What Is Meiosis?
Meiosis is the process whereby chromosomes are copied, paired up and separated to create eggs or sperm.
Credit: NIGMS.

Meiosis is a specialized form of cell division that produces reproductive cells, such as plant and fungal spores, sperm and egg cells.

All cells originate from other cells. The primary mechanism by which this occurs is through cell division. In general, this process involves a “parent” cell splitting into two or more “daughter” cells. In this way, the parent cell is able to pass on its genetic material from generation to generation.

Based on the relative complexity of their cells, all living organisms are broadly classified as either prokaryotes or eukaryotes. Prokaryotes, such as bacteria, consist of a single cell with a simple internal structure. Their DNA floats freely within the cell in a twisted thread-like mass called the nucleoid. 

Eukaryotes, on the other hand, are organisms whose DNA is packed within a central compartment called the nucleus. Animals, plants and fungi are eukaryotes. Eukaryotic cells also usually have specialized components called organelles, such as mitochondria, chloroplasts, the endoplasmic reticulum, the Golgi apparatus and lysosomes. Each performs a specific function. 

Eukaryotes are capable of two types of cell division. Mitosis allows for cells to produce identical copies of themselves. Thus, the genetic material is duplicated between parent and daughter cells. Single-celled eukaryotes, such as amoeba and yeast, use mitosis to reproduce asexually and increase their population numbers. Multicellular eukaryotes use mitosis to grow or heal injured tissues.

The second type of cell division, meiosis, is a specialized form of cell division that occurs in organisms that reproduce sexually (that is, by combining the unique genetic information of two parents). Meiosis produces reproductive cells, such as sperm cells, egg cells and spores from plants and fungi.

Eukaryotic DNA is compartmentalized into the nucleus within the cell. The long double-helical strands of DNA are wrapped tightly around proteins called histones to form a rod-like structure:the chromosome. For example, cells in the human body have 46 chromosomes in total (this includes two sex chromosomes; XX for women and XY for men). 

However, human sperm and egg cells produced through meiosis only have 23 chromosomes. 

“Meiosis is reductional,” said M. Andrew Hoyt, a professor of biology at Johns Hopkins University. Meiosis produces daughter cells that have half the number of chromosomes as the parent cell. Thus, when fertilization occurs, the chromosome number is restored. For example, when the human sperm and egg combine during fertilization, they produce a zygote, a cell with 46 chromosomes. Since sexually reproducing organisms receive a set of chromosomes from each parent, each chromosome has a corresponding pair, or homolog.

The daughter cells produced during meiosis are genetically diverse. Homologous chromosomes (that is, chromosome pairs from the mother and the father) exchange bits of DNA to create genetically unique, hybrid chromosomes destined for each daughter cell.

Meiosis begins with chromosome duplication. Then the cell goes through two consecutive rounds of nuclear divisions, termed meiosis I and meiosis II, according to the journal Nature. As a result, the overall process of meiosis produces four daughter cells from one single parent cell. Each daughter cell has half the number of chromosomes as the original parent cell. The mechanisms of mitosis and meiosis are very similar.

Meiosis I

Prior to meiosis I, the duplicated chromosomes, also known as sister chromatids, fuse together. The point at which they are joined is called the centromere, and the complex resembles the shape of the letter “X.” The chromosomes become compacted, dense structures during each nuclear division, and are visible under the microscope.

There are four stages in meiosis I, according to the authors of “Molecular Biology of the Cell, 4th Ed” (Garland Science, 2002):

Prophase I: During this stage, sister chromatids from the maternal set of chromosomes pair together with their homologs from the paternal set of chromosomes. Together, they resemble two X’s sitting next to each other. The maternal chromatids exchange bits of their DNA with the paternal chromatids and recombine to create genetic variation. Even though the sex chromosomes in a male human (X and Y) are not homologs, they can still pair together and exchange DNA. Recombination only occurs within a small region of the two chromosomes where there is homology.

By the end of prophase I the nuclear membrane breaks down.

Metaphase I: The meiotic spindle, a network of protein filaments, emerges from a structure called the centriole, positioned at either end of the cell. The meiotic spindle latches onto the fused sister chromatids. By the end of metaphase I, all the fused sister chromatids are tethered at their centromeres, and line up in the middle of the cell. The homologs still look like two X’s sitting close together.

Anaphase I: The spindle fibers start to contract, pulling the fused sister chromatids with them. That is to say, each X-shaped complex moves away from the other, toward opposite ends of the cell.

Telophase I: The fused sister chromatids reach either end of the cell. The cell body splits into two. 

Meiosis I results in two daughter cells, each of which contains a set of fused sister chromatids. Furthermore, the genetic makeup of each daughter cell is distinct because of the DNA exchange between homologs.  

The two daughter cells from meiosis I move into meiosis II without any further chromosome duplication.

Meiosis II

“Meiosis II looks like mitosis,” Hoyt told LiveScience. “It’s an equational division.” That means that by the end of the process, the chromosome number is unchanged between the cells that enter meiosis II and the resulting daughter cells.

The four stages of meiosis II are as follows:

Prophase II: The nuclear membrane disintegrates, and meiotic spindles begin to form once again.

Metaphase II: The meiotic spindles latch onto the centromere of the sister chromatids, and they all line up at the center of the cell.

Anaphase II: The spindle fibers begin to contract and the sister chromatids are pulled apart. Each individual chromosome now begins to move to either end of the cell.

Telophase II: The chromosomes reach opposite ends of the cell. The nuclear membrane forms again and the cell body splits into two

Meiosis II results in four daughter cells, each with the same number of chromosomes. However, each chromosome is unique and contains a mix of genetic information from the maternal and paternal chromosomes contained in the original parent cell.

In the case of humans, special cells called germ cells undergo meiosis and ultimately give rise to sperm or eggs. Germ cells contain a complete set of 46 chromosomes (23 maternal chromosomes and 23 paternal chromosomes). By the end of meiosis, the resulting reproductive cells, or gametes, each have 23 genetically unique chromosomes.

Proper segregation of chromosomes during meiosis I and II is essential to generate healthy sperm and egg cells, and by extension, healthy embryos. Failed chromosomal segregation is called nondisjunction, and can result in gametes with missing chromosomes or extra chromosomes, according to the authors of "Molecular Biology of the Cell."

When gametes with abnormal chromosome numbers fertilize, most resulting embryos do not survive and are destroyed through spontaneous abortions (miscarriages). However, not all chromosomal abnormalities are fatal, according to "Molecular Biology of the Cell." For example, Down syndrome is a result of having an extra copy of chromosome 21. People with Klinefelter syndrome are male, but have an extra X chromosome. The occurrence of such segregation errors during meiosis increases with the age of the mother.

One of the most significant impacts of meiosis is that it generates genetic diversity through the recombination of homologous chromosomes. “We are re-assorting the genetic information into new combinations, and that has great benefit,” Hoyt told LiveScience. “Heredity works best when you’re not just making exact duplicates. Shuffling the genetic information allows you to find new combinations which will perhaps be more fit in the real world.”

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