Cell division follows a precise sequence of phases: G1, S, G2, and M. The first three phases (G1, S, and G2) make up interphase, where the cell grows and copies its DNA. The final phase, M, is when the cell actually divides. Within M phase, mitosis itself unfolds through a series of distinct stages that split the duplicated chromosomes into two identical sets.
The Four Phases of the Cell Cycle
Every dividing cell cycles through the same four phases in the same order. After a cell finishes dividing, it enters G1 (gap 1), where it grows, produces proteins, and carries out its normal functions. The cell does not copy its DNA during G1.
G1 is followed by S phase (synthesis), the period when the cell replicates all of its DNA. A human cell enters S phase with 46 chromosomes, each made of a single DNA molecule, and exits with 46 chromosomes that each consist of two identical sister chromatids joined at a central point called the centromere. After S phase, the cell moves into G2 (gap 2), where it continues growing and builds the specific proteins it will need to physically divide. G2 leads directly into M phase: mitosis and the splitting of the cytoplasm.
Most of a cell’s life is spent in interphase (G1 + S + G2). Mitosis is typically the shortest portion of the entire cycle.
The Stages of Mitosis
Mitosis is the portion of cell division where the duplicated chromosomes are separated into two identical sets. It proceeds through five stages, always in this order: prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase
The long, loosely organized DNA fibers condense into tightly packed chromosomes visible under a microscope. Each chromosome appears as two identical sister chromatids connected at the centromere. Meanwhile, the cell begins assembling the mitotic spindle, a structure made of protein filaments called microtubules. The two centrosomes (organizing centers for the spindle) start migrating toward opposite ends of the cell.
Prometaphase
The nuclear envelope, the membrane surrounding the nucleus, breaks apart. This is the pivotal moment that gives the spindle access to the chromosomes. Microtubules from each side of the cell reach into the former nuclear space and attach to the chromosomes at specialized protein structures on each centromere called kinetochores. Some microtubules don’t attach to chromosomes at all; instead, they overlap with microtubules from the opposite side and help push the two poles of the spindle apart.
Metaphase
All the chromosomes line up along an imaginary midline of the cell, equidistant from both poles. This alignment is called the metaphase plate. Every chromosome must be properly attached to microtubules from both poles before the cell proceeds. A built-in checkpoint monitors this: if any chromosome is unattached or incorrectly attached, the cell pauses here until the problem is corrected.
Anaphase
Anaphase is the shortest stage of mitosis. The connection between sister chromatids breaks, and the now-separated chromosomes are pulled toward opposite poles of the cell by the shortening spindle fibers. By the end of anaphase, each side of the cell holds a complete, identical set of chromosomes: 46 in a human cell.
Telophase
Two new nuclear envelopes form around each set of chromosomes. The tightly packed chromosomes begin to relax and decondense back into the loosely organized form used during interphase. The spindle microtubules disassemble. At this point, the division of the nucleus is complete.
Cytokinesis: Splitting the Cell in Two
Cytokinesis overlaps with the final stages of mitosis, beginning during anaphase and finishing as the next interphase begins. It splits the cytoplasm and organelles between the two new daughter cells, but the mechanism differs between animal and plant cells.
In animal cells, a ring of protein filaments tightens around the middle of the cell like a drawstring, creating a visible indentation called a cleavage furrow. The furrow deepens until the cell pinches completely in two. In plant cells, a rigid cell wall prevents pinching. Instead, the cell builds a new wall called a cell plate from the inside out, starting at the center and expanding outward until it reaches the existing cell wall on all sides. The structural preparation for this process actually begins back in G2, when a band of protein fibers forms a ring around the cell to mark the future division site.
Checkpoints That Control the Sequence
The cell doesn’t barrel through these phases automatically. Built-in checkpoints act as quality control gates at three critical points.
- G1 checkpoint: The cell evaluates whether conditions are right to commit to dividing. If DNA damage is detected, a protein called p53 halts the cycle so repairs can be made before DNA replication begins in S phase. If conditions aren’t favorable (insufficient nutrients, lack of growth signals), the cell can exit the cycle entirely and enter a resting state called G0.
- G2 checkpoint: After DNA replication, the cell checks for any errors or damage introduced during copying. If problems are found, the cell pauses here to allow repair before entering mitosis.
- Metaphase checkpoint (spindle checkpoint): The cell verifies that every chromosome is correctly attached to spindle fibers from both poles. If even one chromosome is unattached or improperly connected, the cell blocks the transition to anaphase. This prevents daughter cells from ending up with the wrong number of chromosomes.
How Meiosis Differs From Mitosis
Meiosis is the specialized form of cell division that produces sex cells (eggs and sperm). It uses many of the same stages as mitosis but runs through them twice, with one critical difference in how chromosomes are sorted.
Meiosis begins with a single round of DNA replication, just like mitosis. But instead of one division, the cell divides twice: meiosis I and meiosis II. In meiosis I, homologous chromosomes (the matching pairs you inherited from each parent) line up together and then separate, with one going to each daughter cell. Sister chromatids stay joined. This is the step that halves the chromosome number from 46 to 23. During the extended prophase of meiosis I, homologous chromosomes physically exchange segments of DNA in a process called crossing over, which shuffles genetic material between maternal and paternal chromosomes.
Meiosis II follows immediately, often before the chromosomes have fully decondensed. It closely resembles mitosis: sister chromatids line up at the metaphase plate and then separate in anaphase. The result is four daughter cells, each with 23 chromosomes, half the number of the original cell. This is why combining an egg and a sperm restores the full count of 46.
The complete sequence of meiosis runs: prophase I, metaphase I, anaphase I, telophase I, then prophase II, metaphase II, anaphase II, and telophase II, with cytokinesis occurring after each round.