How do the sperm or egg cells made in meiosis compare to the original cell?

There are two types of cell division: mitosis and meiosis. Most of the time when people refer to “cell division,” they mean mitosis, the process of making new body cells. Meiosis is the type of cell division that creates egg and sperm cells.

Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by certain genes. When mitosis is not regulated correctly, health problems such as cancer can result.

The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of gene shuffling while the cells are dividing.

Living cells go through a series of stages known as the cell cycle. The cells grow, copy their chromosomes, and then divide to form new cells.

• G1 phase. The cell grows.
• S phase. The cell makes copies of its chromosomes. Each chromosome now consists of two sister chromatids.
• G2 phase. The cell checks the duplicated chromosomes and gets ready to divide.
• M phase. The cell separates the copied chromosomes to form two full sets (mitosis) and the cell divides into two new cells (cytokinesis).

The period between cell divisions is known as 'interphase'. Cells that are not dividing leave the cell cycle and stay in G0.

Mitosis and meiosis

Cells divide into two different ways to make new cells.

Mitosis

Mitosis is used to produce daughter cells that are genetically identical to the parent cells. The cell copies - or 'replicates' - its chromosomes, and then splits the copied chromosomes equally to make sure that each daughter cell has a full set.

Your body contains trillions of cells (thousands of millions). But you started life as a single cell - a fertilised egg cell. This cell then divided and divided to make more cells through a process called mitosis.

Mitosis is a way of making more cells that are genetically the same as the parent cell. It plays an important part in the development of embryos, and it is important for the growth and development of our bodies as well. Mitosis produces new cells, and replaces cells that are old, lost or damaged.

In mitosis a cell divides to form two identical daughter cells. It is important that the daughter cells have a copy of every chromosome, so the process involves copying the chromosomes first and then carefully separating the copies to give each new cell a full set.

Before mitosis, the chromosomes are copied. They then coil up, and each chromosome looks like a letter X in the nucleus of the cell. The chromosomes now consist of two sister chromatids. Mitosis separates these chromatids, so that each new cell has a copy of every chromosome. The copied chromosomes consist of two chromatids joined at the centromere.

The process of mitosis involves a number of different stages.

Meiosis

Meiosis is used to make special cells - sperm cells and egg cells - that have half the normal number of chromosomes. It reduces the number from 23 pairs of chromosomes to 23 single chromosomes. The cell copies its chromosomes, but then separates the 23 pairs to ensure that each daughter cell has only one copy of each chromosome. A second division that divides each daughter cell again to produce four daughter cells.

Some simple organisms - such as bacteria - can reproduce by simply dividing into two new individuals. Other organisms, including human beings, reproduce through sexual reproduction. New individuals are formed by the joining together of two special cells: a sperm cell and an egg cell.

The cells in our bodies contain 23 pairs of chromosomes - giving us 46 chromosomes in total. Sperm cells and egg cells contain 23 single chromosomes, half the normal number, and are made by a special form of cell division called meiosis.

Meiosis separates the pairs of matching (or 'homologous') chromosomes, so that sperm cells and egg cells have only one copy of each. That way, when an egg cell fuses with a sperm cell, the fertilised egg has a full set: that is, two copies of every chromosome.

Meiosis involves two cell divisions: Meiosis I and Meiosis II.

Meiosis I separates the matching - or 'homologous' - pairs of chromosomes.

Meiosis II divides each chromosome into two copies (much like mitosis).

In Meiosis I, each daughter cell receives a mix of chromosomes from the two sets in the parent cell. In addition, the chromosomes in each matching pair swap some genetic material before they are parted in a process called crossing over. These processes produce new combinations of genes in the sperm cells and egg cells.

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.