Cell Division

Core Concepts

Why cells divide: the surface-area-to-volume problem

A cell cannot simply grow forever. As a sphere grows, its volume increases with the cube of its radius while its surface area increases only with the square — so the surface-area-to-volume ratio steadily shrinks. Since a cell exchanges nutrients, wastes, and signals across its plasma membrane, a falling ratio eventually means the surface cannot supply the volume: diffusion becomes too slow, and the cell starves or suffocates from the inside out. Division restores a favorable ratio by splitting the volume into two smaller cells, each with proportionally more surface. There is also a second constraint: the cell's DNA must be able to direct the whole cytoplasm, so a growing cell must keep its genome-to-cytoplasm ratio within workable limits. These size limits are why cells are small — and why dividing is not optional.

The genome and the chromosome

The genome is the complete set of genetic instructions of an organism. In a human somatic cell that is about 6 billion base pairs of DNA, organized into 46 chromosomes (23 pairs) — a commonly taught reference count; gametes carry half that number. Chromosomal DNA is wrapped around histone proteins, forming chromatin; during division the chromatin condenses into compact, visible chromosomes so the DNA can be moved without tangling. Before division, each chromosome is replicated: the two identical copies, called sister chromatids, remain attached at a constricted region called the centromere until division separates them. This means a human cell preparing to divide contains 46 chromosomes but 92 chromatids — one of the most frequently tested counting distinctions in the course. After division, each daughter cell receives one chromatid from each replicated chromosome, so each ends up with 46 unreplicated chromosomes again.

Binary fission: prokaryotic division

Prokaryotes have a single circular chromosome (plus sometimes plasmids) and no nucleus, so their division is simpler than mitosis. In binary fission, the chromosome's origin of replication attaches to the plasma membrane; replication begins there and proceeds around the circle while the cell elongates, physically pulling the two copies apart. When replication is complete, the membrane pinches inward at the middle, and the cell splits into two genetically identical daughter cells. Because the chromosome is attached to the membrane during replication, the two copies are guaranteed to end up on opposite sides of the pinch. Binary fission is fast — under good conditions some bacteria can divide in about 20–30 minutes (a commonly taught reference range) — which is why bacterial populations can explode so quickly.

Mitosis: equal distribution of replicated chromosomes

Eukaryotic somatic cells divide by mitosis (nuclear division) followed by cytokinesis (division of the cytoplasm). The purpose is equality: every replicated chromosome is split so that each daughter cell receives one copy of each chromosome — the same 46-chromosome genome as the parent. The mitotic spindle, built from microtubules, attaches to the centromeres and pulls the sister chromatids apart, one to each pole. The result is two genetically identical diploid daughter cells, each with unreplicated chromosomes. The phases of mitosis (prophase, prometaphase, metaphase, anaphase, telophase) and where mitosis fits into the cell cycle are the subject of the next topic; here the key idea is that mitosis is the distribution step, and it is accurate precisely because it is mechanical — spindle fibers, centromeres, and checkpoints all exist to prevent miscounting.

Binary fission, mitosis, and meiosis compared

All three replicate the DNA first, but they differ in how many divisions occur (mitosis: one; meiosis: two), how the chromosomes are distributed (mitosis separates sister chromatids; meiosis also separates homologous chromosomes), and what the ploidy of the products is. The common thread is fidelity: each process exists to pass on genetic information accurately, and each has its own machinery to ensure it.

Genome equivalence: the promise of identical daughters

A defining feature of mitosis is genome equivalence: barring mutation, every somatic cell in an organism carries the same genetic information as the fertilized egg it came from. This is why a skin cell and a liver cell contain the same DNA — they differ only in which genes they express, not in which genes they possess. Mistakes do happen, though: if a chromosome is lost or mis-separated during division, the daughter cells end up with the wrong chromosome number (aneuploidy), which in humans usually causes miscarriage or developmental conditions such as Down syndrome (an extra copy of chromosome 21). The machinery that catches these mistakes — the checkpoints of the cell cycle — is the subject of a later topic in this chapter.

Common Confusions