How do the cells formed as a result of meiosis compare to the original cell?

Your body is made up of trillions of that all originate from just one—a fertilized egg. The massive multiplication of cells after conception is possible thanks to cell division, which occurs when one cell splits into two. Cell division not only enables growth but also replaces damaged or dead cells and makes reproduction possible. There are two kinds of cell division: and .

What’s the Difference?

Mitosis produces two genetically identical “daughter” cells from a single “parent” cell, whereas meiosis produces cells that are genetically unique from the parent and contain only half as much . Most cells in the body regularly go through mitosis, but some do so more often than others. For instance, those that line the stomach replace themselves after just a few days because they’re subjected to strong digestive acids. In contrast, liver cells may wait up to a year to replace themselves. There are also a few types that last a lifetime without dividing, such as some nerve cells and cells that make up the lens of the eye. Meiosis occurs only in the production of sperm and egg cells for sexual reproduction.

The Phases of Cell Division

Before either mitosis or meiosis occurs, cells go through a preparatory process called interphase, where they grow and make a copy of their genetic information.

Mitosis has six phases apart from interphase. The first five phases divide the and its genetic information in half, while the final step splits the entire parent cell into two identical daughter cells. The phases of mitosis are:

1. Prophase: , which contain genetic information, condense and prepare to attach to the spindle—a cellular machine that moves chromosomes during cell division. 
2. Prometaphase: The nuclear —a structure that typically contains the chromosomes—breaks apart, the spindle forms, and chromosomes attach to its strong, hollow fibers.
3. Metaphase: Chromosomes align along the spindle’s center. 
4. Anaphase: Chromosomes pull apart and move toward the spindle’s poles, which also move apart.
5. Telophase: New nuclear envelopes form around the two separated sets of chromosomes. 
6. Cytokinesis: Cells divide. 

Meiosis has similar steps to mitosis but with two sets of divisions. The first division results in two cells that each have two sets of chromosomes, like in mitosis. The second division creates four cells that each contain one set of chromosomes, because the genetic information isn’t copied a second time. One unique feature of meiosis, which takes place during the first round of prophase (prophase I), is a process called . DNA is mixed between matching chromosomes from the different parents, increasing the genetic diversity.

NIGMS-Funded Cell Division Research

Many scientists who are supported by NIGMS study cell division. Some of these researchers are investigating how cells:

• Maintain normal cell division through aligning and separating chromosomes via the spindle during mitosis and meiosis
• Prevent genetic errors from being introduced during crossing over in meiosis
• Decide to either grow and divide or enter a resting phase, which is important in understanding diseases caused by excessive or insufficient cell division

This post is a great supplement to .

Our bodies may not regenerate like some of the research organisms described in Pathways, but they can heal and replace damaged cells through mitosis—the subject of this post.

All cells arise from other cells through the process of cell division. Meiosis is a specialized form of cell division that produces reproductive cells, such as plant and fungal spores and sperm and egg cells.

In general, this process involves a "parent" cell splitting into two or more "daughter" cells. In this way, the parent cell can pass on its genetic material from generation to generation.

Eukaryotic cells and their chromosomes

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.

Animals, plants and fungi are all eukaryotes. Eukaryotic cells have specialized components called organelles, such as mitochondria, chloroplasts and the endoplasmic reticulum. Each of these performs a specific function. Unlike prokaryotes, eukaryotic DNA is packed within a central compartment called the nucleus.

Within the eukaryotic nucleus, long double-helical strands of DNA are wrapped tightly around proteins called histones. This forms a rod-like structure called the chromosome.

Cells in the human body have 23 pairs of chromosomes, or 46 in total. This includes two sex chromosomes: two X chromosomes for females and one X and one Y chromosome for males. Because each chromosome has a pair, these cells are called "diploid" cells.  

On the other hand, human sperm and egg cells have only 23 chromosomes, or half the chromosomes of a diploid cell. Thus, they are called "haploid" cells.

When the sperm and egg combine during fertilization, the total chromosome number is restored. That's because sexually reproducing organisms receive a set of chromosomes from each parent: a maternal and paternal set. Each chromosome has a corresponding pair, orhomolog.

Mitosis vs. meiosis

Eukaryotes (opens in new tab) are capable of two types of cell division: mitosis and meiosis

Mitosis allows for cells to produce identical copies of themselves, which means the genetic material is duplicated from parent to daughter cells. Mitosis produces two daughter cells from one parent cell.

Single-celled eukaryotes, such as amoeba and yeast, use mitosis to reproduce asexuallyand increase their population. Multicellular eukaryotes, like humans, use mitosis to grow or heal injured tissues.

Meiosis, on the other hand, is a specialized form of cell division that occurs in organisms that reproduce sexually. As mentioned above, it produces reproductive cells, such as sperm cells, egg cells, and spores in plants and fungi.

In 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 (opens in new tab), each have 23 genetically unique chromosomes.

The overall process of meiosis produces four daughter cells from one single parent cell. Each daughter cell is haploid, because it has half the number of chromosomes as the original parent cell.

"Meiosis is reductional," said M. Andrew Hoyt, a biologist and professor at Johns Hopkins University.  

Unlike in mitosis, the daughter cells produced during meiosis are genetically diverse. Homologous chromosomes exchange bits of DNA to create genetically unique, hybrid chromosomes destined for each daughter cell.

A closer look at meiosis

Before meiosis begins, some important changes take place within the parent cells. First, each chromosome creates a copy of itself. These duplicated chromosomes are known as sister chromatids. They are fused together and the point where they are joined is known as the centromere. Fused sister chromatids roughly resemble the shape of the letter "X."

Meiosis occurs over the course of two rounds of nuclear divisions, called meiosis I and meiosis II, according to Nature Education's Scitable (opens in new tab). Furthermore, meiosis I and II are each divided into four major stages: prophase, metaphase, anaphase and telophase.

Meiosis I is responsible for creating genetically unique chromosomes. Sister chromatids pair up with their homologs and exchange genetic material with one another. At the end of this division, one parent cell produces two daughter cells, each carrying one set of sister chromatids.

Meiosis II closely resembles mitosis. The two daughter cells move into this phase without any further chromosome duplication. The sister chromatids are pulled apart during this division. A total of four haploid daughter cells are produced during the course of meiosis II.

Meiosis I

The four stages of meiosis Iare as follows, according to "Molecular Biology of the Cell." (Garland Science, 2002):

Prophase I: At this stage, chromosomes become compact, dense structures and are easily visible under the microscope. The homologous chromosomes pair together. The two sets of sister chromatids resemble two X's lined up next to each other. Each set exchanges bits of DNA with the other and recombines, thus creating genetic variation. This process is known as crossing over, or recombination.

Even though in humans the male sex chromosomes (X and Y) are not exact homologs, they can still pair together and exchange DNA. Crossing over occurs within only a small region of the two chromosomes.

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

Metaphase I: The meiotic spindle, a network of protein filaments, emerges from two structures called the centrioles, 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. 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, and the cell body splits into two.

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

Meiosis II

"Meiosis II looks like mitosis," Hoyt told Live Science. "It's an equational division."

In other words, 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, according to “Molecular Biology of the Cell, 4th edition.”

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 start to contract and pull the sister chromatids apart. Each individual chromosome now begins to moves 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 haploid 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 in the original parent cell.

Why is meiosis important?

Proper “chromosomal segregation,” or the separation of sister chromatids during meiosis I and II is essential for generating healthy sperm and egg cells, and by extension, healthy embryos. If chromosomes fail to segregate completely, it's called nondisjunction and can result in the formation of gametes that have missing or extra chromosomes, according to "Molecular Biology of the Cell, 4th edition."

When gametes with abnormal chromosome numbers fertilize, most of the resulting embryos don't survive. However, not all chromosomal abnormalities are fatal to the embryo. For example, Down syndrome occurs as a result of having an extra copy of chromosome 21. And, people with Klinefelter syndrome are genetically male but have an extra X chromosome.

The most significant impact of meiosis is that it generates genetic diversity, and that's a major advantage for species survival.

"Shuffling the genetic information allows you to find new combinations which will perhaps be more fit in the real world," Hoyt said. 

How does it compare to the original number of chromosomes in meiosis?

What is the relationship between the cells at the end of meiosis and the original cell?