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.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
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).
Examples in context
Example 1. Skin healing after a Wilsons Promontory bushwalk. A grazed shin on the Wilsons Promontory Lighthouse track heals because basal keratinocytes in the epidermis cycle through G1, S, G2 and M phases every 20 to 25 hours. The G1/S checkpoint, regulated by the p53 protein, halts cells whose DNA is damaged by UV exposure on the walk; the S-phase damage checkpoint pauses replication so DNA can be repaired. If checkpoints pass, mitosis produces two daughter cells that migrate to seal the wound. Sunburnt skin sheds because p53 triggers apoptosis when damage exceeds the cell's repair capacity, which is the body's first line of defence against melanoma.
Example 2. Cyanobacteria binary fission in Lake Eildon. When summer water temperatures rise above 22 degrees C in Lake Eildon, blue-green algae (Microcystis) divide by binary fission roughly every 6 hours, doubling the population in that time. Binary fission is simpler than mitosis: the circular chromosome is replicated, the two copies separate, the plasma membrane invaginates and a new cell wall is laid down. There is no nuclear envelope to break down, no spindle to form, no chromatin to condense. This simplicity is why one Microcystis cell can become a million within 5 days, producing the toxic blooms managed by Goulburn-Murray Water under the National Health and Medical Research Council guidelines.
Try this
Q1. Outline the four phases of mitosis and identify the key event in each. [4 marks]
- Cue. Prophase: chromosomes condense, nuclear envelope breaks down. Metaphase: chromosomes align at equator. Anaphase: sister chromatids separate. Telophase: nuclear envelopes reform.
Q2. A cell biologist measures DNA content per cell across the cell cycle: G1 = 2C, S = increases to 4C, G2 = 4C, M = 4C until anaphase, then 2C per daughter cell. Sketch in words how a graph of DNA content versus time would look, identifying each phase. [3 marks]
- Cue. Flat at 2C during G1, sloping up during S to 4C, flat at 4C through G2 and M (until anaphase), then drops to 2C at cytokinesis.
Q3. Compare prokaryotic binary fission with eukaryotic mitosis. (a) State two similarities. (b) State two differences. (c) Explain why binary fission is faster than mitosis. [2+2+2 marks]
- Cue. (a) Both replicate DNA before division; both produce genetically identical daughters. (b) Prokaryotes use circular DNA, no spindle, no nuclear envelope; eukaryotes use linear chromosomes, spindle, nuclear envelope. (c) Fewer steps and structures so cycle time is hours not days.
Exam-style practice questions
Practice questions written in the style of VCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
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.
2025 VCAA-style3 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.
Related dot points
- cells as the basic structural feature of life on Earth, including the distinction between prokaryotic and eukaryotic cells
A focused answer to the VCE Biology Unit 1 dot point on cells as the basic unit of life. Covers the cell theory, the distinction between prokaryotic and eukaryotic cells, and the structural features (nucleus, membrane-bound organelles, ribosomes, cell wall) that separate them.
- apoptosis as a regulated process of programmed cell death, including the role of caspases, and the consequences of disruption to the regulation of the cell cycle and apoptosis with reference to the development of cancer
A focused answer to the VCE Biology Unit 1 dot point on apoptosis and cancer. Covers programmed cell death through initiator and effector caspases, the intrinsic and extrinsic pathways, and how loss of checkpoint control (mutations in tumour suppressor genes such as p53, or activation of proto-oncogenes) leads to cancer.
- the structure and specialisation of plant and animal cell organelles for distinct functions, including chloroplasts and mitochondria, and the suggested origins of mitochondria and chloroplasts as described by the endosymbiotic theory
A focused answer to the VCE Biology Unit 1 dot point on cell organelles. Covers the structure and function of the nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuole, cytoskeleton and cell wall, and the endosymbiotic theory for the origin of mitochondria and chloroplasts.