The cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life, and all living things are composed of cells. Most animal cells are between 0.2 and 8 ?m in diameter and between 0.01 and 1 ?m in thickness, while plant cells are between 0.2 and 200 ?m in diameter and 1 and 10 ?m in thickness. The cell is the fundamental unit of life. All living organisms are composed of trillions of cells, each with a unique set of DNA instructions. A single human cell may have an estimated 50,000 genes, and all cells contain the same basic components: a plasma membrane, a cell nucleus, and a cytoplasm, which contains various organelles. All cells are derived from pre-existing cells, through cell division. The cell was discovered by Robert Hooke in 1665, although others had previously described the components of cells, such as Antonie van Leeuwenhoek who is credited with the discovery of red blood cells. The term cell comes from the Latin word cella, meaning "small room", because of its small size within the body. The smallest unit of a cell is called a granule, which is a spherical particle of RNA or DNA. The cell is composed of a cytoplasm enclosed by a cell membrane, which contains a semi-permeable lipid bilayer called a plasma membrane. The plasma membrane is a phospholipid bi-layer which acts as a barrier for controlling what materials can enter and leave the cell. The nucleus is a membrane-bound organelle inside the cell, in which the cell's DNA is housed. The nucleus is separated from the cytoplasm by a nuclear envelope. The DNA is tightly coiled with many strands of DNA tightly coiled around each other. The nucleus is the command center of the cell, controlling the cell cycle, gene expression, cell division and protein synthesis. The cytoplasm is a fluid compartment of the cell, which contains the cell organelles. The cytoplasm is usually separated from the cell membrane by a cell membrane. The organelles are made of different materials and are involved in many different cellular functions. The organelles are the following: the Golgi apparatus, the endoplasmic reticulum, the ribosomes, the lysosomes, the peroxisomes, the mitochondria and the vacuoles. The organelles are used for housekeeping functions and they also help with the cell's function. The cell cycle is a sequence of events that a cell goes through as it divides. The cell cycle is divided into four different phases: the G1 phase, S phase, the G2 phase and the M phase. There are four main stages in the cell cycle, G1, S, G2 and M. The cell cycle is also called the eukaryotic cell cycle, because humans and other eukaryotes have cell cycles. The cell cycle is regulated by the cell cycle checkpoints. The cell cycle checkpoints are the following: the G1/S checkpoint, the G2/M checkpoint and the mitotic entry checkpoint. The G1/S checkpoint is the first checkpoint, which controls when a cell enters the G1 phase of the cell cycle. The G2/M checkpoint controls when the cell enters the M phase of the cell cycle. The mitotic entry checkpoint controls when a cell enters mitosis.