Mitosis steps consist of prophase, metaphase, anaphase, and telophase; the cell undergoes nucleus division and split into two identical daughter cells.
The stages of mitosis occur in sequence with specific events in each one. By observing the chromosome, one can identify the mitotic process. Remember that the DNA is in the nucleus, which is surrounded by the nuclear membrane. The DNA needs to be free from the nucleus so it can evenly distribute to two daughter cells.
The events of mitosis describe the processes of splitting and moving nuclear DNA to opposite ends of the parent cell, where the nuclear membranes will reform. Then the cell membrane can split the cytoplasm and organelles (termed cytokinesis). The two daughter cells will each have the same genetic code. The significant mitosis stages are:
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
Mitosis begins in prophase, where the chromosome condenses into chromatids. A centromere connects each chromatid to its copy, making the linked pairs look like X’s. The nuclear envelope starts to disintegrate, mitotic spindle begins to assemble, and centriole pairs move toward opposite poles of the cell. This stage is a preparatory stage. “Pro-” means “before;” “phase” means “stage.” So, this is the stage before the process gets into full swing.
In prophase,
- chromosomes condense and become visible
- spindle fibers emerge from the centrosomes
- nuclear envelope breaks down
- nucleolus disappears
Metaphase
Metaphase is so named because the chromosomes line up in the middle of the cell. The centromeres attach to the spindle fibers originating at opposite poles. The root “meta-” means “middle.”
In metaphase,
- mitotic spindle is fully developed, and centrosomes are at opposite poles of the cell
- chromosomes line up at the metaphase plate
- each sister chromatid is attached to a spindle fiber originating from opposite poles
Anaphase
During anaphase, proteins that bind the sister chromatids together break down. The connected sister chromatids, now called chromosomes, pull toward opposite poles and cause the chromosomes to split. The nonkinetochore spindle fibers from one pole also push on the other pole, which causes cell elongation. By the end of anaphase, each pole of the cell contains an identical set of chromosomes. The root “ana-” refers to “apart”.
In anaphase,
- cohesin proteins binding the sister chromatids together break down
- sister chromatids (now called chromosomes) pull toward opposite poles
- non-kinetochore spindle fibers lengthen, elongating the cell
Telophase
In telophase, the nuclei at each pole form again. The chromosomes separate into two identical nuclei. That implies the end of mitosis. However, the two nuclei are still in a single cell. “Telo” comes from the Greek word for “end”.
In telophase,
- chromosomes arrive at opposite poles and begin to condense
- nuclear envelope material surrounds each set of chromosomes
- the mitotic spindle breaks down
The next step will separate the cytoplasm into two compartments.
Cytokinesis
Cytokinesis is the separation of the cytoplasm into two new daughter cells. Animal cells divide when proteins pinch in the center of the cell until it separates into two. This region is called the cleavage furrow.
Plant cells divide when new cell wall components lay down in the center of the cell. This part is called the cell plate.
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Key Points
• During prophase, the nucleus disappears, spindle fibers form, and DNA condenses into chromosomes (sister chromatids).
• During metaphase, sister chromatids align along the middle of the cell by attaching their centromeres to the spindle fibers.
• During anaphase, sister chromatids physically separate at the centromere and pull towards opposite poles of the cell by the mitotic spindle.
• During telophase, chromosomes arrive at opposite poles and unwind into thin strands of DNA, the spindle fibers disappear, and the nuclear membrane reappears.
• Cytokinesis is the actual splitting of the cell membrane; animal cells pinch apart, while plant cells form a cell plate that becomes the new cell wall.
• Cells enter the G0 (inactive) phase after they exit the cell cycle when they are not actively preparing to divide; some cells remain in G0 phase permanently.
Key Terms
centrosome: an organelle near the nucleus in the cytoplasm of most organisms that control the organization of its microtubules and gives rise to the mitotic spindle
cytokinesis: the second portion of the mitotic phase in which the cytoplasm of a cell divides following the division of the nucleus
nuclear membrane: the membrane around the nucleus
cell plate: where plant cells divide when new cell wall are made
mitotic spindle: the apparatus that orchestrates the movement of chromosomes during mitosis
sister chromatid: the identical copies formed by the DNA replication of a chromosome
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.
