What is Cell cycle? — Biology

Definition 12.1 High School Biology · Chapter 12 — The Cell Cycle and Mitosis

The cell cycle is the sequence of events from the birth of a cell, by division, to its own division into two. It comprises the interphase, during which the cell grows and copies its DNA, and the M phase, during which the nucleus divides (mitosis) and then the cytoplasm (cytokinesis). Interphase has three parts: $\mathrm{G_1}$ (growth), S (DNA replication, Chapter 11) and $\mathrm{G_2}$ (preparation for division).

The cell cycle of a typical dividing human cell, about 24 hours. Interphase (G_1, S, G_2) takes 23 of them; the division itself, one. Many cells leave the cycle after G_1 and never divide again. — The cell cycle of a typical dividing human cell, about 24 hours. Interphase ( $\mathrm{G_1}$ , S, $\mathrm{G_2}$ ) takes 23 of them; the division itself, one. Many cells leave the cycle after $\mathrm{G_1}$ and never divide again.

Amount of DNA in one nucleus over two successive cycles of 24 hours. It doubles during S and halves at the end of mitosis; a daughter cell always starts with q. — Amount of DNA in one nucleus over two successive cycles of 24 hours. It doubles during S and halves at the end of mitosis; a daughter cell always starts with $q$ .

Examples

Example 12.3 (Reading the curve)

A cell measured at $2q$ may be in $\mathrm{G_2}$ or in early mitosis; at $1.5q$ it is certainly in S, half-way through replication; at $q$ it is in $\mathrm{G_1}$ — or has left the cycle for good, like a neuron or a muscle fibre, which stay at $q$ for life. A tissue in which every cell is at $q$ is a tissue that has stopped dividing.

Example 12.6 (Counting in the root tip)

Of 600 cells counted in a root tip, 540 are in interphase and 60 in mitosis: a mitotic index of 10%. If the cycle lasts 20 hours, mitosis lasts about $0.10 \times 20 = 2\,\mathrm{h}$ ; and if 36 of the 60 dividing cells are in prophase, prophase takes $36/60$ of those two hours, some 72 minutes, while anaphase, seen in only 4 cells, takes about 8 minutes. The rarest stage is the fastest.

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Examples

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