The Cell Cycle
The cell cycle is a series of events that occur once a cell divides until it divides again. A cell is the basic unit of life that makes up an organism. Organisms can contain anywhere from one cell like in yeast to billions of cells like in humans. In humans there are two main types of cells, germline cells and somatic cells. Germline cells are the cells used for reproduction (sperm and eggs) and somatic cells are the other cells that make up the body. Each cell contains many parts, referred to as organelles. Each organelle has a specific function that contributes to the cells survival. There are organelles and structures that process nutrients, produce energy, give structure, contain the genetic material, and process this genetic material in the cell. A cell’s genetic material, its DNA, is found in its nucleus (an organelle in the cell) and includes a complete set of chromosomes (46 chromosomes). This genetic material controls everything the cell does including how it proceeds through the cell cycle.
The cell cycle is a made up of two major phases, interphase and mitosis (this can be seen on Figure 1). Interphase is the phase when the cell grows and replicates its genetic material (DNA) and is separated into three stages called Gap 1 (G1; the gap between mitosis and S phase), DNA synthesis (S phase), and Gap 2 (G2; the gap between S phase and mitosis). Mitosis is the phase when the cell divides into two cells that are identical to the original cell (two identical daughter cells).
During the cell cycle a new cell begins in G1 phase where the cell grows and gets ready to make duplicate copies of its DNA through a process known as DNA replication. In G1 phase the cell has one complete set of chromosomes. After G1 phase the cell must decide if it will proceed to S phase. The cell can either enter into a resting state, known as G0 phase, where it continues with its biological function but does not go through the rest of the cell cycle, or the cell can continue with the cycle. If it continues with the cell cycle then it must pass through a checkpoint between the G1 phase and S phase before it can proceed. At this point the cell looks at all its DNA to determine if there are any problems. If it finds a problem then the cell will not continue to S phase and will go through programmed cell death or apoptosis. If it does not find a problem then the cell proceeds to S phase. S phase is when the cell replicates its DNA. During this phase the cell goes from having one set of chromosomes to two sets of chromosomes. Once the cell has replicated its DNA it proceeds to G2 phase where it goes through a period of further growth. Then the cell must pass through another checkpoint to determine if there were problems during replication. If the cell detects a problem then it will go through programmed cell death but if no problem is detected then it will proceed into mitosis or M phase. During M phase the cell goes through a series of additional steps which ultimately forms two daughter cells.
Somatic cells go through a process called mitosis in M phase. Mitosis has five stages that a cell must proceed through: prophase, prometaphase, metaphase, anaphase, and telophase (Figure 2). During prophase the chromosomes condense and mitotic spindles (which will later separate the chromosomes) begin to form. In prometaphase the nuclear membrane breaks down and the chromosomes attach to the spindle. In metaphase the pairs of chromosomes are entirely condensed and line up along the midline of the cell. Then in anaphase the chromosome pairs are pulled apart to separate ends of the cell by the spindles. In telophase the chromosomes begin to decondense and the cell goes through a process called cytokinesis where the rest of the cell divides and membranes reform to make two identical daughter cells. Then each new cell begins the entire cell cycle again beginning in G1 phase.
Germline cells proceed through a process similar to somatic cells, but instead of repeating the cycle germline cells proceed through a modified second round of the cycle. This entire process is referred to as meiosis (Figure 3). Meiosis is divided into meiosis 1 and meiosis 2. The cell proceeds through interphase and then enters meiosis 1 which consists of the same stages as mitosis. Then instead of going through interphase again to replicate the DNA, the cell proceeds into meiosis 2 which produces cells with half the number of chromosomes (23 chromosomes) referred to as a haploid cell. Like it was previously mentioned there are two types of germline cells, sperm and egg cells. In meiosis, sperm cells complete the first round of the cell cycle like somatic cells but then they do another round of cell division without replicating their DNA again. At the end of meiosis, the one cell has produced four identical haploid sperm. However, egg cells complete meiosis a little differently then sperm cells. An egg cell completes the first part of meiosis but then enters into a resting state and will continue through the second part of meiosis only if it is fertilized. However, eggs do not produce four identical cells at the end. Instead one round of meiosis produces one egg cell for reproduction and some polar bodies that are not used.
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Wikipedia article on this topic