BIO101: Introduction to Molecular and Cellular Biology (2022.A.01)

The cell cycle includes all parts of the normal lifetime of a single, eukaryotic cell. It starts when a cell is created through the division of a previous cell, and it ends when that cell undergoes its own cell division to produce two new cells.

A single cell cycle consists of four major phases of unequal length:

1. The G₁ phase includes most of the normal lifetime of the cell. During G1, the cell goes about using its DNA as instructions for building proteins that allow the cell to metabolize and function for its specific purpose. During this time, the DNA has not yet replicated.
2. The S phase is the first step in a cell's preparation for cell division. During the S phase, the DNA is replicated, yielding two identical copies of the DNA (one for each of two cells that will be created when this cell divides).
3. The G₂ phase follows the S phase. Like the G1 phase, the G2 phase features much protein synthesis and metabolism, but most of this activity is concentrated on preparing for cell division.
4. The M phase includes mitosis (the division of the nucleus) and cytokinesis (the division of the cytoplasm). By the end of the M phase, two separate cells have been created from the original cell, and each of these two cells enters its own cell cycle. 

Mitosis is the division of a eukaryotic cell's nucleus. One of the hallmarks of mitosis is that when the original nucleus divides, two genetically identical nuclei are produced. That is, the chromosomes in one nucleus are exactly the same as the chromosomes in the other nucleus. Each nucleus is genetically identical to the nucleus in the original cell (before mitosis). Therefore, mitosis creates two nuclei from one, and each of those nuclei can serve as the nucleus of a new cell. When cytokinesis accompanies mitosis, the cytoplasm divides, forming two distinct cells. Each cell contains its own nucleus (created by mitosis). Cell division that features mitosis allows one parent cell to divide into two, genetically identical daughter cells.

Creating new cells is important for several reasons. Mitosis allows for cell proliferation for the purpose of developing a unicellular zygote into a multicellular organism. Mitosis also allows for cell proliferation for the purpose of growing a multicellular organism. It also allows for the creation of new cells to replace cells that have been damaged by injury or infection in a multicellular organism. Mitosis is a means for a unicellular, eukaryotic organism to reproduce.

As you review mitosis, think about how mitosis (as part of the M phase) fits into the cell cycle. Mitosis occurs in five phases.

1. Prophase: During this first phase of mitosis, the microtubules that make up the mitotic spindle begin forming on the two centrioles, and those centrioles start to move to opposite poles.
2. Prometaphase: Construction of the mitotic spindle is completed, and the nuclear envelope disintegrates, allowing microtubules of the mitotic spindle to connect to replicated chromosomes.
3. Metaphase: Replicated chromosomes (each consisting of two identical sister chromatids) move along the spindle tubules until all replicated chromosomes are aligned at the metaphase plate, midway between the poles.
4. Anaphase: Each pair of sister chromatids (one pair for each replicated chromosome) separate and move toward opposite poles. At this point, they are no longer called chromatids; rather each is an unreplicated chromosome.
5. Telophase: The unreplicated chromosomes reach opposite poles. Each pole becomes a new nucleus, as each pole becomes enclosed by a new nuclear envelope. In most cases, cytokinesis (the division of the cytoplasm) occurs near the end of telophase, in which case the original cell is separated into two distinct cells, each with its own nucleus.