HiSET Science: Cell Cycle and Cellular Division

Cell Cycle Stages
The cell cycle consists of three stages: interphase, mitosis, and cytokinesis. Interphase is the longest stage of the cell cycle and involves the cell growing and making a copy of its DNA.

Cells typically spend more than 90% of the cell cycle in interphase. Interphase includes two growth phases called  and .

The order of interphase is the first growth cycle, GAP 1 ( phase), followed by the synthesis phase (S), and ending with the second growth phase, GAP 2 ( phase).

During the  phase of interphase, the cell increases the number of organelles by forming diploid cells.

During the S phase of interphase, the DNA is replicated, and the chromosomes are doubled. During the  phase of interphase, the cell synthesizes needed proteins and organelles, continues to increase in size, and prepares for mitosis. Once the  phase ends, mitosis can begin.


Mitosis
Mitosis is the asexual process of cell division. During mitosis, one parent cell divides into two identical daughter cells. Mitosis is used for growth, repair, and replacement of cells. Some unicellular organisms reproduce asexually by mitosis. Some multicellular organisms can reproduce by fragmentation or budding, which involves mitosis. Mitosis consists of four phases: prophase, metaphase, anaphase, and telophase. During prophase, the spindle fibers appear, and the DNA is condensed and packaged as chromosomes that become visible. The nuclear membrane also breaks down, and the nucleolus disappears. During metaphase, the spindle apparatus is formed and the centromeres of the chromosomes line up on the equatorial plane. During anaphase, the centromeres divide and the two chromatids separate and are pulled toward the opposite poles of the cell.

During telophase, the spindle fibers disappear, the nuclear membrane reforms, and the DNA in the chromatids is decondensed.



Cytokinesis
Cytokinesis is the dividing of the cytoplasm and cell membrane by the pinching of a cell into two new daughter cells at the end of mitosis. This occurs at the end of telophase when the actin filaments in the cytoskeleton form a contractile ring that narrows and divides the cell. In plant cells, a cell plate forms across the phragmoplast, which is the center of the spindle apparatus. In animal cells, as the contractile ring narrows, the cleavage furrow forms. Eventually, the contractile ring narrows down to the spindle apparatus joining the two cells and the cells eventually divide. Diagrams of the cleavage furrow of an animal cell and cell plate of a plant are shown below.



Meiosis
Meiosis is a type of cell division in which the number of chromosomes is reduced by half. Meiosis produces gametes, or egg and sperm cells. Meiosis occurs in two successive stages, which consist of a first mitotic division followed by a second mitotic division. During meiosis I, or the first meiotic division, the cell replicates its DNA in interphase and then continues through prophase I, metaphase I, anaphase I, and telophase I. At the end of meiosis I, there are two daughter cells that have the same number of chromosomes as the parent cell.
During meiosis II, the cell enters a brief interphase but does not replicate its DNA. Then, the cell continues through prophase II, metaphase II, anaphase II, and telophase II. During prophase II, the unduplicated chromosomes split. At the end of telophase II, there are four daughter cells that have half the number of chromosomes as the parent cell.



Cell Cycle Checkpoints
During the cell cycle, the cell goes through three checkpoints to ensure that the cell is dividing properly at each phase, that it is the appropriate time for division, and that the cell has not been damaged. The first checkpoint is at the end of the  phase just before the cell undergoes the S phase, or synthesis. At this checkpoint, a cell may continue with cell division, delay the division, or rest. This resting phase is called . In animal cells, the  checkpoint is called restriction. Proteins called cyclin D and cyclin E, which are dependent on enzymes cyclin-dependent kinase 4 and cyclin-dependent kinase 2 (CDK4 and CDK2), respectively, largely control this first checkpoint. The second checkpoint is at the end of the
 phase just before the cell begins prophase during mitosis. The protein cyclin A, which is dependent on the enzyme CDK2, largely controls this checkpoint. During mitosis, the third checkpoint occurs at metaphase to check that the chromosomes are lined up along the equatorial plane. This checkpoint is largely controlled by cyclin B, which is dependent upon the enzyme CDK1.