How do the daughter cells produced by mitosis compare to the original cell
Cell division cycle, figure from Wikipedia. Cells that stop dividing exit the G1 phase of the cell cycle into a so-called G0 state. Show
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 PloidyHumans are diploid, meaning we have two copies of each chromosome. We inherited one copy of each chromosome from other mother, and one copy of each from our father. Gametes (sperm cells or egg cells) are haploid, meaning that they have just one complete set of chromosomes. Chromosomes that do not differ between males and females are called autosomes, and the chromosomes that differ between males and females are the sex chromosomes, X and Y for most mammals. Humans most commonly have 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY), for a total of 46 chromosomes. We say that humans have 2N = 46 chromosomes, where N = 23, or the haploid number of chromosomes. Cells with complete sets of chromosomes are called euploid; cells with missing or extra chromosomes are called aneuploid. The most common aneuploid condition in people is variation in the number of sex chromosomes: XO (having just one copy of the X), XXX, or XYY. Having no X chromosome results in early embryonic death. The two copies of a particular chromosome, such as chromosome 1, are called homologous. The karyotype image above shows the homologous pairs for all the autosomes. Homologous chromosomes are not identical to each other, unlike sister chromatids. They frequently have different variants of the same hereditary information, such as blue eye color vs brown eye color or blood type A versus blood type B. MitosisMitosis produces two daughter cells that are genetically identical to each other, and to the parental cell. A diploid cell starts with 2N chromosomes and 2X DNA content. After DNA replication, the cells is still genetically diploid (2N chromosome number), but has 4X DNA content because each chromosome has replicated its DNA. Each chromosome now consists of a joined pair of identical sister chromatids. During mitosis the sister chromatids separate and go to opposite ends of the dividing cell. Mitosis ends with 2 identical cells, each with 2N chromosomes and 2X DNA content. All eukaryotic cells replicate via mitosis, except germline cells that undergo meiosis (see below) to produce gametes (eggs and sperm).
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. Meiosis sets the stage for Mendelian genetics. Students need to know that most of the genetics action occurs in the first meiotic division:
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. One way of thinking about ploidy is the number of possible alleles for each gene a cell can have. Right after meiosis I, the homologous chromosomes have separated into different cells. Each homolog carries one copy of the gene, and each gene could be a different allele, but these two homologs are now in two different cells. Though it looks like there are two of each chromosome in each cell, these are duplicated chromosomes; ie, it is one chromosome which has been copied, so there is only one possible allele in the cell (just two copies of it). The second meiotic division is where sister (duplicated) chromatids separate. It resembles mitosis of a haploid cell. At the start of the second division, each cell contains 1N chromosomes, each consisting of a pair of sister chromatids joined at the centromere. Meiosis Overview from Wikipedia by Rdbickel And here is a video that walks through the steps of meiosis: It is very important that you recognize how and why cells become haploid after meiosis I. To confirm for yourself that you understand meiosis, work through one or more of these interactive tutorials:
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, N, in eukaryotes, refers to the number of chromosomes in a haploid cell, or gamete (sperm or egg cell). Diploid cells (all the cells in our body except our gametes) have 2N chromosomes, because a diploid organism is created by union of 2 gametes each containing 1N chromosomes. In terms of chromosome number (ploidy), it’s useful to think of chromosomes as packages of genetic information. A pair of sister chromatids is one chromosome because it has genetic information (alleles) inherited from only one parent. A pair of homologous chromosomes, each consisting of a single chromatid in a daughter cell at the end of mitosis, has alleles from the father and from the mother, and counts as 2 chromosomes. This chromosome number stays the same after chromosome replication during S phase: each chromosome entering cell division now consists of a pair of sister chromatids joined together at the centromere. Then in mitosis, the sister chromatids of each chromosome separate, so each daughter cell receives one chromatid from each chromosome. The result of mitosis is two identical daughter cells, genetically identical to the original cell, all having 2N chromosomes. So during a mitotic cell cycle, the DNA content per chromosome doubles during S phase (each chromosome starts as one chromatid, then becomes a pair of identical sister chromatids during S phase), but the chromosome number stays the same. A chromatid, then, is a single chromosomal DNA molecule. The number of chromatids changes from 2X in G1 to 4X in G2 and back to 2X, but the number of chromosomes stays the same. 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: How does the original cell compared to the daughter cells that are produced?The dividing of the DNA ensures that both the “old” cell (parent cell) and the “new” cells (daughter cells) have the same genetic makeup and both will be diploid, or containing the same number of chromosomes as the parent cell.
How does the DNA of daughter cells compared to the original?Answer and Explanation: Because mitotic daughter cells are copies of the parent cell, the genetic material is also copied over with exactness from the parent. Therefore, the genetic material in daughter cells does not differ at all from that of the original cell.
How does the original cell compare to the daughter cells that are produced at the end of cytokinesis?The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell.
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