Cell Division & Cell Cycle

What is cell division?

Cell Division - cell division is a process by which a cell, called the parent cell, divides into two/four cells, called daughter cells. - in mitotic division, the daughter cells can become parent cells and go into subsequence division. - in meiotic division, the daughter cells will be permanently transformed and cannot divide again.

Why cell division?

Cell Division - for unicellular organisms such as the Amoeba, one cell division reproduces an entire organism. - cell division can create progeny from multi- cellular organisms, such as plants that grow from cuttings. - cell division also enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by cell division from gametes. - cell division allows for continual renewal and repair of the organism.

Cell Cycle - cell cycle is the series of events that take place in a eukaryotic cell leading to its duplication through cell division. - certain fly embryos sport cell cycles that last only 8 minutes per cycle. - some mammals take much longer than that - up to a year in certain liver cells. - for fast-dividing mammalian cells, the length of the cycle is approximately 24 hours.

Cell Cycle - the cell cycle consists of four distinct phases: G 1 phase, S phase, G 2 phase (collectively known as interphase) and M phase. - cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G 0 phase. What types of cell permanently in G 0 phase?

G 0 Phase - nonproliferative cells in multicellular eukaryotes generally enter the quiescent G 0 state from G 1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). - this is very common for cells that are fully differentiated. - some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G 0 phase semi-permanently and can only be induced to begin dividing again under very specific circumstances.

G 1 Phase - the first phase within interphase, from the end of the previous M phase till the beginning of DNA synthesis is called G 1. - during this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. - this phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication. - duration of G 1 is highly variable, even among different cells of the same species.

S Phase - S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e. each chromosome has two (sister) chromatids. - during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. - rates of RNA transcription and protein synthesis are very low during this phase. - an exception to this is histone production, most of which occurs during the S phase. - the duration of S phase is relatively constant among cells of the same species.

G 2 Phase - cell undergoes a period of rapid growth to prepare for mitosis. - significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. - inhibition of protein synthesis during G 2 phase prevents the cell from undergoing mitosis.

Regulation of Cell Cycle - Regulation of the cell cycle involves steps crucial to the cell, including detecting and repairing genetic damage, and provision of various checks to prevent uncontrolled cell division. - The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to "reverse" the cycle. - Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine a cell's progress through the cell cycle.

Cell Cycle Checkpoints - cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle. - If a cell fails to meet the requirements of a phase it will not be allowed to proceed to the next phase until the requirements have been met. - several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to daughter cells. - two main checkpoints exist: the G 1 /S checkpoint and the G 2 /M checkpoint. -G 1 /S transition is also known as restriction point. Lack of growth factors causes some cells to arrest at the restriction point. If a cell is not ready, or external conditions are not appropriate for the S phase, then the cell may enter G 0 phase.