The Cell Cycle

What you’ll learn to do: Identify the stages of the cell cycle, by picture and by description of major milestones

The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to 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. The cell cycle has two major phases: interphase and the mitotic phase (Figure 1). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell divides.

The diagram shows Mitosis and cell division as a cyclic process. The cycle starts at interphase. Interphase is divided into three stages: G1, S, and G2. During interphase, the cell is preparing to divide; DNA is duplicated, creating sister chromatids. Interphase is followed by the mitotic cycle, mitosis and cytokinesis. Mitosis is a process in which a diploid cell nucleus divides itself (duplicates) into 2 diploid nuclei. The first stage is prophase, during which the nucleus begins to disappear. The next stage is metaphase. During this phase, all sister chromatids are aligned in the center of the cell. Next is anaphase. During this phase, the sister chromosomes are all split. Next is telophase. A new nucleus is formed around each set of chromatids. The two nuclei are divided from each other, creating two new cells in a process called cytokinesis. Each new cell is now ready to being the cycle again.

Figure 1. The cell cycle

Learning Outcomes

  • Identify the characteristics and sub-phases of interphase
  • Identify the characteristics and stages of mitosis
  • Identify the characteristics of cytokinesis

Interphase

Stages of Interphase

During interphase, the cell undergoes normal growth processes while also preparing for cell division. It is the longest phase of the cell cycle, cell spends approximately 90% of its time in this phase.  In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met.

The three stages of interphase are called G1, S, and G2.

G1 Phase (First Gap)

The first stage of interphase is called the G1 phase (first gap) where the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus.

S Phase (Synthesis of DNA)

In the S phase, DNA replication results in the formation of identical pairs of DNA molecules—sister chromatids—that are firmly attached to the centromeric region.

G2 Phase (Second Gap)

In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase.

Mitosis

The mitotic phase (also known as M phase) is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis, or nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells.

Karyokinesis (Mitosis)

Karyokinesis, also known as mitosis, is divided into a series of phases—prophase, metaphase, anaphase, and telophase—that result in the division of the cell (Figure 2).

The cell cycle, with each step labelled. The six steps in order are prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Mitosis ends when two daughter cells are produced.

Figure 2. Stages of mitosis

During prophase, the “first phase,” the nuclear envelope starts to dissociate into small vesicles, and the membranous organelles fragment and disperse toward the periphery of the cell. The nucleolus disappears . The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly and become visible under a light microscope.  Each sister chromatid develops a protein structure called a kinetochore in the centromeric region (Figure 3). The proteins of the kinetochore attract and bind mitotic spindle microtubules.

During prometaphase, the nuclear envelope is fully broken down and chromosomes are attached to microtubules from both poles of the mitotic spindle, which begin to move them toward the middle of the cell.

This illustration shows two sister chromatids. Each has a kinetochore at the centromere, and mitotic spindle microtubules radiate from the kinetochore.

Figure 3. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles.

During metaphase, all the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, midway between the two poles of the cell.  At this time, the chromosomes are maximally condensed.

During anaphase, the sister chromatids separate at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated (oval shaped) as the polar microtubules slide against each other at the metaphase plate where they overlap.

During telophase,  the chromosomes reach the opposite poles and begin to decondense (unravel), relaxing into a chromatin configuration.  Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.

The activity below will walk you through mitosis—providing you with the chance to review the different steps of the process and how they work together.

Click here for a text-only version of the activity.

Cytokinesis

Cytokinesis is the second main stage of the mitotic phase during which cell division is completed via the physical separation of the cytoplasmic components into two daughter cells. Division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.

In cells such as animal cells that lack cell walls, cytokinesis follows the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. This fissure, or “crack,” is called the cleavage furrow. The furrow deepens as the actin ring contracts, and eventually the membrane is cleaved in two (Figure 4).

Part a: This illustration shows cytokinesis in a typical animal cell. Part b: Cytokinesis is shown in a typical plant cell. In an animal cell, a contractile ring of actin filaments forms a cleavage furrow that divides the cell in two. In a plant cell, Golgi vesicles coalesce at the metaphase plate. A cell plate grows from the center outward, and the vesicles form a plasma membrane that divides the cytoplasm.

Figure 4. During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring contracts, forming a cleavage furrow, which divides the cell in two. In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast. A cell plate formed by the fusion of the vesicles of the phragmoplast grows from the center toward the cell walls, and the membranes of the vesicles fuse to form a plasma membrane that divides the cell in two.

In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast (a vesicular structure) at the metaphase plate. There, the vesicles fuse and coalesce from the center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma membrane on either side of the new cell wall (Figure 4).

The Complete Cell Cycle

Remember, mitosis is the process of cell division, but it’s just a portion of the full cell cycle. Figure 5 shows approximately how long a cell spends in each stage of the cell cycle:

Like a clock, the cell cycles from interphase to the mitotic phase and back to interphase. Most of the cell cycle is spent in interphase, which is subdivided into G_{1}, S, and G_{2} phases. Cell growth occurs during G_{1}, DNA synthesis occurs during S, and more growth occurs during G_{2}. The mitotic phase consists of mitosis, in which the nuclear chromatin is divided, and cytokinesis, in which the cytoplasm is divided, resulting in two daughter cells.

Figure 5. 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.

As you can see, cells spend most of their time in interphase.

Video Review: The Cell Cycle

This video reviews all the steps of mitosis; seeing it all together is a great review at this stage.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.