Meiosis is a special type of cell division that produces sex cells, also called gametes. It takes one cell and divides it twice, ultimately creating cells with half the original number of chromosomes. In humans, that means going from 46 chromosomes down to 23. This reduction is essential for sexual reproduction: when a sperm and egg combine at fertilization, the resulting embryo gets the full set of 46 back.
Why Halving the Chromosomes Matters
Your body’s regular cells contain chromosomes in pairs, 23 pairs for a total of 46. One set came from your mother, the other from your father. If sex cells also carried 46 chromosomes, every generation would double the count. Meiosis solves this by splitting the pairs apart so each sperm or egg carries only one chromosome from each pair.
In men, meiosis produces four functional sperm cells from a single starting cell. In women, the same process technically produces four cells, but only one becomes a viable egg. That one cell receives most of the original cell’s contents, while the other three (called polar bodies) are tiny and break down.
Meiosis I: Separating Chromosome Pairs
Before meiosis begins, the cell copies all of its DNA, so each chromosome exists as two identical sister chromatids joined together. That means the cell temporarily holds 92 chromatids organized into 46 doubled chromosomes. During the first division, matching chromosomes (called homologs) pair up side by side, forming groups of four chromatids known as tetrads.
The longest and most complex phase is prophase I. This is where the paired homologs physically swap segments of DNA in a process called crossing over. The cell builds a protein scaffold between the paired chromosomes that holds them in precise alignment while the exchange happens. Sections of one chromosome’s DNA are cut, swapped with the corresponding section on its partner, and reattached. The result is chromosomes that carry new combinations of genetic information, blending traits inherited from each parent.
After crossing over, the cell pulls the homologous pairs apart. Each daughter cell ends up with 23 chromosomes instead of 46, but each chromosome still consists of two joined sister chromatids. This is the step that actually halves the chromosome number.
Meiosis II: Splitting Sister Chromatids
The second division works much like ordinary cell division (mitosis). The two sister chromatids in each chromosome are pulled to opposite sides of the cell. There is no further reduction in chromosome number. The chromatids simply separate into individual chromosomes, one per cell. At the end, four cells exist, each carrying 23 single chromosomes.
How Meiosis Creates Genetic Diversity
If meiosis just split chromosomes in half the same way every time, siblings with the same parents would be nearly identical. Two mechanisms prevent that.
- Independent assortment. When the 23 homologous pairs line up during meiosis I, which chromosome from each pair goes to which daughter cell is random. With 23 pairs, that creates over 8 million possible combinations in a single sex cell.
- Crossing over. The DNA swapping that happens during prophase I reshuffles gene versions along each chromosome. Every crossover event creates a chromosome that never existed before in either parent, multiplying the possible genetic combinations far beyond 8 million.
Crossing over tends to happen more frequently in certain regions of chromosomes. It is inhibited near the centers (centromeres) and tips (telomeres) of chromosomes in many species, including humans.
Timing Differs Between Sperm and Eggs
In males, the entire process of going from a precursor cell to mature sperm takes about 65 days, and production runs continuously from puberty onward.
In females, the timeline is dramatically different. Egg precursor cells enter meiosis before birth but pause partway through prophase I. They remain frozen at that stage for years, sometimes decades, until ovulation. When a particular egg is selected during a menstrual cycle, it resumes meiosis I and then immediately pauses again, this time at the second division. It only completes meiosis II if a sperm fertilizes it. This means an egg released at age 35 has been sitting in a paused state of meiosis for roughly 35 years.
That long pause is one reason the risk of chromosomal errors in eggs increases with maternal age. The cellular machinery holding chromosomes in place can deteriorate over time.
What Happens When Meiosis Goes Wrong
The most common error in meiosis is called nondisjunction, where chromosomes fail to separate properly. Instead of each daughter cell getting one copy, one cell ends up with two copies and the other gets none. If a sex cell with an extra or missing chromosome is fertilized, the resulting embryo will have an abnormal chromosome count.
Several well-known conditions result from these errors:
- Down syndrome occurs when there are three copies of chromosome 21 instead of two.
- Turner syndrome occurs in females who have only one X chromosome (45 total) instead of two.
- Klinefelter syndrome occurs in males who carry an extra X chromosome (47 total, with XXY).
Most other types of whole-chromosome errors are so severe that the embryo cannot develop, which is why these three conditions are among the few that survive to birth. Nondisjunction can happen during either meiosis I or meiosis II, but errors during the first division are more common and tend to have broader consequences because entire homologous pairs fail to separate.