The cell cycle has two major phases: interphase (G0, G1, S, G2) and the mitotic phase (M).
The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells undergoing cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells.
During interphase, the cell grows, and DNA replicates. In the mitotic phase, the replicated DNA and cytoplasmic contents separate, and the cell divides.
The Stages of Interphase and the Cell Cycle: The cell cycle consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei. The cytoplasm is usually divided as well, resulting in two daughter cells
G0 Phase
Not all cells undergo mitotic phase. Cells in the G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. No more DNA replication or cell division happens at this phase. The cells that never or rarely divide include mature cardiac muscle and nerve cells, and they remain in G0 permanently.
G1 Phase (First Gap)
The first stage of interphase is the G1 phase (first gap), the growing phase. All cells undergo G1. Here, the cell is quite active at the biochemical level. The cell grows and accumulates the building blocks of chromosomal DNA and the associated proteins as well as sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. Cells increase in size and produce organelles.
The cell has two choices at this point: to divide or not to divide. Between G1 and S phase, the cell decides if it wants to grow.
Some cells that do not divide include bone cells and blood cells (they do not undergo mitosis). These cells do not go through S or G2. They stop at G1 or G0.
S Phase (Synthesis of DNA)
The synthesis phase of interphase takes the longest because of the complexity of the duplicated genetic material. The S phase is where DNA replication occurs, and centrioles replicate. The two centrosomes give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes of animal cells associate with a pair of rod-like objects, the centrioles, which are at right angles to each other. Centrioles help organize cell division.
G2 Phase (Second Gap)
In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. This phase is where the cell prepares for division. Here, the cell has double the DNA and again increase in size. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G2.
M Phase
Following the interphase, the cell enters the multistep mitotic phase, where cell nucleus divides, and the cell components split into two identical daughter cells.
Practice Questions
MCAT Official Prep (AAMC)
Biology Question Pack, Vol. 1 Passage 4 Question 22
Biology Question Pack, Vol 2. Question 41
Biology Question Pack, Vol 2. Passage 17 Question 111
Practice Exam 1 B/B Section Passage 8 Question 40
Practice Exam 4 B/B Section Question 59
Key Points
• There are three stages of interphase: G1 (first gap), S (synthesis of new DNA ), and G2 (second gap).
• Cells spend most of their lives in interphase, specifically in the S phase where genetic material must be copied.
• Some cells that do not divide or replicate stops at G1 or G0; G0 and G1 are sometimes the same thing.
• The cell grows and carries out biochemical functions, such as protein synthesis, in the G1 phase.
• During the S phase, DNA as well as centrioles are replicated.
• In the G2 phase, energy is replenished, new proteins are synthesized, and additional growth occurs.
• After interphase, mitosis follows.
Key Terms
interphase: the stage in the life cycle of a cell where the cell grows and DNA is replicated
centrosome: an organelle near the nucleus of a cell that contains the centrioles (in animal cells) and from which the spindle fibers develop in cell division.
mitotic spindle: the apparatus that orchestrates the movement of chromosomes during mitosis
quiescent: in a state or period of inactivity or dormancy
centrioles: the main centers that help in the formation of microtubule fiber
