Meiosis has eight main phases, split across two rounds of cell division called meiosis I and meiosis II. A single cell with 46 chromosomes (in humans) goes through both rounds and produces four cells, each with 23 chromosomes. These are the cells that become sperm or eggs. Here’s what happens at each stage.
How Meiosis Differs From Regular Cell Division
Normal cell division (mitosis) copies a cell into two genetically identical daughters. Meiosis is fundamentally different: it cuts the chromosome count in half and shuffles genetic material so that every resulting cell is unique. It accomplishes this by dividing twice without copying its DNA in between. The first division separates paired chromosomes; the second division separates the two copies within each chromosome. When a sperm and egg later fuse at fertilization, the full chromosome count is restored.
Meiosis I: The Four Phases
Meiosis I is called the “reductional division” because it’s the step that actually halves the chromosome number. Of the two rounds, this one takes far longer and does most of the genetic shuffling.
Prophase I
Prophase I is the longest and most complex phase of the entire process. In human egg cells, it can last years. During this phase, chromosomes condense and become visible, and each chromosome already consists of two identical sister chromatids joined at a central point called the centromere, giving it an X shape.
The defining event of prophase I is that matching chromosomes (one inherited from your mother, one from your father) find each other and physically pair up. This pairing, called synapsis, is held together by a zipper-like protein structure that forms along the full length of the paired chromosomes. Each paired unit, called a bivalent, contains four chromatids total.
While the chromosomes are tightly paired, something remarkable happens: segments of DNA break and swap between the maternal and paternal chromosomes. This is called crossing over, or recombination. It creates chromosomes with entirely new combinations of genetic information that didn’t exist in either parent. The visible connection points where this exchange occurred are called chiasmata, and they serve double duty: they’re also what physically holds the paired chromosomes together until it’s time to separate them. By the end of prophase I, the nuclear membrane breaks down and the spindle (a structure of protein fibers) forms to prepare for pulling chromosomes apart.
Metaphase I
The paired chromosomes line up along the middle of the cell. Each pair orients randomly, meaning the maternal chromosome can face either side of the cell, and so can the paternal one. This random orientation is called independent assortment, and it’s a major source of genetic diversity. With 23 pairs of chromosomes in humans, there are over 8 million possible combinations of maternal and paternal chromosomes in each resulting cell, before even counting the variation from crossing over.
Spindle fibers from opposite sides of the cell attach to each member of the pair. Critically, both sister chromatids of one chromosome are attached to the same side, so they’ll travel together. This is the opposite of what happens in regular cell division, where sister chromatids get pulled apart.
Anaphase I
The chiasmata break, and the spindle fibers pull the two members of each chromosome pair toward opposite ends of the cell. The homologous chromosomes separate, but the sister chromatids within each chromosome stay connected. Each side of the cell now has 23 chromosomes (in humans), down from 46.
Telophase I
The chromosomes arrive at opposite poles, and the cell divides in two. Each daughter cell has half the original chromosome number, though each chromosome still consists of two joined sister chromatids. Importantly, these two daughter cells are not genetically identical to each other because of the crossing over and random assortment that occurred earlier.
Between the Two Divisions
There is a brief gap between meiosis I and meiosis II, sometimes called interkinesis. The critical point is that no DNA replication occurs during this interval. This is what makes the second division a true reduction: the cell enters meiosis II with half the chromosomes and doesn’t duplicate them first.
Meiosis II: The Four Phases
Meiosis II closely resembles a normal mitotic division. Its job is to separate the sister chromatids that have been joined together since before meiosis I began. Both daughter cells from meiosis I go through this process simultaneously.
Prophase II
Chromosomes condense again (if they had decondensed), the nuclear membrane breaks down, and a new spindle forms. No pairing or crossing over occurs this time.
Metaphase II
The individual chromosomes (each still made of two sister chromatids) line up along the middle of the cell. This time, spindle fibers from opposite poles attach to each sister chromatid, setting them up to be pulled apart.
Anaphase II
The sister chromatids finally separate, each one now an independent chromosome. They are pulled to opposite sides of the cell.
Telophase II
The chromosomes arrive at the poles, nuclear membranes re-form, and each cell divides. The end result: four haploid cells, each with 23 single chromosomes. Every one of these four cells is genetically unique.
Chromosome Count at Each Stage
Tracking chromosome and chromatid counts helps clarify what each division actually accomplishes. In human cells:
- Start of meiosis I (prophase I): 46 chromosomes, 92 chromatids (each chromosome is doubled)
- After meiosis I (telophase I): 23 chromosomes per cell, 46 chromatids per cell
- After meiosis II (telophase II): 23 chromosomes per cell, 23 chromatids per cell
Where Meiosis Pauses in Egg Cells
In females, meiosis doesn’t run straight through. Egg cells pause at two specific points. The first arrest happens during prophase I, at a substage called diplotene, while the person is still an embryo. Those cells stay frozen in prophase I until puberty, and some remain paused for nearly 50 years. A hormonal signal triggers each egg to resume meiosis during ovulation.
The second arrest occurs at metaphase II. The egg stays locked at this stage until fertilization, when a flood of calcium inside the cell breaks the block and allows meiosis II to finish. This means the final phase of meiosis in an egg cell is only completed after a sperm arrives.
What Happens When Phases Go Wrong
The most common error in meiosis is called nondisjunction, which means chromosomes fail to separate properly. The consequences depend on when the error happens.
If chromosomes fail to separate during anaphase I, all four resulting cells end up abnormal: two will have an extra chromosome and two will be missing one. If the error happens later, during anaphase II, only two of the four cells are affected (one with an extra chromosome, one missing one), while the other two are normal.
When a gamete with the wrong chromosome number fuses with a normal gamete during fertilization, the resulting embryo has an abnormal total. An extra copy of chromosome 21, for example, causes Down syndrome. Most chromosomal imbalances are far more severe and result in early pregnancy loss. The risk of nondisjunction increases with the age of the egg cell, which is one reason that the chance of chromosomal conditions rises with maternal age.