Mitosis vs. Meiosis: How Human Cells Divide to Grow and Reproduce

Every cell in the human body traces back to a single fertilized egg, and the instructions for turning one cell into trillions rely on two distinct types of division: mitosis and meiosis. The U.S. National Institutes of Health's National Human Genome Research Institute (NHGRI) and National Institute of General Medical Sciences (NIGMS) both describe these as the two core mechanisms cells use to multiply or to create the sex cells needed for reproduction.

The Cell Cycle: Where Division Fits In

According to NHGRI's genetics glossary, a dividing cell first passes through interphase, made up of three stages: G1, when the cell prepares to divide; S, when it duplicates its entire set of DNA (S stands for DNA synthesis); and G2, when it organizes and condenses the newly copied genetic material. Division itself happens in the M phase, when the cell partitions its two DNA copies into two new daughter cells.

Mitosis: Copying a Cell Exactly

Mitosis produces two daughter cells that are genetically identical to the parent cell and to each other, NHGRI states. NIGMS notes that this is the division responsible for ordinary body growth and tissue replacement — stomach lining cells, for instance, replace themselves every few days, while liver cells can take up to a year to turn over.

NIGMS lists six stages of mitosis following interphase: prophase, when chromosomes condense and attach to the spindle apparatus; prometaphase, when the nuclear membrane breaks down and the spindle fully forms; metaphase, when chromosomes line up at the cell's center; anaphase, when sister chromatids separate and move toward opposite poles; telophase, when new nuclear envelopes form around each set; and cytokinesis, when the cell physically splits in two.

Meiosis: Making Egg and Sperm Cells

Meiosis is used exclusively to produce gametes — eggs and sperm — and works differently. NHGRI explains that a diploid cell, which carries 46 chromosomes (two sets of 23), copies its DNA once but then divides twice, yielding four haploid daughter cells, each with just 23 chromosomes. When a sperm cell and an egg cell unite at fertilization, their combined 23 chromosomes from each parent restore the normal diploid number of 46.

The Embryo Project Encyclopedia at Arizona State University describes meiosis as occurring in two stages. Meiosis I is the reduction division, split into prophase I, metaphase I, anaphase I, and telophase I; it is during prophase I that matching (homologous) chromosomes pair up and exchange segments of DNA in a process called crossing over, which NIGMS says increases genetic diversity. Meiosis II then resembles mitosis, separating sister chromatids to complete the formation of four haploid cells from one original diploid cell.

According to the Embryo Project Encyclopedia, egg and sperm development diverge sharply in timing. Spermatogenesis begins at puberty and continuously produces four viable sperm from each starting cell. Oogenesis, by contrast, begins before birth but pauses at a specific point in prophase I until puberty; when division resumes, the cell divides unevenly, producing one large, nutrient-rich egg and small discarded structures called polar bodies, so only one usable gamete results per cycle.

A Brief History

The Embryo Project Encyclopedia credits biologist Oscar Hertwig with first describing meiosis in the mid-1870s using sea urchin cells. Evolutionary biologist August Weismann recognized in 1890 that the process reduced chromosome number by half, and geneticist Thomas Hunt Morgan demonstrated chromosomal crossing over in 1911.

When Meiosis Goes Wrong

Errors during meiosis can have medical consequences. The U.S. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) states that Down syndrome results from a random error in cell division called nondisjunction, in which a chromosome pair fails to separate properly — both copies end up in one resulting cell and none in the other. The most common form, complete trisomy 21, occurs when every cell carries three copies of chromosome 21 instead of two.

NICHD reports that in most trisomy 21 cases the extra chromosome comes from the mother's egg, while less than 5% originate from the father's sperm. A separate form, mosaic trisomy 21, also accounts for under 5% of cases and occurs when only some of a person's cells carry the extra chromosome. NICHD states plainly that no parental behavior or environmental factor is known to cause Down syndrome, and that the condition arises randomly during the formation of eggs or sperm.