Difference Between G1 G2 and S Phase

The key difference between G1 G2 and S phase is the growth that occurs.  During the G1 phase, the cell shows first growth by copying organelles and making the molecular building blocks which are necessary for later steps; during the G2 phase, the cell shows second growth by making proteins and organelles and beginning to reorganize its contents in preparation for mitosis; during the S phase, the cell copies or duplicates all the DNA in the cell, making an extra set of chromosomes.

Eukaryotic cells undergo cell divisions as they grow and develop. There are two major phases in the cell cycle. They are interphase and mitosis. A dividing cell spends most of its time in interphase. In other words, interphase is the longest phase of the cell cycle in which the cell prepares for nuclear division and produce new daughter cells. Interphase further subdivides into three phases as Gap 1 (G1) phase, Gap 2 (G2) phase and Synthesis (S) phase. G1 phase is the first stage of interphase which is considerably a longer process. S phase is the middle phase in which the cell makes an extra copy of its chromosome set. G2 phase is the last stage of interphase which is relatively a short phase.

CONTENTS

1. Overview and Key Difference
2. What is G1 Phase
3. What is G2 Phase
4. What is S Phase
5. Similarities Between G1 G2 and S Phase
6. Side by Side Comparison – G1 vs G2 vs S Phase in Tabular Form
7. Summary

What is G1 Phase?

Gap 1 or G1 phase is the first cell growth phase of the interphase of the cell cycle. Significant development processes take place within the cell during the G1 phase. The cell size will increase due to the extensive synthesis of proteins and RNA. Synthesis of proteins and RNA is required prior to S phase where the replication of DNA takes place. Proteins synthesized during the G1 phase mainly include histone proteins, and the majority of RNA synthesized is mRNA. Histone proteins and mRNA participate in the S phase for DNA replication.

The duration of the cell cycle varies according to the type of organism. Some organisms have a longer G1 phase before entering the S phase while other organisms may have a shorter G1 phase. In humans, a typical cell cycle runs for 18 hours. Out of the total time of the cell cycle, G1 phase normally takes 1/3 of the time. However, this time can change due to certain factors. These factors are referred to as growth factors, and some of them are cellular environment, availability of nutrients such as proteins and specific amino acids and cellular temperature. The temperature mainly affects the proper growth of the organism, and this value varies from organism to organism. In humans, the optimum temperature for cellular growth is roughly 37 ⁰C.

The cell cycle regulatory mechanism controls the G1 phase. During the regulation, the control of duration and coordination between other phases take place. G1 phase is considered to be an important phase because it is the point which determines the fate of the cell. In this phase, the cell decides whether it proceeds with the rest of the phases of the cell cycle or leave the cell cycle. If the cell receives a signal to keep it at an un-dividing stage, the cell will not enter into the S phase. It will move into the dormant phase called G0 phase. G0 phase is a state of cell cycle arrest.

What is G2 Phase (aka Gap 2 phase)?

G2 phase, also known as the Gap 2 phase, is the final phase of interphase. G2 phase is a shorter phase, but it is an important phase of the cell cycle. During the G2 phase, extensive cellular growth takes place under high synthesis rate of proteins. With the synthesis of necessary RNA and proteins, it also helps the formation of the spindle apparatus during mitosis. Even though this phase is important, a cell can bypass this stage and directly enter mitosis once the cell completes its S phase. But by completing the G2 phase, the cell can become fully prepared for mitosis or nuclear division.

If a cell enters the G2 phase, it confirms the fact that the cell has successfully completed the S phase where DNA replication has taken place. Therefore, all cells in the G2 phase will progress into mitosis where the cell will be divided into two identical daughter cells. In G2 phase, the cell size increases along with the different components, including the nucleus and almost all the other cellular organelles. Similar to the G1 phase, G2 phase is also regulated by cell cycle regulatory mechanisms.

What is S Phase?

S phase or synthesis phase is the middle stage of the interphase. During the S phase, the cell copies all its DNA and makes an extra copy. Hence, a complete copy of the chromosome set is synthesized in S phase. Moreover, centrosomes are also synthesized during the S phase. These centrosomes are for future use during the mitosis. Centrosomes help the separation of DNA.

A successful cell division depends on the duplication of its genome. It takes place during the S phase. S phase is the most critical phase of the cell cycle. Therefore, this phase is tightly regulated and widely conserved.

What are the Similarities Between G1 G2 and S Phase?

• G1, G2 and S phases are three phases of the interphase of the cell cycle.
• All three phases occur in almost all cell cycles.
• During these phases, the cell grows, replicates its chromosomes and prepares for cell division.
• The duration of all three phases is greater than the M phase.
• All three phases are critical and very important for a successful cell division.

What is the Difference Between G1 G2 and S Phase?

G1 phase is the first phase of the interphase in which the cell grows by copying organelles and synthesizing proteins and RNA. G2 phase is the third phase of the interphase in which cell makes proteins and organelles and RNA and reorganizes cell content. S phase is the middle phase of the interphase in which cell duplicates its DNA and centrosomes. So, this is the key difference between G1 G2 and S phase. G1 phase is the longest phase of interphase while G2 phase is the shortest phase of interphase and S phase is the second-longest phase of the interphase.

Below is a detailed side-by-side comparison of all 3 phases to discern the difference between G1 G2 and S phase.

Summary – G1 G2 vs S Phase

Interphase is the longest phase in the cell cycle. During the interphase, the cell grows, replicates its DNA, accumulates nutrients and duplicates its organelles. By doing all these, the cell prepares for cell division and the making of new cells. There are three major stages in interphase as G1, S and G2 phases. G1 is the first phase, and during this phase, the cell grows larger, copies its organelles and makes the molecular building blocks. During the S phase, cell synthesizes a complete extra copy of its DNA. The cell also duplicates centrosomes that are required for DNA separation in mitosis. G2 is the last phase of the interphase, and during it, the cell grows more, synthesizes proteins and organelles and reorganizes cell contents. Thus, this is the summary of the difference between G1 G2 and S phase.

Reference:

1. “Phases of the Cell Cycle (Article).” Khan Academy, Khan Academy, Available here.
2. “The Cell Cycle.” Lumen, College of the Redwoods & Northern Virginia Community College, Available here.

Image Courtesy:

1. “Eukaryotic replication cycle” By Boumphreyfr – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Asymmetry in the synthesis of leading and lagging strands”By David O Morgan – The Cell Cycle. Principles of Control. (Attribution) via Commons Wikimedia