By Kevin Beck
Updated Aug 30, 2022

Andrew Brookes/Image Source/GettyImages

Cell division is the cornerstone of life, occurring in two distinct modes that serve different biological functions. Mitosis, the non‑sexual division of somatic cells, ensures growth, repair, and asexual reproduction. Meiosis, confined to the gonads, creates haploid gametes and drives genetic diversity through recombination and independent assortment.

What Are Cells?

Cells are the fundamental units of life, possessing five core capabilities: environmental sensing, growth, reproduction, homeostasis, and complex chemistry. Despite the remarkable diversity of organisms, at the microscopic level the architecture of a human cell resembles that of a plant cell—both contain a nucleus, cytoplasm, and defined boundaries.

Prokaryotes vs. Eukaryotes

Prokaryotes (bacteria and archaea) are almost exclusively unicellular, reproduce asexually via binary fission, and lack membrane‑bound organelles. Eukaryotes—animals, plants, fungi, and protists—are largely multicellular, possess complex organelles, and rely on mitosis and meiosis to propagate and diversify.

The Cell Cycle

In eukaryotes the cell cycle orchestrates growth and division. Interphase (G1, S, G2) prepares the cell: G1 involves growth and organelle duplication; S replicates the DNA; G2 assembles the machinery for division. The cycle culminates in the M phase—mitosis—followed by cytokinesis, which physically separates the two daughter cells.

Chromosome Basics

Genetic material in eukaryotes is packaged as chromatin—DNA wrapped around histone proteins—forming distinct chromosomes. Humans have 46 chromosomes: 22 pairs of autosomes and a pair of sex chromosomes (XX or XY). Although homologous chromosomes share a similar structure, their nucleotide sequences differ, enabling recombination during meiosis.

Mitosis vs. Meiosis

Both processes start with duplicated chromosomes (92 chromatids in a diploid cell). In mitosis, the 46 pairs of sister chromatids segregate to produce two genetically identical daughter cells. Meiosis, however, reduces the chromosome number by half and introduces genetic variation.

The Phases of Mitosis

• Prophase: Chromosomes condense; the nuclear envelope dissolves; the mitotic spindle forms.
• Prometaphase: Chromosomes attach to spindle microtubules and begin to move.
• Metaphase: Chromosomes align at the metaphase plate.
• Anaphase: Sister chromatids separate toward opposite poles.
• Telophase: Nuclear envelopes reform around each set of chromosomes, which decondense.

Cytokinesis follows, producing two daughter cells that each enter a new cell cycle.

The Two Stages of Meiosis

Meiosis consists of two consecutive divisions—meiosis I and meiosis II—producing four haploid gametes. Each division contains phases analogous to mitosis but with unique features.

Meiosis I

• Prophase I: Homologous chromosomes pair to form bivalents (tetrads) and undergo crossing over, shuffling genetic material.
• Metaphase I: Bivalents align at the metaphase plate; the orientation of each homolog is random, creating up to 2²³ (≈8.4 million) possible gamete combinations.
• Anaphase I: Homologous chromosomes segregate to opposite poles, while sister chromatids remain attached.
• Telophase I: Two haploid cells form, each still containing duplicated chromatids.

Meiosis II

• Meiosis II mirrors mitosis: the duplicated chromatids separate, resulting in four genetically distinct haploid cells.
• These cells become gametes—sperm in males and eggs in females—each carrying a single copy of the 23 chromosomes.
• Sex determination occurs during this stage: sperm carrying Y lead to male offspring; sperm with X can only contribute to daughters.

Meiosis and Genetic Diversity

Meiosis integrates two mechanisms—crossing over and independent assortment—to generate the vast genetic diversity seen in eukaryotes. Understanding these processes is essential for grasping concepts in genetics, evolution, and medicine.