AP Syllabus focus:
‘During G1 phase, the cell is metabolically active, duplicating organelles and cytosolic components before DNA replication.’
G1 is the major growth interval of interphase, when a cell builds the materials and internal capacity needed to copy its genome later.
Labeled cell-cycle diagram showing interphase subdivided into G1, S, and G2, followed by M phase, with an exit to G0. It visually reinforces that G1 is a growth-and-preparation phase that occurs before DNA synthesis (S phase) and that cells can pause the cycle by entering G0. Source
It emphasises biosynthesis, organelle production, and expansion of cytoplasm and membranes.
What G1 Phase Is
G1 phase is the first gap phase after cell division and before DNA synthesis, characterized by high metabolic activity and growth.
G1 phase: The interphase stage in which a cell grows and carries out biosynthesis, increasing its cytoplasm and duplicating cellular components in preparation for later DNA replication.
A key idea for AP Biology is that “growth” in G1 is not only an increase in size; it is coordinated production and allocation of cell parts so future genome replication and cell division can proceed efficiently.
Metabolic Activity During G1
Cells in G1 are metabolically active, meaning they use energy and raw materials to build macromolecules and maintain homeostasis while increasing overall cellular capacity.
Graph of cyclin protein concentrations across the cell cycle (G1, S, G2, and M), illustrating that regulatory proteins rise and fall in a timed pattern. This helps explain how sustained biosynthesis in G1 supports the buildup of specific regulators required to pass checkpoints and commit to DNA replication. Source
Energy and building blocks
ATP production rises to support biosynthetic work (e.g., polymer formation, active transport).
Uptake and processing of nutrients supply carbon skeletons and nitrogen for macromolecule synthesis.
Redox cofactors (e.g., NADH/NADPH) support both energy metabolism and anabolic reactions.
Macromolecule biosynthesis
Protein synthesis increases, including enzymes for metabolism, structural proteins, and transport proteins.
RNA synthesis supports new ribosomes and mRNA production for expanded translation.
Lipid synthesis supports expansion of cellular membranes as the cell grows.
Duplicating Organelles and Cytosolic Components
The specification emphasises that, in G1, the cell duplicates organelles and cytosolic components to restore and expand cellular infrastructure before DNA replication begins.
Cytosol: The aqueous interior of the cell (excluding membrane-bound organelles) that contains dissolved ions, metabolites, and many proteins that carry out metabolic and signalling functions.
Between organelles and cytosol, the cell must rebuild what was distributed during the previous division and also add extra capacity for the next cycle.
Organelle duplication (functional emphasis)
Mitochondria increase in number and/or mass to match rising energy demand.
Endomembrane system (e.g., ER and Golgi) expands to support heightened protein and lipid processing.
Ribosomes become more abundant as rRNA and ribosomal proteins are produced, boosting translation.
Cytoskeleton components are produced and reorganised to support cell shape, intracellular transport, and growth.
Cytosolic components (examples)
Enzymes for glycolysis, respiration, and biosynthetic pathways increase to raise throughput.
Cytosolic structural proteins and regulatory proteins accumulate to support cell growth and responsiveness.
Ions and metabolites are balanced and stockpiled to maintain osmotic stability and fuel synthesis.
Growth as Expansion of Cellular Structure
G1 growth includes coordinated increases in volume and surface area so the cell can sustain larger cytoplasmic content.
Membrane and transport capacity
Plasma membrane expansion requires phospholipid and cholesterol synthesis.
Transport proteins (channels, carriers, pumps) increase to import nutrients and export wastes at higher rates.
Surface-area-to-volume constraints can shape how quickly a cell can grow and how much it must invest in membrane production.
Biosynthetic coordination
Building more cytoplasm without proportionally increasing organelles can limit function; G1 growth therefore couples bulk growth with organelle biogenesis.
Many proteins made in G1 are “capacity builders” (e.g., metabolic enzymes, ribosomes), not just short-term responders.
“Before DNA Replication”: Why Timing Matters
Diagram of the three major cell-cycle checkpoints (G1, G2, and M), emphasizing that the G1 checkpoint evaluates readiness before S phase begins. It clarifies that progression into DNA replication is regulated rather than automatic, linking preparation in G1 to controlled entry into S phase. Source
G1 prepares the cell for the later challenge of copying its genome by ensuring sufficient:
Nucleotides and replication-related proteins can be produced when needed.
Energy supply can support high-cost processes.
Organelle and cytoplasmic content can be distributed to future daughter cells without shortages.
FAQ
Common approaches include DNA-content staining (G1 cells have baseline DNA content) and measuring synthesis markers.
Flow cytometry with DNA dyes distinguishes G1 from post-replication states.
Protein or RNA synthesis assays can support G1 identification.
G1 duration reflects how quickly a cell can accumulate biomass and organelles.
Cells with high nutrient access and strong growth signals may shorten G1, while specialised cells may maintain a prolonged growth state tailored to function.
Ribosome biogenesis is the production and assembly of rRNA and ribosomal proteins into ribosomes.
Greater ribosome numbers increase translation capacity, enabling the broad protein synthesis needed to expand cytoplasm and organelle systems during G1.
Nutrient availability can shift allocation among biosynthetic pathways.
High amino acids support protein synthesis.
Adequate carbon sources support lipid synthesis for membranes.
Limitation can prioritise maintenance over growth-related production.
Constraints include transport efficiency and scaling of membrane area relative to volume.
As volume increases faster than surface area, cells must invest heavily in membrane expansion and transport proteins to maintain adequate exchange and internal distribution.
Practice Questions
Describe two events that occur in G1 phase and explain how each helps the cell prepare for DNA replication. (3 marks)
Identifies a correct G1 event (e.g., organelle duplication; increased protein/RNA synthesis; membrane expansion) (1)
Explains preparation link for that event (e.g., more mitochondria provide ATP; more ribosomes increase protein production needed for replication) (1)
Provides a second correct event with a valid preparation link (1)
A researcher compares two cell populations. Population A shows high rates of protein synthesis and increasing mitochondrial number. Population B shows low biosynthesis and stable organelle number. Using G1 phase concepts, explain which population is more likely to be in G1 and justify your reasoning with three distinct points. (6 marks)
Correctly identifies Population A as more consistent with G1 (1)
Links high protein synthesis to G1 metabolic activity/growth (1)
Links increasing mitochondrial number to organelle duplication in G1 (1)
Explains that G1 builds cytosolic components (e.g., enzymes, ribosomes) needed before DNA replication (1)
Notes Population B is less consistent due to low biosynthesis/lack of growth-associated duplication (1)
Uses correct biological terminology and clear, logically connected reasoning (1)
