Protein production, which encompasses the processes of transcription and translation, is generally a continuous activity in a living cell, but its rate fluctuates dramatically depending on the cell’s immediate needs. The question of whether protein production is high during mitosis, the phase of the cell cycle where a cell physically divides into two daughter cells, requires a careful distinction between the different stages. The overall rate of synthesis is substantially reduced during the active physical division, or M phase. However, the period immediately preceding mitosis sees an intense, highly specific burst of production for regulatory proteins necessary to initiate the division process itself. The answer is therefore nuanced: high production of specific molecules occurs just before the M phase, while global production plummets during the M phase.
Protein Synthesis During Interphase
The vast majority of a cell’s growth and preparation for division occurs during Interphase (G1, S, and G2 phases), and this is when protein synthesis is at its peak. The G1 phase, or “Gap 1,” is the primary stage for cellular growth following division, demanding a high rate of general protein production to double the cell’s mass. During this time, the cell rapidly synthesizes components for new organelles and accumulates resources needed for the subsequent phases.
This intense synthesis is required to meet the demands of duplicating the entire cellular contents. For example, before DNA synthesis can begin in the S phase, the cell must produce massive quantities of histone proteins. Histones are the structural components around which DNA wraps to form chromatin, and their production is tightly linked to the need to package the newly replicated DNA.
Protein synthesis remains elevated throughout the S phase (DNA synthesis) and G2 phase (Gap 2) as the cell prepares its final checks and machinery for division. This sustained high rate ensures that all the necessary structural components are available for two complete daughter cells. The total synthesis capacity of the cell is at its maximum during these interphase stages.
Essential Proteins Orchestrating Mitotic Entry
While general protein production is a hallmark of Interphase, the synthesis of a select group of regulatory proteins increases dramatically in the G2 phase to directly trigger the onset of mitosis. These proteins act as molecular switches, ensuring the cell only enters division when all prior steps, such as DNA replication, are complete. The most recognized of these molecules are the mitotic cyclins, particularly Cyclin B, which must accumulate to a certain threshold level.
Cyclin B partners with and activates the Cyclin-Dependent Kinase 1 (CDK1) enzyme, forming the complex that drives the cell across the G2/M border. This complex initiates a cascade of phosphorylation events that cause the nuclear envelope to break down and the chromosomes to begin condensation. The accumulation of this regulatory protein is a necessary event, and synthesis is completed before the cell even reaches prophase.
The production of these regulatory molecules must be precisely balanced by their targeted destruction. For example, the Anaphase-Promoting Complex/Cyclosome (APC/C) is a large enzyme complex that tags Cyclin B with ubiquitin molecules, marking it for rapid degradation by the proteasome. This switch-like balance between Cyclin B synthesis and degradation creates the abrupt, irreversible transition into and out of the M phase.
Gene Expression Reduction During Active M Phase
Once the cell is fully engaged in the physical process of mitosis—from prophase through metaphase and anaphase—the overall rate of gene expression, including transcription and translation, plummets dramatically. This reduction provides the counter-answer to the initial question. During metaphase and anaphase, the overall rate of transcription can drop to as low as 13 to 16% of the interphase rate.
This profound transcriptional shutdown is directly caused by the physical restructuring of the cell’s genetic material. Chromosomes condense into their compact, recognizable shapes, which makes the DNA template inaccessible to the RNA polymerase machinery. The tight coiling and compaction effectively prevent the enzymes responsible for reading genes from binding to the DNA, thus halting the creation of new messenger RNA (mRNA).
Translation, the process of synthesizing protein from existing mRNA, also experiences a significant slowdown, decreasing by approximately 50 to 70% in many mammalian cells. This translational inhibition is driven by the global reorganization of the cell’s internal machinery and specific inhibitory signals from the activated CDK1. Cap-dependent translation, the main mechanism for protein synthesis, is often inhibited, although a small number of specific mRNAs may still be translated using alternative mechanisms.