SIG - Cell Cycle [LESSON]

Cell Cycle

The cell cycle can be thought of as the life cycle of a cell. In other words, it is the series of growth and development steps a cell undergoes between its “birth”—formation by the division of a mother cell—and reproduction—division to make two new daughter cells. Much of this lesson should be a review from your introductory biology class. Here’s a great introductory video to the broad concepts involved with mitosis. Go ahead and watch and take notes. Pay close attention to the condensation of DNA throughout the cell cycle.

Watch the Mitosis video below.

The cell cycle is split into three overall parts: interphase, mitosis, and cytokinesis.

The image shows the three major parts of the cell cycle – interphase, mitosis, and cytokinesis.

Interphase

Preparation for division happens in three steps:

  • G1 phase. During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps. 
  • S phase. In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase.
  • G2 phase. During the second gap phase, or G2phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis.

These three phases together are known as interphase.

Mitotic Phase

During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division-related processes: mitosis and cytokinesis.

In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis takes place in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase.

Mitosis

Click through the Mitosis Stages interactive below to learn more about the stages of mitosis.

Cytokinesis

In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells.

the image shows animal and plant cells.

What happens to the two daughter cells produced in one round of the cell cycle? This depends on what type of cells they are. Some types of cells divide rapidly, and in these cases, the daughter cells may immediately undergo another round of cell division. For instance, many cell types in an early embryo divide rapidly, and so do cells in a tumor.

Other types of cells divide slowly or not at all. These cells may exit the G1 phase and enter a resting state called G0 phase. In G0 a cell is not actively preparing to divide, it’s just doing its job. For instance, it might conduct signals as a neuron or store carbohydrates as a liver cell. G0 is a permanent state for some cells (example: most muscle cells), while others may re-start division if they get the right signals.

Practice below Matching the phases of the cell cycle to the descriptions.

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