Concepts: The cell cycle and interphase

1.1 From one cell to a whole body

Every organism that starts life as a zygote reaches its final size through the same two processes: cells increase in number and cells increase in size. The increase in number happens by cell division. In all of them, one parent cell gives rise to two daughter cells, and each daughter can grow and divide again. A single fertilised egg therefore produces the millions of cells of a mature body through rounds of growth and division.

Division by itself would cut the cell into smaller pieces, so it has to be coordinated with growth and with the copying of DNA. A cell must duplicate its genome before it can hand one complete copy to each daughter. The cytoplasm also needs to grow enough to supply both daughters. The sequence of events through which a cell duplicates its genome, grows and then divides is called the cell cycle.

1.1.1 Two main phases

The cell cycle has two broad phases. The M phase is the phase of actual division, also called the mitotic phase. Interphase is the long stretch between two M phases, during which the cell prepares for division. In a human cell dividing once in about 24 hours at a cultured-cell pace, the division proper takes barely an hour. Interphase occupies more than 95 per cent of the cycle. The name resting phase for interphase is misleading, because this is the busiest period of preparation.

The M phase begins with nuclear division, karyokinesis, and usually ends with the division of the cytoplasm, cytokinesis. Cells whose nuclei divide without cytoplasmic division end up as a multinucleate mass, a syncytium.

1.1.2 The three parts of interphase

Interphase is divided into G1, S and G2.

G1 is the first gap phase. It begins right after a cell divides and lasts until DNA replication starts. The cell is metabolically active, makes RNA and proteins, grows in size and duplicates most of its organelles. DNA is not copied in this phase.

The S phase is the synthesis phase. DNA replication happens here, so the DNA content of the cell doubles. If the DNA amount at the start of S is written as 2C, it becomes 4C by the end of S. The chromosome number does not change. A diploid cell with 2n chromosomes before S still has 2n chromosomes after S, because the newly copied DNA stays attached to the original DNA as two sister chromatids of the same chromosome. In animal cells, DNA replication happens in the nucleus, and the centriole pair duplicates in the cytoplasm during the same phase.

G2 is the second gap phase. Proteins needed for mitosis are made here, the cell keeps growing, and the duplicated DNA is checked before division begins.

1.1.3 The G0 stage

Some cells stop dividing once they are mature. Heart muscle cells of an adult and most neurons do not divide again. Such cells leave the cycle after G1 and enter a quiescent stage called G0. A G0 cell is not dead and not inactive: it carries on its normal metabolism, often for the whole life of the organism, but it does not replicate DNA or divide unless a signal calls it back. Liver cells are a standard example of cells that stay largely in G0 but re-enter the cycle when the liver is injured and the tissue has to be rebuilt.

Cell cycle length is not fixed across life. A human cell in culture takes about a day, yeast manages the same cycle in roughly 90 minutes, and an early embryo can divide even faster because its G1 and G2 phases are very short. The length also differs from one tissue to another in the same body.

1.1.4 Practice from the NCERT exercises, questions 1 to 4 and 16

Exercise 1. A mammalian cell in culture has an average cycle of about 24 hours.

Exercise 2. Karyokinesis is the division of the nucleus, and cytokinesis is the division of the cytoplasm after the nucleus has divided.

Exercise 3. Interphase has G1, S and G2. G1 carries growth and organelle duplication, S carries DNA replication from 2C to 4C, and G2 carries the protein synthesis and growth that prepare the cell for division.

Exercise 4. G0 is the quiescent stage entered by cells that stop dividing after G1. These cells remain metabolically active but do not divide unless the organism needs new cells.

Exercise 16. At G1 the chromosome number is 2n and DNA content is 2C. After S phase the chromosome number stays 2n while DNA rises to 4C. At G2 the number stays 2n with 4C DNA. After M phase each daughter cell has 2n chromosomes and 2C DNA.