Presynthetic period ($G_1$)
This stage is also known as the growth period or postmitotic period. It begins immediately after the cell is formed as a result of division.
Process characteristics:
- Intensive growth: the cell actively increases in mass. Large-scale synthesis of proteins, enzymes, fats, and carbohydrates takes place. Nucleotides are also accumulated, which will later be necessary for RNA and DNA synthesis.
- Organelle restoration: a large number of ribosomes are formed to ensure protein synthesis. New membranous organelles are formed: mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, vesicular vesicles, and chloroplasts in plant cells.
- Energetics and regulation: active ATP synthesis is underway. In addition, specific substances are produced that can either stimulate or inhibit the onset of the next phase of the cell cycle.
Genetic status: chromosomes consist of a single chromatid (single-chromatid). The genetic set is designated by the formula 2n2c, where n is the number of chromosomes and c is the amount of DNA. This is a classic diploid set with a normal amount of genetic material.
Synthetic period ($S$)
The synthetic stage is the central event of the entire interphase, as it is here that the duplication of genetic information takes place.
Process characteristics:
- DNA replication: the process proceeds in a semiconservative manner, ensuring accurate copying of the parental molecule.
- Protein synthesis: parallel with replication, special proteins — histones — are formed. They are extremely important because they are involved in the formation of the correct structure of new chromosomes and the spatial packing of the doubled DNA.
Genetic status: at the very beginning of the period, chromosomes remain single-chromatid (2n2c). However, by the end of the $S$-period, after the completion of replication, each chromosome acquires a sister chromatid. Now the chromosomes consist of two identical chromatids (double-chromatid), and the set formula changes to 2n4c.
Postsynthetic period ($G_2$)
The postsynthetic or premitotic period represents the final stage of cell preparation for division.
Process characteristics:
- Specific synthesis: the cell produces RNA and actively stores energy in the form of ATP (it will be completely consumed during mitosis). The most important event is the synthesis of tubulin proteins, from which the mitotic spindle will later be built.
- Structural rearrangements: the increase in cytoplasm mass is finally completed. There is a sharp increase in the volume of the cell nucleus. Duplication of centrioles, necessary for the organization of division poles, occurs.
Genetic status: chromosomes remain double-chromatid, the genetic set remains stably at 2n4c until the onset of mitosis.
Summary table of periods
| Period | Synonym | Genetic set | Key events |
|---|---|---|---|
| $G_1$ | Presynthetic | 2n2c | Cell growth, organelle and nucleotide synthesis |
| $S$ | Synthetic | Transition from 2n2c to 2n4c | Semiconservative DNA replication, histone synthesis |
| $G_2$ | Postsynthetic | 2n4c | Centriole duplication, tubulin and ATP synthesis for mitosis |
Briefly: answers to topical questions
What happens to the cell if it enters the G0 phase (resting period)?
Upon entering the $G_0$ phase (reproductive rest period), the cell stops dividing and is in a "resting" state. In this phase, cells differentiate (specialize) and behave differently depending on their type:
- Non-dividing — terminally differentiated cells that finally lose the ability to enter the proliferation stage (e.g., neurons, cardiomyocytes).
- Rarely dividing — conditionally renewing cells that are able to re-enter the cell cycle after a long resting period (e.g., hepatocytes).
Also, a cell from this resting phase can simply transition back into the active division cycle.
What specific protein-enzymes regulate the passage of interphase checkpoints by the cell?
The passage of interphase periods by the cell is regulated by complexes of activator proteins (cyclins) and enzymes — cyclin-dependent kinases (CdKs). Cyclins are synthesized in waves and activate kinases, which phosphorylate target proteins.
Phase transitions are determined by specific complexes:
- Initial G1 period and exit from G0 — "Cyclin D — Cdk 4" and "Cyclin D — Cdk 6" complexes.
- Final part of G1 — "Cyclin E — Cdk 2" complex.
- S-period — "Cyclin A — Cdk 2" complex.
- G2-period and Mitosis — "Cyclin B/A — Cdk 1" complexes.
After performing their function, cyclins are degraded in proteasomes.
What is the mechanism of semiconservative DNA replication in the S-period (what enzymes are involved)?
The semiconservative mechanism of replication implies that each of the two parent DNA strands serves as a template for the formation of a new daughter strand. Each daughter DNA molecule consists of one parental strand and one newly synthesized strand.
The process includes the following stages:
- Initiation: initiator proteins bind to the DNA molecule, complementary strands uncoil and hydrogen bonds between nitrogenous bases break; a replication fork is formed.
- Elongation: elongation of the DNA polynucleotide chain, synthesis of new DNA strands. Deoxyribonucleoside triphosphates serve as substrates and energy sources.
- Termination: end of replication. Primer removal and completion of the lagging DNA strand formation are also described.
Enzymes involved in DNA replication: DNA polymerases. 7 types of DNA polymerases are known in eukaryotes: $\alpha, \beta, \gamma, \delta, \epsilon, \zeta, \eta$; they are involved in DNA replication and repair.
It is easy to remember the change in the set: the letter "c" (amount of disease/DNA) doubles only after the "S"-period (Synthesis). Before it — 2n2c, after it — 2n4c.
Class and exam questions
In which period of interphase does DNA replication occur?
DNA replication (doubling) occurs in the synthetic ($S$) phase.
Why does the cell synthesize tubulins during the G2 phase?
Tubulin proteins are necessary for the subsequent formation of mitotic spindle fibers during mitosis.
How does chromosome structure change throughout the interphase?
In the $G_1$-period, chromosomes consist of a single chromatid. In the $S$-period, they duplicate, and by the $G_2$-period they become double-chromatid.
What proportion of the cell cycle does interphase occupy?
Interphase is the longest stage, taking up about 90% of the entire cell's existence from division to division.
What else to study on this topic
- Mechanism of semiconservative DNA replication
- Functions and structure of histone proteins
- Structure of centrioles and formation of the mitotic spindle from tubulins
- Regulatory proteins and checkpoints of the cell cycle
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