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High school biology

Course: High school biology > Unit 4

Lesson 3: The cell cycle and mitosis
Science
High school biology
Reproduction and cell division
The cell cycle and mitosis
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The cell cycle and mitosis review

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Key terms

TermMeaning
Cell cycleThe series of growth and development steps a cell undergoes between its formation and reproduction
InterphasePhase of the cell cycle where the cell grows and makes a copy of its DNA
MitosisPhase of the cell cycle where the cell separates its DNA into two sets and divides, forming two new cells
CancerA disease of uncontrolled cell growth

The cell cycle

In eukaryotic cells, the cell cycle is divided into two major phases: interphase and mitosis (or the mitotic (M) phase).
Interphase is the longest part of the cell cycle. This is when the cell grows and copies its DNA before moving into mitosis. During mitosis, chromosomes will align, separate, and move into new daughter cells.
The prefix inter- means between, so interphase takes place between one mitotic (M) phase and the next.
Image of the cell cycle. Interphase is composed of G1 phase (cell growth), followed by S phase (DNA synthesis), followed by G2 phase (cell growth). At the end of interphase comes the mitotic phase, which is made up of mitosis and cytokinesis and leads to the formation of two daughter cells. Mitosis precedes cytokinesis, though the two processes typically overlap somewhat.
Image credit: "The cell cycle: Figure 1" by OpenStax College, Biology (CC BY 3.0).

Interphase

Interphase consists of three steps:
  • G1 phase: first gap phase; the cell grows larger and organelles are copied
  • S phase: synthesis phase; the cell synthesizes a complete copy of the DNA in its nucleus
  • G2 phase: second gap phase; the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis
Cells that are meant to divide will complete G2 and enter mitosis. Other types of cells that divide slowly or not at all may exit the G1 phase and enter a non-dividing state called G0. Some cells remain here indefinitely, while others may re-enter division under the right conditions.

Mitosis (the M phase)

The process of mitosis, or cell division, is also known as the M phase. This is where the cell divides its previously-copied DNA and cytoplasm to make two new, identical daughter cells.
Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase.

Stages of mitosis

Early prophase. The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears.
Late prophase (prometaphase). The nuclear envelope breaks down and the chromosomes are fully condensed.
Metaphase. Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles.
Anaphase. The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The microtubules that are not attached to chromosomes push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles.
Telophase. The spindle disappears, a nuclear membrane re-forms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense.
Cytokinesis in animal and plant cells.
Cytokinesis in an animal cell: an actin ring around the middle of the cell pinches inward, creating an indentation called the cleavage furrow.
Cytokinesis in a plant cell: the cell plate forms down the middle of the cell, creating a new wall that partitions it in two.

Cancer and cell cycle regulation

Checkpoints

The cell cycle is usually regulated by checkpoints. These are the factors that a cell considers when deciding whether or not to move forward through the cell cycle, and include both external cues (like molecular signals) and internal cues (like DNA damage).
Diagram of cell cycle with checkpoints marked. G1 checkpoint is near the end of G1 (close to the G1/S transition). G2 checkpoint is near the end of G2 (close to the G2/M transition). Spindle checkpoint is partway through M phase, and more specifically, at the metaphase/anaphase transition.

Cancer

Cancer is a term that describes many different diseases caused by the same problem: uncontrolled cell growth.
Most cancers occur due to a series of mutations that make them divide more quickly, bypass checkpoints during cell division, and avoid apoptosis (programmed cell death).
Generally, mutations of two types of cell cycle regulators can promote the development of cancer:
  • Positive regulators, which normally promote cell growth, may become hyperactivated (oncogenic).
  • Negative regulators (tumor suppressors), which prevent the formation of tumors, may become inactivated.

Common mistakes and misconceptions

  • Interphase is not part of mitosis. Although we often talk about interphase and mitosis together, interphase is technically not part of mitosis. However, both processes are part of the larger cell cycle, where interphase consists of the G1, S, and G2 stages of the cell cycle.
  • DNA replication occurs during interphase, not prophase. A common misconception is that DNA copies itself during prophase, but this is not true. In prophase, the DNA has already been copied while the cell was in interphase.
  • The chromosome number is the same in the daughter cells as it was in the parent cell. Because DNA is duplicated during interphase before the cell undergoes mitosis, the amount of DNA in the original parent cell and the daughter cells are exactly the same.
  • Both genetics, as well as external factors, can play a role in the development of cancer. Many types of cancer have a genetic component, so inheriting certain genes may make someone more likely to get these types of cancer. However, having these genes does not necessarily mean that cancer will develop, as factors such as lifestyle and environment also play a part.

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