Lesson 1 of 13 · 6 min

The cell cycle and its phases

NCERT §10.1

Ishaan has an onion root tip under the school microscope and is annoyed: out of hundreds of cells, only a handful look like they are doing anything. His sister Diya says that is exactly what the cell cycle predicts.

The story this chapter follows: Ishaan's onion root tip slide

Ishaan has to find and draw dividing cells in an onion root tip for his practical record, and he keeps getting stuck. His older sister Diya, who studied this chapter two years ago, sits beside him at the microscope and explains what each cell is doing and how many chromosomes it holds.

The lesson in notes

In short

Cells reproduce by splitting into two, and the daughters can grow and split again. Repeated rounds of growth and division let one cell build a body of millions of cells.

During a division the cell must also copy its DNA and grow, and these three processes are coordinated so that every daughter receives an intact genome.

The cell cycle is the ordered series of events in which a cell copies its genome, makes its other components and finally divides into two daughter cells.

Growth of the cytoplasm goes on continuously, but DNA is synthesised in only one particular stage of the cycle. The copied chromosomes are then shared out to the daughter nuclei, and all of these events are under genetic control.

Human cells grown in culture, a typical eukaryotic example, complete one cycle in about 24 hours. The length varies between organisms and between cell types: yeast can finish a cycle in only about 90 minutes.

The cycle has two basic phases: interphase, the stretch between two successive divisions, and the M phase (mitosis phase), when the cell actually divides.

Of the roughly 24-hour human cycle, division proper takes only about an hour. Interphase takes up more than 95% of the cycle.

The M phase begins with karyokinesis, the division of the nucleus in which the daughter chromosomes separate, and usually ends with cytokinesis, the division of the cytoplasm.

Watch a class

The cell cycle and its phases

Prefer a video? Watch this