Cell Cycle

Phases of the Cell Cycle

The cell cycle consists of interphase and mitosis.
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. Interphase is characterized by cell growth and DNA replication, while M is the phase of the cell cycle in which the cell divides to make two new cells. The G in the G1 and G2 phases stands for gap or growth. Some scientists also recognize a fifth phase, G0, in which the cell is resting just after mitosis. However, this brief period can be considered part of G1 in cells that are still dividing. Some cells, such as mature nerve or muscle cells, actually 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 on the type of cell and the environmental conditions in which the cell lives.
Interphase comprises the (G1), S, and (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

During interphase, comprised of the G1, S, and G2 phases of the cell cycle, the cell grows, replicates its DNA, then grows some more.
Interphase is collectively the Gap1 (G1), Synthesis (S), and Gap2 (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 phase, growing in size and carrying out the normal functions of the cell. The length of this phase is the most variable among different cell types. When the cell receives signals for division, it moves to the S phase.

1. During S phase, the replication of the cell's genetic material, known as DNA, occurs. In eukaryotic cells, DNA is found in structures called chromosomes. Before replication, these chromosomes exist as long, thin chromatin fibers. After replication, each chromosome is condensed into one of two identical halves of a replicated chromosome called a chromatid. Sister chromatids are two identical copies of DNA, the original strand of DNA, and the newly created copy. Sister chromatids separate during mitosis. A centromere, the point on a chromosome that attaches to the spindle fibers during cell division, attaches the sister chromatids. The centromere is aided in binding sister chromatids together by one of several proteins called a cohesin. Once the DNA has been replicated, the cell moves to the second gap phase.

2. 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 this 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, which assembles into condensed, tight bundles called sister chromatids. Each is joined by a region of repetitive DNA called a centromere. The collection of microtubules called a centrosome also replicates. There are two gap stages during interphase. During Gap1 (G1), the cell grows in size, while during Gap2 (G2), the cell finishes growing and performs a quick check of the replicated DNA to make sure it was copied correctly.

Mitosis

Mitosis divides the DNA in the steps prophase, prometaphase, metaphase, anaphase, and telophase. The end of mitosis is followed by cytokinesis, forming two cells identical to the parent cell.
Mitosis is cellular division that occurs in somatic (non-reproductive) cells that are growing or repairing a multicellular organism. Mitosis begins after the G2 phase. Mitosis consists of five distinct steps, followed by cytokinesis, the pinching off of the cytoplasm (the jellylike fluid that fills the spaces around internal cell structures) to form two new cells. DNA and other cellular structures are visible with a light microscope during mitosis, so the steps of mitosis have been understood since the late 19th century. These steps occur only in eukaryotic cells. Prokaryotic cells undergo a different division process called binary fission.

Stages of Mitosis

Mitosis consists of five continuous steps that involve the movement and separation of chromosomes in the cell. 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, where the nucleus dissolves and the DNA condenses into chromosomes.
Prophase is the first phase of mitosis, in which sister chromatids condense, the mitotic spindle begins to form, and centrosomes (the structures that coordinate the formation of microtubules, which allow cell division to proceed) segregate to opposite poles. 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 segregate to the ends of the cell, called the poles. 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 to the centromeres, the point on a chromosome that attaches to the spindle fibers and at which the sister chromatids are attached. The mitotic spindle helps align the sister chromatids correctly for proper cell division, ensuring each daughter cell gets one copy.

Prophase of Mitosis

In prophase, the first phase of mitosis, the sister chromatids condense, the mitotic spindle begins to form, and the centromeres move to opposite poles of the cell.

Prometaphase

During prometaphase, the nuclear membrane starts to dissolve and the chromosomes start to move towards the spindle fibers.
The second phase of mitosis in which the nuclear membrane breaks down and spindle fibers attach to the centromere is called prometaphase. The breakdown of the membrane creates a vesicle, a small, leftover piece of the nuclear membrane. The formation of many vesicles give 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. 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 additional spindle fiber called an interpolar microtubule extends from the centrosome across the cell equator. A third type of spindle fiber, called the astral microtubule, attaches the centrosome to the cell membrane.

Prometaphase

In prometaphase the nuclear envelope begins to break down, allowing the kinetochore microtubules to attach to the centromeres via kinetochores. They tug the chromosomes to the middle of the cell. 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, where the chromosomes align themselves along the equator of the cell.
The third stage of mitosis, in which the sister chromatids line up along the cell equator is metaphase. In this stage, the chromosomes line up along the cell equator (also called the metaphase plate), 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. The sister chromatids are attached by their centromere 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.

Metaphase

In metaphase the third stage of mitosis, the sister chromatids of the replicated chromosomes line up along the cell equator.

Anaphase

Anaphase is the fourth phase of mitosis, where the sister chromatids separate and start to move towards opposite sides of the cell.
Anaphase, the fourth phase of mitosis in which sister chromatids separate and are pulled to the opposite poles of the cell, 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 fourth phase of mitosis, the sister chromatids separate, forming single daughter chromosomes. The kinetochore microtubules shorten, pulling each daughter chromosome toward opposite poles. The other microtubules also shorten, elongating the cell and segregating the chromosomes to each pole.

Telophase and Cytokinesis

During telophase the nuclear membrane reforms and prepares the cell to divide in half during 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 DNA 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. This final cell division marks the end of 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.

Telophase and Cytokinesis

In telophase, the final phase of mitosis, the mitotic spindle breaks down and the nuclear envelope forms around the chromosomes. A cleavage furrow forms along the cell equator, which pinches off the two new cells. This is known as cytokinesis.