The Eukaryotic Cell Cycle and Interphase

The eukaryotic cell cycle is the ordered sequence of events by which a cell grows, copies its DNA and divides into two daughter cells, and most of the cycle is spent in interphase, the stage between divisions. In brief: interphase has three stages, G1 (growth), S (DNA synthesis, when DNA is replicated) and G2 (preparation for division), and it is followed by the M phase, which includes mitosis and cytokinesis. In a typical human cell that divides every 24 hours, interphase takes about 23 hours and mitosis only about 1 hour.

Understanding the cell cycle is essential for the study of growth, repair and cancer. This guide describes each phase of the cycle, explains how DNA replication occurs before division, introduces the checkpoints that control the cycle, works through a timing example using the proportion of cells in each phase and corrects common mistakes.

What is the cell cycle?

The cell cycle is the life cycle of a dividing cell. It starts when a new cell is formed by division, and ends when that cell divides to make two new cells. The cycle allows organisms to grow, replace worn-out cells and repair damage, and it allows single-celled organisms to reproduce.

Cycle lengths vary. Human skin and gut lining cells may divide every day or two, liver cells divide only occasionally, and fully developed nerve and muscle cells usually do not divide at all. Cells that leave the cycle and stay in a resting state are said to be in the G0 phase.

What happens in interphase?

Interphase is the longest part of the cycle, and it is when the cell is carrying out its normal functions while also preparing to divide. It is not a “resting” phase. Under the light microscope, a cell in interphase has a visible nucleus with a nucleolus, and the chromosomes are not visible because they are uncoiled as chromatin.

G1 phase (first gap)

The cell grows in size, makes proteins and organelles, and carries out its normal functions. It also decides whether to divide. This phase usually takes about 8 to 12 hours in human cells in culture, but varies greatly. At the end there is a major checkpoint, and cells that pass it are committed to division.

S phase (synthesis)

The cell copies its DNA. At the start, each chromosome consists of one DNA molecule (one chromatid). After replication, each chromosome consists of two identical sister chromatids joined at the centromere. The number of chromosomes is unchanged (46 in humans), but the amount of DNA doubles. The centrosome is also duplicated. The S phase takes about 6 to 8 hours.

G2 phase (second gap)

The cell continues to grow, makes proteins needed for mitosis and, importantly, checks that the DNA has been replicated correctly and repairs any damage. It lasts around 3 to 4 hours.

How is DNA replicated in the S phase?

DNA replication is semiconservative: each new DNA molecule consists of one old strand and one new strand. The main steps are:

1. The enzyme helicase unwinds the double helix and breaks the hydrogen bonds between the bases at many sites called origins of replication, forming replication forks.
2. The enzyme DNA polymerase adds free nucleotides to each exposed strand following the base-pairing rules (A with T, C with G).
3. The new strands form, and DNA ligase joins the fragments on one of the strands.

DNA polymerase has a proofreading function and makes only about one error in 10⁹ to 10¹⁰ nucleotides copied. Errors that remain are mutations. Replication is covered in more detail in the article on semiconservative DNA replication.

What is the M phase?

After interphase, the cell enters the mitotic (M) phase. Mitosis divides the nucleus into two identical nuclei, in four stages: prophase, metaphase, anaphase and telophase. Cytokinesis then divides the cytoplasm. The two daughter cells are genetically identical to the parent cell and to each other. These stages are described in the next article.

How is the cell cycle controlled?

The cycle is controlled by regulatory proteins called cyclins and cyclin-dependent kinases (CDKs). The levels of cyclins rise and fall during the cycle, and each cyclin activates a CDK at the appropriate time. In addition, there are checkpoints at which the cell stops if something is wrong.

• G1 checkpoint: checks cell size, nutrients, growth signals and DNA damage. The protein p53 stops the cycle if the DNA is damaged and can trigger repair or cell death (apoptosis).
• G2 checkpoint: checks that DNA replication is complete and undamaged.
• Spindle (metaphase) checkpoint: checks that all chromosomes are attached correctly to the spindle before they are separated.

When control fails, cells may divide uncontrollably, which is the basis of cancer. Mutations in genes that stimulate division (proto-oncogenes) or that stop it (tumor suppressor genes such as p53) are often involved.

Phases of the cell cycle

In the table, 2C is the DNA content of a normal diploid cell, and 4C is double that amount after replication.

Worked example: estimating the length of mitosis

A student examines a slice of onion root tip under the microscope and counts 200 cells in the field of view. Of these, 160 are in interphase, and 40 are in some stage of mitosis. The root tip cells divide about once every 20 hours.

1. Proportion in mitosis: 40 ÷ 200 = 0.20, or 20 percent. This is called the mitotic index.
2. Time in mitosis: if the cells are evenly distributed through the cycle, the proportion of cells in a phase equals the proportion of time spent in it. Time = 0.20 × 20 h = 4 hours.
3. Time in interphase: 160 ÷ 200 × 20 h = 16 hours.

The 20 percent mitotic index is high because root tips are actively growing regions. In many adult tissues, the index is below 1 percent. A higher than normal mitotic index in a tissue sample may point to a tumor. The method assumes that the cells are not synchronized, and it counts all cells in the sample, which means the results are estimates. For a sample of 200 cells, the result is approximate, and counting more cells would improve accuracy.

How was the cell cycle discovered?

Walther Flemming described mitosis in detail in the 1870s and 1880s by staining dividing cells of salamander larvae, and he introduced the terms mitosis and chromatin. Alma Howard and Stephen Pelc showed in 1953 that DNA is replicated during a particular part of interphase, which defined the S phase. Leland Hartwell, Tim Hunt and Paul Nurse discovered the key regulators of the cell cycle, including cyclins and CDKs, in the 1970s and 1980s and shared the Nobel Prize in Physiology or Medicine in 2001.

Which mistakes are common?

Mistake 1: Calling interphase a resting phase

The cell is very active in interphase: it grows, copies its DNA and prepares for division.

Mistake 2: Saying that mitosis is the longest part of the cycle

Interphase takes up most of the time, often 90 percent or more.

Mistake 3: Saying that chromosome number doubles in S phase

The DNA content doubles, but the number of chromosomes stays the same, because each chromosome now has two sister chromatids.

Mistake 4: Using chromatid and chromosome as synonyms

A replicated chromosome consists of two sister chromatids. After they separate, each chromatid is counted as a chromosome.