The cell cycle is a series of carefully regulated events, including periods of growth, rest, and cell division.
The cell cycle consists of four distinct phases: the G1 phase, the first phase of the cell cycle, when a cell grows; the S phase, the second phase of the cell cycle, when DNA is replicated; the G2 phase, the third phase of the cell cycle, when a cell grows some more; and the M phase, the fourth phase of the cell cycle, when a cell divides. The G1, S, and G2 phases of the cell cycle are collectively known as interphase. A cell spends most of its time in interphase, during which it mostly performs its normal functions, such as protein synthesis. Interphase is characterized by cell growth and DNA replication, while M is the phase of the cell cycle in which the chromosomes of the cell prepare for the cell to divide into two new cells. The G in the G1 and G2 phases stands for "gap" or "growth." A fifth phase has been identified, G0, in which the cell is resting just after mitosis but still carrying out normal functions. A cell can remain in the G0 phase indefinitely, awaiting a signal to return to the normal cell cycle. However, in cells that are still dividing, G0 is a brief period and can be considered part of G1. Some cells, such as mature nerve and muscle cells, leave the cell cycle and remain at rest until they die. The amount of time spent in any given phase of the cell cycle is dependent upon the type of cell and the environmental conditions in which the cell lives.
Interphase comprises the gap 1 (G1), synthesis (S), and gap 2 (G2) phases, in which the cell grows and replicates its genetic material. The M phase consists of mitosis, in which the cell divides, producing two new, identical cells.
Interphase
Interphase, the part of the cell cycle between division events, includes the G1, S, and G2 phases, during which the cell grows, replicates its DNA, and undergoes its usual functions.
Interphase is collectively the gap 1 (G1), synthesis (S), and gap 2 (G2) phases of mitosis, in which a cell grows, replicates its DNA, and grows again. Most cells spend nearly all of their time in this part of cell division, growing in size and carrying out the normal functions of the cell. The length of this particular phase is the most variable among different cell types. When the cell receives signals for division, it moves to the S phase.
During the S phase, the replication of the cell's genetic material, DNA, occurs. In eukaryotic cells, DNA is found in structures called chromosomes. Before replication, these chromosomes exist as long, thin chromatin fibers. Each chromosome is condensed into a structure of two sister chromatids during prophase of mitosis. A chromatid is one of two identical halves of a replicated chromosome. Sister chromatids are identical copies of DNA that remain connected until they are separated during mitosis. A centromere, the point on a chromosome that attaches to the spindle fibers with a kinetochore during cell division, attaches the sister chromatids. The centromere is aided in binding sister chromatids together by several proteins called cohesins and condensins. Once the DNA has been replicated, the cell moves to the second gap phase.
During the G2 phase, a structure in the cytoplasm of animal cells that coordinates the formation of microtubules, called a centrosome, allows cell division to proceed during reproduction. The centrosome will organize a complex structure of microtubules, the mitotic spindle, involved in mitosis. Other cellular structures are duplicated during G2, such that each replicated daughter cell produced during mitosis will have all necessary organelles (such as mitochondria, endoplasmic reticulum, etc.). For most cells the G2 phase is relatively short; once complete, the cell is ready to divide. Interphase takes most of the time in the cell cycle, comprising more than 95% of the duration of the cell cycle in most eukaryotes.
During interphase, a cell grows larger. The cell replicates its DNA, forming sister chromatids. Each is joined by a centromere. A collection of microtubules (structural proteins) called a centrosome also replicates. There are two gap stages during interphase. During gap 1 (G1), the cell grows in size, while during gap 2 (G2), the cell finishes growing and performs a quick check of the replicated DNA to make sure it was copied correctly.
Mitosis
Mitosis includes prophase, prometaphase, metaphase, and anaphase, as well as telophase, during which chromosome copies are carefully separated in preparation for cytokinesis, where the cytoplasm divides.
Mitosis is a form of nuclear division in which replicated chromosomes are carefully organized and separated in preparation for cytoplasmic division. This process results in two cells that are genetic clones of the parent cell. Mitosis occurs in somatic (nonreproductive) cells that are growing or repairing a multicellular organism. Mitosis begins after the G2 phase. Mitosis consists of five distinct steps, followed by cytokinesis, which is the pinching off of the cytoplasm to form two new cells at the end of mitosis.
During mitosis, the genetic material that has been replicated during the synthesis phase is affected by the actions of two types of proteins called cohesins and condensins. Prior to mitosis, the DNA is found spread out inside the nucleus. A cohesin protein helps bind sister chromatids together at the centromere until they separate during anaphase. A condensin is a protein that helps condense DNA into chromosomes during prophase of mitosis. Condensin reorganizes chromosomes when they get compacted during prophase. This reduces the amount of space the DNA takes up. Both of these proteins have been shown to regulate gene expression by controlling the entire chromosome or a single gene. They have also been shown to play roles in DNA repair and in organizing the centromere so that the spindle fibers properly attach during mitosis.
Mitosis consists of five continuous steps that involve the organization and separation of chromosome copies in preparation for cell division. Prior to mitosis, the chromosomes are duplicated so that each of the resulting daughter cells will be identical to the parent cell.
Prophase
Prophase is the first phase of mitosis in which sister chromatids condense, the mitotic spindle begins to form, and centrosomes segregate to opposite poles. Centrosomes are the structures that coordinate the formation of microtubules, which allow cell division to proceed. During prophase, the sister chromatids condense until they are tightly packed. This makes them appear X-shaped, which is the representation of chromosomes with which most people are familiar. The centrosomes then start to organize the formation of the mitotic spindle, the bundle of spindle fibers attached at one end to the centrosome. The mitotic spindle is composed of microtubules that elongate from the centrosome and aim to reach the centromeres of a chromosome and the ends of opposing microtubules.
In prophase, the first phase of mitosis, the sister chromatids (chromosome copies) condense, the mitotic spindle begins to form, and the centrosomes move to opposite poles of the cell.
Prometaphase
Prometaphase is the second phase of mitosis in which the nuclear membrane breaks down and spindle fibers attach to the centromere. The mitotic spindle helps align the sister chromatids correctly for proper cell division, ensuring each daughter cell gets one copy. The breakdown of the membrane creates a vesicle, a small piece of cell membrane used to transport materials within a cell. The formation of many vesicles gives the mitotic spindle access to the centromeres of the chromosomes. The spindle fibers of the mitotic spindle elongate outward from the centrosomes. When a spindle fiber finds a centromere, it attaches at the kinetochore, a group of proteins bound at the centromere to which the spindle fibers attach. The spindle fibers tug the chromosomes back and forth as they position them correctly for the next stage. The spindle fiber that extends from the centrosome to the kinetochore on the centromere is known as a kinetochore microtubule. An interpolar microtubule is a spindle fiber that extends from one centrosome to the other across the cell equator. A third type of fiber is called the astral microtubule. The astral microtubule is a spindle fiber that attaches the centrosome to the cell membrane.
In prometaphase the nuclear envelope begins to break down, allowing microtubules to attach to the centromeres via kinetochores. Other microtubules stretch across the middle of the cell (interpolar microtubules) or attach to the cell membrane (astral microtubules).
Metaphase
Metaphase is the third phase of mitosis in which the sister chromatids line up along the cell equator. In this stage, the chromosomes line up along the cell equator (also called the metaphase plate). The cell equator is an imaginary line in the center of a cell during mitosis, along which sister chromatids align. Each chromosome is composed of a pair of identical sister chromatids linked by the centromere. Spindle fibers attached to the centromeres of each sister chromatid play tug-of-war to align the chromosomes. The sister chromatids are attached by their centromere via the kinetochore to two spindle fibers: one leading to each centrosome at opposite ends—each pole—of the cell. The chromosomes no longer move back and forth but stay aligned along the equator.
In metaphase, the third stage of mitosis, the replicated chromosomes line up along the cell equator.
Anaphase
Anaphase is the fourth phase of mitosis in which sister chromatids separate and are pulled to the opposite poles of the cell. Anaphase follows metaphase. In anaphase, the proteins binding sister chromatids together at their centromeres break down. This leads to separation of the sister chromatids into individual chromosomes. Each chromosome is still attached to a centrosome by spindle fibers. At the same time the chromosomes separate, the kinetochore microtubules begin to shorten, pulling the chromosomes toward their respective poles. Some scientists classify this portion of anaphase as anaphase A. During the second half of anaphase, sometimes known as anaphase B, the astral microtubules begin to shorten, while the interpolar microtubules, those extending from each centrosome, slide past each other. This pulls the centrosomes farther away from each other and begins segregating each set of chromosomes to the opposite poles of the cell.
In anaphase, the sister chromatids separate, forming single daughter chromosomes. The kinetochore microtubules shorten, pulling each daughter chromosome toward opposite poles. Some of the other microtubules also shorten, elongating the cell and segregating the chromosomes to each pole.
Telophase and Cytokinesis
In the final stage of mitosis, telophase, the chromosomes arrive at their respective poles. The vesicles formed during prometaphase reassemble into a new nuclear membrane surrounding the chromosomes of each daughter cell. The mitotic spindle breaks down, releasing the chromosomes to be bound in their new nuclear membrane. At the end of telophase, the cell has two distinct nuclei, and mitosis is complete. Mitosis, the division of the nucleus, is then followed by cytokinesis, the pinching off of the cytoplasm to form two new cells. In animal cells, a cleavage furrow, the indentation in an animal cell along which cytokinesis occurs, begins to form along the cell equator during telophase. It is created by a ring of shortening microfilaments attached to the cell membrane. Two identical daughter cells are produced, each having the same number of chromosomes as the original parent cell. This final cell division marks the end of the M phase of the cell cycle. At this point, each cell resumes the G1 phase or exits the cell cycle completely if the cell will no longer divide.
In telophase, the final phase of mitosis, the mitotic spindle breaks down, and the nuclear envelope (membrane) forms around the chromosomes. A cleavage furrow forms along the cell equator, which pinches off the two new cells. This is known as cytokinesis.
Plant cell cytokinesis is slightly different from that of animal cells. Plant cells have a rigid cell wall and need to stay attached to one another for the continuity of the plant. As such, during cytokinesis, plant cells do not separate completely. Instead, vesicles containing the cellulose (a large sugar molecule) used to build cell walls move into position along the equator of the cell. These vesicles are formed from parts of the Golgi apparatus. Here, the vesicles fuse together, with the vesicle membranes forming the cell plate between the two new plant cells. The cell plate is a partition that forms along the midline of a dividing plant cell. The cell plate fuses with the cell membrane to form two daughter cells. A new cell wall forms between the membranes of the cell plate.
Cytokinesis in Plant Cells
During cytokinesis in plant cells, small membrane compartments called vesicles join together to form a structure called a cell plate. These vesicles then form a new cell wall between the daughter cells.
