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How do cells function?
the binary fission of prokaryotic cells and the eukaryotic cell cycle, including interphase (G1, S and G2), mitosis (prophase, metaphase, anaphase and telophase) and cytokinesis in plant and animal cells, with reference to checkpoints that regulate the cycle
A focused answer to the VCE Biology Unit 1 dot point on cellular reproduction. Covers prokaryotic binary fission, the eukaryotic cell cycle (G1, S, G2, M), the four phases of mitosis (PMAT), cytokinesis in plant and animal cells, and the checkpoints that regulate the cycle.
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What this dot point is asking
VCAA wants the prokaryotic mechanism (binary fission), the eukaryotic cell cycle (interphase + M phase), the four phases of mitosis (PMAT), the difference in cytokinesis between plant and animal cells, and the checkpoints that regulate the cycle.
The answer
Prokaryotic binary fission
Prokaryotes have a single circular chromosome and no nucleus, so they reproduce by a simpler process called binary fission.
- The circular DNA is replicated, producing two identical chromosomes attached to the plasma membrane.
- The cell grows and the two chromosomes move to opposite ends.
- The plasma membrane pinches inward and a new cell wall forms in the middle, splitting the cell into two genetically identical daughter cells.
Binary fission is fast: E. coli can divide every 20 minutes under ideal conditions.
The eukaryotic cell cycle
The cell cycle has two main parts: interphase (preparing for division) and the mitotic phase (dividing).
Interphase (about 90% of the cycle) has three sub-phases:
- G1 (Gap 1). The cell grows, makes proteins, organelles duplicate. Most of the cell's lifespan is spent here.
- S (Synthesis). DNA is replicated. Each chromosome now consists of two identical sister chromatids joined at the centromere. DNA content doubles.
- G2 (Gap 2). Final growth and preparation for division. The cell synthesises proteins and organelles needed for mitosis.
M phase consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division).
Cells that have stopped dividing (such as mature neurons or muscle cells) exit the cycle into G0.
The four phases of mitosis (PMAT)
A mnemonic: Prophase, Metaphase, Anaphase, Telophase.
Prophase. Chromosomes condense and become visible under the microscope as pairs of sister chromatids. The nuclear envelope breaks down. The mitotic spindle starts to form from microtubules organised by centrioles (in animal cells) at opposite poles. In plants, the spindle forms without centrioles.
Metaphase. Spindle fibres attach to the centromere of each chromosome. Chromosomes line up along the cell's equator at the metaphase plate. This is the diagnostic image of metaphase.
Anaphase. Centromeres divide. The two sister chromatids of each chromosome are pulled apart and dragged to opposite poles by shortening spindle fibres. The cell now has two identical sets of chromosomes, one at each pole.
Telophase. Chromosomes arrive at the poles and decondense back into chromatin. A new nuclear envelope re-forms around each set, and nucleoli reappear. The spindle dissolves. The cell now has two nuclei, ready for the cytoplasm to split.
Cytokinesis
In animal cells. A ring of actin and myosin proteins beneath the plasma membrane contracts, forming a cleavage furrow that deepens around the cell's equator until the cell pinches into two daughter cells.
In plant cells. The rigid cellulose cell wall prevents pinching. Instead, vesicles full of cell-wall material bud off the Golgi and gather at the cell's equator. They fuse to form a cell plate that grows outwards until it meets the existing cell wall, dividing the cell into two and laying down a new wall between them.
Cytokinesis usually overlaps with telophase but is considered a separate step.
Outcome of mitosis
Two daughter cells, each genetically identical to the parent, each with the full diploid number of chromosomes (2n in humans, 46). Mitosis underpins growth, tissue repair, asexual reproduction in some organisms, and the maintenance of multicellular bodies.
Checkpoints
The cell cycle is tightly regulated by checkpoints that pause the cycle to verify conditions before proceeding. The three major checkpoints:
- G1 checkpoint (restriction point). End of G1. Checks: cell size, nutrient supply, growth-factor signals, DNA integrity. If satisfied, the cell commits to S phase and division. If not, the cell pauses or exits to G0.
- G2 checkpoint. End of G2. Checks: DNA has been replicated correctly and is undamaged, cell size, organelle replication. If satisfied, the cell enters mitosis.
- M checkpoint (spindle checkpoint). During metaphase. Checks: all chromosomes are correctly attached to spindle fibres at the metaphase plate. If not, anaphase is delayed to prevent aneuploidy.
Checkpoints use proteins called cyclins and cyclin-dependent kinases (CDKs). Tumour suppressor genes such as p53 trigger checkpoint pauses, DNA repair, or apoptosis when damage is detected. Loss of checkpoint control is the key step in cancer (see the apoptosis and cancer dot point).
Worked example
A liver cell is exposed to a chemical that damages DNA. At the G1 checkpoint, the cell detects the damage. p53 is activated; the cyclin/CDK complex needed to enter S phase is inhibited. The cell pauses to repair the DNA. If the repair succeeds, the cycle resumes. If the damage is too severe, p53 triggers apoptosis, preventing the damaged cell from replicating and forming a tumour. A different cell, in S phase when damaged, may stall at the G2 checkpoint instead.
Common traps
Confusing chromosomes and chromatids. Before S phase: each chromosome is one chromatid. After S phase and through G2 and most of mitosis: each chromosome is two sister chromatids joined at the centromere. After anaphase: each chromatid is now its own chromosome.
Skipping interphase. Mitosis is only 10% of the cell cycle. Most of a cell's life is spent in interphase.
Saying binary fission is "the same as mitosis". Binary fission is simpler: no spindle, no chromosomes condensing visibly, no PMAT phases. It is the prokaryotic equivalent.
Saying cytokinesis is part of mitosis. Mitosis is nuclear division (PMAT). Cytokinesis is cytoplasmic division. Together they make M phase.
Forgetting checkpoints. A question on cell cycle regulation expects G1, G2 and M checkpoints, not just a list of phases.
In one sentence
Prokaryotes divide by binary fission (replicate the circular DNA, then split), while eukaryotes follow a regulated cell cycle of interphase (G1, S, G2) and M phase (mitosis: prophase, metaphase, anaphase, telophase, then cytokinesis with a cleavage furrow in animals or a cell plate in plants), with G1, G2 and M checkpoints (using cyclins, CDKs and p53) ensuring the cell only divides when conditions and DNA are sound.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2022 VCE4 marksDescribe the four phases of mitosis.Show worked answer →
A 4-mark answer needs all four phases with the key event in each.
- Prophase. Chromosomes condense and become visible as sister chromatids joined at the centromere. The nuclear envelope breaks down. The mitotic spindle starts to form from centrioles (in animal cells) at opposite poles.
- Metaphase. Spindle fibres attach to centromeres. Chromosomes line up along the cell's equator (the metaphase plate).
- Anaphase. Centromeres divide. Sister chromatids are pulled to opposite poles by the shortening spindle fibres. The cell briefly contains two complete sets of chromosomes, one at each pole.
- Telophase. Chromosomes arrive at the poles and decondense. A nuclear envelope re-forms around each set. The spindle breaks down. The cell now has two nuclei, ready for cytokinesis.
Markers reward all four phases in order with the diagnostic event for each.
2024 VCE3 marksCompare cytokinesis in plant and animal cells.Show worked answer →
A 3-mark answer needs the mechanism in each cell type and the reason for the difference.
In animal cells, a ring of actin and myosin under the membrane contracts, pinching the cell inward to form a cleavage furrow. The furrow deepens until the cell splits into two.
In plant cells, the rigid cell wall prevents pinching. Instead, vesicles from the Golgi gather at the cell equator and fuse to form a cell plate. The cell plate grows outwards until it meets the existing cell wall, becoming the new cell wall between the two daughter cells.
The difference is driven by the presence or absence of a cell wall.
2026 VCE3 marksExplain the role of the G1 checkpoint in regulating the cell cycle.Show worked answer →
A 3-mark answer needs the location, the assessments made, and the consequence.
The G1 checkpoint (also called the restriction point) occurs at the end of G1, just before the cell commits to DNA replication.
The cell assesses: cell size (is the cell big enough?), nutrient availability (are enough materials present?), DNA integrity (is the DNA undamaged?), and the presence of growth factors signalling division.
If all conditions are met, the cell enters S phase and replicates its DNA. If conditions are not met, the cell pauses (or enters a non-dividing G0 state). If DNA damage is too severe, the cell triggers apoptosis. This checkpoint prevents cells from replicating damaged DNA, which is the central protection against cancer.
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