The Cell Cycle

The process of cell growth and division-The cell cycle starts when a cell is produced by cell division and ends with the cell dividing to produce two identical cells.-The cell cycle consists of a period of cell growth and DNA replication, called interphase, and a period of cell division, called mitosis.-Interphase (cell growth) is sub-divided into three separate growth stages. These are called G1, S and G2.

Mitosis is needed for the growth of multicellular organisms (like us) and for repairing damaged tissues.Some organisms (e.g. some plants and fungi) reproduce asexually (without sex) using mitosis. This means any new organisms produced are genetically identical to the original, parent organism.

Mitosis is really one continuous process, but it's described as a series of division stages - prophase, metaphase, anaphase and telophase.

Interphase comes before mitosis in the cell cycle - it's when cells grow and replicate their DNA ready for division.

Interphase - The cell carries out normal functions, but also prepares to divide. The cell's DNA is unravelled and replicated, to double its genetic content. The organelles are also replicated so it has spare ones, and its ATP content is increased (ATP provides the energy needed for cell division).

Prophase - The chromosomes condense, getting shorter and fatter. The centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle. The nuclear envelope (the membrane around the nucleus) breaks down and chromosomes lie free in the cytoplasm.

Metaphase - The chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere.

As mitosis begins, the chromosomes are made of two strands joined by a centromere. The separate strands are called chromatids.There are two strands because each chromosome has already made an identical copy of itself during interphase. When mitosis is over, the chromatids end up as one-strand chromosomes in the new daughter cells.

Anaphase - The centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite ends of the cell, centromere first.

Telophase - The chromatids reach the opposite poles on the spindle. They uncoil and become long and thin again. They're now called chromosomes again. A nuclear envelope forms around each group of chromosomes, so there are now two nuclei. The cytoplasm divides and there are now two daughter cells that are genetically identical to the original cell and to each other. Mitosis is finished and each daughter cell starts the interphase part of the cell cycle to get ready for the next round of mitosis.

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