytokinesis, is the shortest part of the cell cycle. Most of the time, the cells are in interphase, which is composed of three different subphases: the G1 phase (first gap), the S phase (synthesis), and the G2 phase (second gap).

The goal of interphase is to grow by producing proteins and cytoplasmic organelle such as the endoplasmic reticulum. DNA is replicated during the S phase only. During the M phase, the replicated chromosomes are segregated into individual nuclei (mitosis), and the cell splits into two (cytokinesis).

The image of the cell cycle shows a circle divided into 4 pieces like a pie graph with a red arrow that goes clockwise. The arrow begins during the section that takes up 40% of pie graph labelled Cell Growth. Cell Growth phase contains G1 checkpoint or restriction. Arrow continues to section labelled DNA synthesis that takes up 35% of the pie graph. Arrow continues to small section labelled Growth that takes up 20% of pie graph. Growth phase contains G2 checkpoint. Arrow continues to small section that takes up 5% labelled my-totic phase. My-totic phase contains M checkpoint causing formation of two daughter cells.

Figure 1. Phases of the cell cycle: the key steps a cell must go through in order to divide.

Cell division occurs during the M phase whereby the replicated chromosomes from the S phase are split in half into individual nuclei, followed by cytokinesis where the cell itself splits in half forming two daughter cells.

A cell doesn’t continuously divide. Some cells, such as nerve cells and muscle cells, do not divide at all or they do it a very low rate in a mature humans. Cell division rates and timing are crucial for development, growth and maintenance. Breakdown in cell cycle control plays a major role in cancer development.

S phase

During the S phase, the chromosomes and chromatin protein that govern various aspects of chromosomes are duplicated accurately. Chromosome duplication is triggered by the activation of S-Cdk.


Cytokinesis is slightly different in plant and animal cells. In animal cells, a contractile ring of actin and myosin divides the cytoplasm into two. By contrast in plants, a growth plate comprised of fused golgi vesicles grows outward from the middle of the cell, dividing the old cell in two.