DNA, in the form of chromosomes, is divided so that each daughter cell has a complete copy of the genetic material (or genome). Organisms that reproduce sexually have two copies of each chromosome, one from their father and one from their mother. Show MeiosisA special form of cell division needed to produce sex cells - for example, sperm and eggs with only one copy of each chromosome. Fusion of the sex cells creates a new individual with two copies of each chromosome. Many single-celled organisms reproduce by cell division and have a single copy of each chromosome. 
Cells divide and reproduce in two ways, mitosis and meiosis. Mitosis results in two identical daughter cells, whereas meiosis results in four sex cells. Below we highlight the keys differences and similarities between the two types of cell division.
This page was last updated on 2021-07-21 Í The life cycle of eukaryotic cells can generally be divided into four stages and a typical cell cycle is shown in Figure \(\PageIndex{13}\). When a cell is produced through fertilization or cell division, there is usually a lag before it undergoes DNA synthesis (replication). This lag period is called Gap 1 (G1), and ends with the onset of the DNA synthesis (S) phase, during which each chromosome is replicated. Following replication, there may be another lag, called Gap 2 (G2), before mitosis (M). Cells undergoing meiosis do not usually have a G2 phase. Interphase is as term used to include those phases of the cell cycle excluding mitosis and meiosis. Many variants of this generalized cell cycle also exist. Some cells never leave G1 phase, and are said to enter a permanent, non-dividing stage called G0. On the other hand, some cells undergo many rounds of DNA synthesis (S) without any mitosis or cell division, leading to endoreduplication. Understanding the control of the cell cycle is an active area of research, particularly because of the relationship between cell division and cancer.
The amount of DNA within a cell changes following each of the following events: fertilization, DNA synthesis, mitosis, and meiosis (Fig 2.14). We use “c” to represent the DNA content in a cell, and “n” to represent the number of complete sets of chromosomes. In a gamete (i.e. sperm or egg), the amount of DNA is 1c, and the number of chromosomes is 1n. Upon fertilization, both the DNA content and the number of chromosomes doubles to 2c and 2n, respectively. Following DNA replication, the DNA content doubles again to 4c, but each pair of sister chromatids is still counted as a single chromosome (a replicated chromosome), so the number of chromosomes remains unchanged at 2n. If the cell undergoes mitosis, each daughter cell will return to 2c and 2n, because it will receive half of the DNA, and one of each pair of sister chromatids. In contrast, the 4 cells that come from meiosis of a 2n, 4c cell are each 1c and 1n, since each pair of sister chromatids, and each pair of homologous chromosomes, divides during meiosis.
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Cell division cycle, figure from Wikipedia. Cells that stop dividing exit the G1 phase of the cell cycle into a so-called G0 state. Cells reproduce genetically identical copies of themselves by cycles of cell growth and division. The cell cycle diagram on the left shows that a cell division cycle consists of 4 stages:
ChromosomesChromosomes were first named by cytologists viewing dividing cells through a microscope. The modern definition of a chromosome now includes the function of heredity and the chemical composition. A chromosome is a DNA molecule that carries all or part of the hereditary information of an organism. In eukaryotic cells, the DNA is packaged with proteins in the nucleus, and varies in structure and appearance at different parts of the cell cycle. In G1, each chromosome is a single chromatid. In G2, after DNA replication in S phase, as cell enter mitotic prophase, each chromosome consists of a pair of identical sister chromatids, where each chromatid contains a linear DNA molecule that is identical to the joined sister. The sister chromatids are joined at their centromeres, as shown in the image below. A pair of sister chromatids is a single replicated chromosome, a single package of hereditary information. Human karyotype “painted” using fluorescent DNA probes. These mitotic chromosomes each consist of a pair of sister chromatids joined at their centromeres. The images of the homologous chromosome pairs (e.g., 2 copies of chromosome 1) have been lined up next to each other. Image from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157 Ploidy
Here is a simplified diagram illustrating the overall process and products of mitosis: Source: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:MajorEventsInMeiosis_variant_int.svg) Questions or points to ponder or note about the figure above (answers at bottom of page):
This animation below shows the packaging of DNA and condensation of chromosomes as a cell undergoes mitosis. MeiosisThis is a special sequence of 2 cell divisions that produces haploid gametes from diploid germline cells. It starts with a diploid cell that has undergone chromosomal DNA replication: 2N chromosomes, 4X DNA content. Two successive divisions, with no additional DNA replication, results in 4 haploid gametes: 1N chromosomes, 1X DNA content.
The last point appears to be the most difficult for students to grasp. Consider the X and Y chromosomes. They pair in prophase I, and then separate in the first division. The daughter cells of the first meiotic division have either an X or a Y; they don’t have both. Each cell now has only one sex chromosome, like a haploid cell. Here is a simplified diagram illustrating the overall process and products of meiosis: Meiosis Overview from Wikipedia by Rdbickel And here is a video that walks through the steps of meiosis:
Chromosomes, chromatids, what is the difference and how many chromosomes are there at different times of the cell cycle and after mitosis and meiosis?Chromosomes by definition contain the DNA that makes up the fundamental genome of the cell. In a prokaryote, the genome is usually packaged into one circular chromosome consisting of a circular DNA molecule of a few million base pairs (Mbp). In eukaryotes, the genome is packaged into multiple linear chromosomes, each consisting of a linear DNA molecule of tens or hundreds of Mbp. Chromosomes exist at all different phases of the cell cycle. They condense and become visible to light microscopy in prophase of mitosis or meiosis, and they decondense during interphase, in the form of chromatin (DNA wrapped around nucleosomes, like “beads on a string”). The chromosome number is reduced from 2N to 1N in the first meiotic division, and stays at 1N in the second meiotic division. Because homologous chromosomes separate in the first division, the daughter cells no longer have copies of each chromosome from both parents, so they have haploid genetic information, and a 1N chromosome number. The second meiotic division, where sister chromatids separate, is like mitosis. Chromosome number stays the same when sister chromatids separate. Using the information above, compare these two simplified diagrams of mitosis and meiosis to visualize why cells are haploid after meiosis I. Specifically, compare the chromosomes in cells at the end of mitosis vs the end of meiosis I, recognizing that the diagram of mitosis tracks just a single pair of homologous chromosomes, whereas the diagram of meiosis tracks two pairs of homologous chromosomes (one long chromosome and short chromosome): Meiosis Overview from Wikipedia by Rdbickel The video below is geared toward a high school audience, but it does present a helpful way for recognizing how many chromosomes are present in a cell (and thus the ploidy level of that cell). While watching, see if you can recognize why the products of meiosis 1 are haploid cells: Answers to questions about the mitosis figure:
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How does the amount of DNA produced in a cell during mitosis compared with that produced in meiosis
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