Cell Cycle and Cell Division · Lesson 2 of 9
Interphase: G1, S, G2 and the G0 State
“Explain the chronological preparation for division, regulated DNA replication, different cycle durations and quiescence.”
• Trace the sequence G1 → S → G2 → M and explain the role of each interval. • Describe DNA replication and animal-cell centriole duplication during S. • Distinguish G0 quiescence from inactivity or cell death. • Compare dividing, occasionally dividing and long-lived non-dividing cells. • Interpret the differing durations of interphase and M phase.
A complete cycle contains interphase and division
A cell about to divide must grow, duplicate its DNA and make enough proteins, membranes and organelles to support daughter cells. These tasks are not all accomplished while visible chromosomes line up or separate. Most of the time lies between divisions, in interphase. The eukaryotic cell cycle is conventionally divided into interphase and M phase. Interphase comprises G1, S and G2. M phase includes nuclear chromosome segregation and usually cytokinesis. The textbook describes a typical cultured human cell that completes the whole cycle in about 24 hours, with actual cell division taking roughly one hour; this is an illustrative example, not a clock for every human tissue.
The interval between successive M phases when a cell grows, carries out normal activities and, if it is proceeding through a division cycle, replicates its DNA.
The word 'resting' is historically associated with interphase because conspicuous chromosome movement is absent, but the cell is not generally resting. RNA transcription, protein synthesis, energy metabolism and organisation of cell components continue. DNA replication is confined to the S part in a typical normal cycle. This separation helps reduce the risk of making incomplete genomes. Interphase can be much longer than mitosis, and the relative times vary with cell type and external conditions.
G1: growth after the preceding division
The interval after mitosis and before DNA replication, during which the cell grows and performs much of its normal metabolism.
A newly formed daughter cell may be smaller than its parent had been at the start of division. In G1 it increases its mass, synthesises proteins, generates RNA and expands or renews organelles. DNA content remains at the unreplicated level during G1, typically 2C in a diploid cell. The cell may respond to nutrients, growth signals and its specialised functions. For many animal cells G1 is a key interval in which continuing toward another division or entering a non-dividing state depends on biological conditions.
S: one complete round of DNA replication
The synthesis phase during which chromosomal DNA is replicated once, generating sister chromatids.
During S, each chromosome's DNA is copied, so the DNA quantity per diploid cell doubles from 2C to 4C. The chromosome number remains 2n because each new sister pair is still joined as one replicated chromosome. The replication takes place in the nucleus in eukaryotes. In many animal cells, centrosome duplication also occurs during the S-period sequence, with centrioles duplicating as part of preparation for bipolar spindle assembly. Do not assume that every eukaryotic cell possesses centrioles: most higher plant cells assemble spindles without the typical animal-cell centriolar arrangement.
The doubled DNA amount does not mean that 'two nuclei are already present'. The nucleus can contain a full set of duplicated chromosomes while its envelope is still intact. Each DNA molecule is copied with mechanisms that help preserve sequence accuracy. Replication provides copies to be segregated; it does not itself separate sister chromatids into daughters. Separation occurs later, during mitosis or meiosis II, after spindle attachment.
G2: final preparation for chromosome segregation
The interval after DNA synthesis but before mitosis, during which continued growth and synthesis of division-related components occur.
During G2 the diploid cell has 2n chromosomes with 4C DNA. It synthesises proteins and organises cellular structures required for chromosome condensation, spindle function and division. This phase is not another S phase: DNA content normally does not double to 8C. Proper preparation matters because each chromosome must attach to the correct spindle arrangement, and cell components must be available for daughter-cell function. Cells may also delay transition when replication is incomplete or DNA damage has been detected.
Sequence diagram and what its arrows mean
Start at G1 after an earlier cell division. The cell proceeds through S and G2 before entering M. Successful M phase, including cytokinesis, creates daughter cells that can start G1 again. The extra branch to G0 represents a cell leaving the active proliferation cycle. The arrows show conceptual sequence rather than precise sizes or durations: a cell can spend many hours preparing but far less time actually partitioning chromosomes.
G0 is metabolically active quiescence
A quiescent condition in which a cell is outside the actively progressing division cycle, usually after leaving G1; it may retain the ability to re-enter in some tissues.
Many adult animal cells are not dividing continuously. Some are highly specialised and ordinarily remain outside the cycle for long periods, whereas others re-enter as needed to replace dead or damaged cells. A quiescent cell can still carry out metabolism, respond to signals and maintain its specialised function. G0 therefore does not mean death, absolute chemical inactivity or 'a special kind of mitosis'. Some cells can re-enter the cell cycle, while others are terminally differentiated and have little or no normal capacity to do so.
Plant meristems show a different developmental pattern. Apical and lateral meristematic regions contain dividing cells that support lengthening and thickening of growing plant tissues. Mature plant cells elsewhere may divide rarely or not at all. In animals, cells in renewing epithelial tissues frequently divide, whereas most mature cardiac muscle cells do not commonly divide. It is incorrect to treat the entire plant or animal as one synchronized culture of cells passing through the same phase.
Cycle duration differs between organisms and cell types
For an illustrative cultured human cell, the source gives approximately 24 hours per cycle; for some yeast cells it gives about 90 minutes. Within an organism, different tissues also divide at different rates. The duration depends on a cell's growth needs, nutrient state, environmental conditions and genetic regulation. The claim that interphase lasts more than 95% refers to the illustrative human 24-hour cycle with about one hour in mitosis; it is not a universal percentage for every cell.
| Phase | DNA amount for typical diploid cell | Key events | Common error |
|---|---|---|---|
| G1 | 2C | Growth, metabolism and organelle/protein synthesis | Calling it a period of total inactivity |
| S | 2C increases to 4C | DNA replication; in animal cells centrosome duplication sequence | Doubling chromosome number |
| G2 | 4C | Preparation, growth, protein synthesis | Assuming DNA replication repeats |
| M | 4C before daughter separation; 2C per completed daughter | Karyokinesis, normally followed by cytokinesis | Treating M as only cytokinesis |
| G0 | Usually 2C in quiescent diploid cells | Normal metabolism without ongoing proliferation | Equating quiescence with death |
Regulatory checkpoints: an additional conceptual connection
The chapter states that cell-cycle events are under genetic control. A useful extension is that cellular regulatory systems monitor readiness to enter the next stage, completion of replication and correct spindle attachment. These are often termed checkpoints. Their function is to avoid proceeding with obvious problems, not to guarantee that every error in every cell will be corrected. For the present lesson, what matters is understanding that G1, S, G2 and M cannot safely be reordered; the molecular details of particular regulatory proteins belong to more advanced study.
Problem
A cell has replicated only part of its genome but attempts to enter mitosis. Why is this dangerous?
- 1.A chromosome copy may be incomplete because S phase has not finished.
- 2.Spindle segregation could distribute fragments or missing genomic regions to daughter nuclei.
- 3.A normal division cycle therefore includes regulatory mechanisms that help prevent entry into mitosis when replication is incomplete.
Interphase is not universally 23 hours and mitosis not universally one hour. These are illustrative values for a particular average cultured human-cell example; some yeast cells cycle much faster.
Quick Quiz
Quiz
During which phase does the amount of DNA double?
Which interval ordinarily follows S phase?
A cell in G0 is best described as:
Which statement about G1 is accurate?
What is the usual DNA content of a diploid G2 cell if its G1 amount was 2C?
Which part of a typical cell cycle is most active metabolically?
Which is a correct statement about centriole duplication?
What is the key reason cell-cycle regulatory controls matter?
Practice Problems
- Trace the G1, S, G2 and M sequence, explaining what happens to chromosome number and DNA amount in a typical diploid cell.
- Explain why interphase should not be described as metabolically inactive.
- Compare a renewing gut epithelial cell with a quiescent adult heart muscle cell.
- What would happen to DNA amount if a diploid cell underwent two complete S phases without cell division? Is this a normal cycle?
- A student claims that human cells always spend exactly 23 hours in interphase. Explain the problem.
G1: 2n, 2C; cell grows. S: DNA doubles to 4C while chromosomes stay 2n. G2: 2n, 4C; preparation for mitosis. M: duplicated chromosomes are segregated and daughter cytoplasm divides, yielding two 2n, 2C cells.
Key Takeaways
• G1 supports growth, S replicates DNA and G2 prepares for M. • In a typical diploid cell, S changes 2C to 4C without changing 2n. • G0 is quiescence with continuing metabolism, not death. • Cell cycles vary greatly in duration and depend on genetic controls. • DNA replication normally happens once before a sequence of divisions.